If you're used to using raw SQL to find database records, then you will generally find that there are better ways to carry out the same operations in Rails. Active Record insulates you from the need to use SQL in most cases.
Code examples throughout this guide will refer to one or more of the following models:
All of the following models use id
as the primary key, unless specified otherwise.
class Client < ActiveRecord::Base has_one :address has_many :orders has_and_belongs_to_many :roles end
class Address < ActiveRecord::Base belongs_to :client end
class Order < ActiveRecord::Base belongs_to :client, counter_cache: true end
class Role < ActiveRecord::Base has_and_belongs_to_many :clients end
Active Record will perform queries on the database for you and is compatible with most database systems (MySQL, PostgreSQL and SQLite to name a few). Regardless of which database system you're using, the Active Record method format will always be the same.
1 Retrieving Objects from the Database
To retrieve objects from the database, Active Record provides several finder methods. Each finder method allows you to pass arguments into it to perform certain queries on your database without writing raw SQL.
The methods are:
bind
create_with
eager_load
extending
from
group
having
includes
joins
limit
lock
none
offset
order
preload
readonly
references
reorder
reverse_order
select
distinct
uniq
where
All of the above methods return an instance of ActiveRecord::Relation
.
The primary operation of Model.find(options)
can be summarized as:
- Convert the supplied options to an equivalent SQL query.
- Fire the SQL query and retrieve the corresponding results from the database.
- Instantiate the equivalent Ruby object of the appropriate model for every resulting row.
- Run
after_find
callbacks, if any.
1.1 Retrieving a Single Object
Active Record provides five different ways of retrieving a single object.
1.1.1 Using a Primary Key
Using Model.find(primary_key)
, you can retrieve the object corresponding to the specified primary key that matches any supplied options. For example:
# Find the client with primary key (id) 10. client = Client.find(10) # => #<Client id: 10, first_name: "Ryan">
The SQL equivalent of the above is:
SELECT * FROM clients WHERE (clients.id = 10) LIMIT 1
Model.find(primary_key)
will raise an ActiveRecord::RecordNotFound
exception if no matching record is found.
1.1.2 take
Model.take
retrieves a record without any implicit ordering. For example:
client = Client.take # => #<Client id: 1, first_name: "Lifo">
The SQL equivalent of the above is:
SELECT * FROM clients LIMIT 1
Model.take
returns nil
if no record is found and no exception will be raised.
The retrieved record may vary depending on the database engine.
1.1.3 first
Model.first
finds the first record ordered by the primary key. For example:
client = Client.first # => #<Client id: 1, first_name: "Lifo">
The SQL equivalent of the above is:
SELECT * FROM clients ORDER BY clients.id ASC LIMIT 1
Model.first
returns nil
if no matching record is found and no exception will be raised.
1.1.4 last
Model.last
finds the last record ordered by the primary key. For example:
client = Client.last # => #<Client id: 221, first_name: "Russel">
The SQL equivalent of the above is:
SELECT * FROM clients ORDER BY clients.id DESC LIMIT 1
Model.last
returns nil
if no matching record is found and no exception will be raised.
1.1.5 find_by
Model.find_by
finds the first record matching some conditions. For example:
Client.find_by first_name: 'Lifo' # => #<Client id: 1, first_name: "Lifo"> Client.find_by first_name: 'Jon' # => nil
It is equivalent to writing:
Client.where(first_name: 'Lifo').take
1.1.6 take!
Model.take!
retrieves a record without any implicit ordering. For example:
client = Client.take! # => #<Client id: 1, first_name: "Lifo">
The SQL equivalent of the above is:
SELECT * FROM clients LIMIT 1
Model.take!
raises ActiveRecord::RecordNotFound
if no matching record is found.
1.1.7 first!
Model.first!
finds the first record ordered by the primary key. For example:
client = Client.first! # => #<Client id: 1, first_name: "Lifo">
The SQL equivalent of the above is:
SELECT * FROM clients ORDER BY clients.id ASC LIMIT 1
Model.first!
raises ActiveRecord::RecordNotFound
if no matching record is found.
1.1.8 last!
Model.last!
finds the last record ordered by the primary key. For example:
client = Client.last! # => #<Client id: 221, first_name: "Russel">
The SQL equivalent of the above is:
SELECT * FROM clients ORDER BY clients.id DESC LIMIT 1
Model.last!
raises ActiveRecord::RecordNotFound
if no matching record is found.
1.1.9 find_by!
Model.find_by!
finds the first record matching some conditions. It raises ActiveRecord::RecordNotFound
if no matching record is found. For example:
Client.find_by! first_name: 'Lifo' # => #<Client id: 1, first_name: "Lifo"> Client.find_by! first_name: 'Jon' # => ActiveRecord::RecordNotFound
It is equivalent to writing:
Client.where(first_name: 'Lifo').take!
1.2 Retrieving Multiple Objects
1.2.1 Using Multiple Primary Keys
Model.find(array_of_primary_key)
accepts an array of primary keys, returning an array containing all of the matching records for the supplied primary keys. For example:
# Find the clients with primary keys 1 and 10. client = Client.find([1, 10]) # Or even Client.find(1, 10) # => [#<Client id: 1, first_name: "Lifo">, #<Client id: 10, first_name: "Ryan">]
The SQL equivalent of the above is:
SELECT * FROM clients WHERE (clients.id IN (1,10))
Model.find(array_of_primary_key)
will raise an ActiveRecord::RecordNotFound
exception unless a matching record is found for all of the supplied primary keys.
1.2.2 take
Model.take(limit)
retrieves the first number of records specified by limit
without any explicit ordering:
Client.take(2) # => [#<Client id: 1, first_name: "Lifo">, #<Client id: 2, first_name: "Raf">]
The SQL equivalent of the above is:
SELECT * FROM clients LIMIT 2
1.2.3 first
Model.first(limit)
finds the first number of records specified by limit
ordered by primary key:
Client.first(2) # => [#<Client id: 1, first_name: "Lifo">, #<Client id: 2, first_name: "Raf">]
The SQL equivalent of the above is:
SELECT * FROM clients ORDER BY id ASC LIMIT 2
1.2.4 last
Model.last(limit)
finds the number of records specified by limit
ordered by primary key in descending order:
Client.last(2) # => [#<Client id: 10, first_name: "Ryan">, #<Client id: 9, first_name: "John">]
The SQL equivalent of the above is:
SELECT * FROM clients ORDER BY id DESC LIMIT 2
1.3 Retrieving Multiple Objects in Batches
We often need to iterate over a large set of records, as when we send a newsletter to a large set of users, or when we export data.
This may appear straightforward:
# This is very inefficient when the users table has thousands of rows. User.all.each do |user| NewsLetter.weekly_deliver(user) end
But this approach becomes increasingly impractical as the table size increases, since User.all.each
instructs Active Record to fetch the entire table in a single pass, build a model object per row, and then keep the entire array of model objects in memory. Indeed, if we have a large number of records, the entire collection may exceed the amount of memory available.
Rails provides two methods that address this problem by dividing records into memory-friendly batches for processing. The first method, find_each
, retrieves a batch of records and then yields each record to the block individually as a model. The second method, find_in_batches
, retrieves a batch of records and then yields the entire batch to the block as an array of models.
The find_each
and find_in_batches
methods are intended for use in the batch processing of a large number of records that wouldn't fit in memory all at once. If you just need to loop over a thousand records the regular find methods are the preferred option.
1.3.1 find_each
The find_each
method retrieves a batch of records and then yields each record to the block individually as a model. In the following example, find_each
will retrieve 1000 records (the current default for both find_each
and find_in_batches
) and then yield each record individually to the block as a model. This process is repeated until all of the records have been processed:
User.find_each do |user| NewsLetter.weekly_deliver(user) end
1.3.1.1 Options for find_each
The find_each
method accepts most of the options allowed by the regular find
method, except for :order
and :limit
, which are reserved for internal use by find_each
.
Two additional options, :batch_size
and :start
, are available as well.
:batch_size
The :batch_size
option allows you to specify the number of records to be retrieved in each batch, before being passed individually to the block. For example, to retrieve records in batches of 5000:
User.find_each(batch_size: 5000) do |user| NewsLetter.weekly_deliver(user) end
:start
By default, records are fetched in ascending order of the primary key, which must be an integer. The :start
option allows you to configure the first ID of the sequence whenever the lowest ID is not the one you need. This would be useful, for example, if you wanted to resume an interrupted batch process, provided you saved the last processed ID as a checkpoint.
For example, to send newsletters only to users with the primary key starting from 2000, and to retrieve them in batches of 5000:
User.find_each(start: 2000, batch_size: 5000) do |user| NewsLetter.weekly_deliver(user) end
Another example would be if you wanted multiple workers handling the same processing queue. You could have each worker handle 10000 records by setting the appropriate :start
option on each worker.
1.3.2 find_in_batches
The find_in_batches
method is similar to find_each
, since both retrieve batches of records. The difference is that find_in_batches
yields batches to the block as an array of models, instead of individually. The following example will yield to the supplied block an array of up to 1000 invoices at a time, with the final block containing any remaining invoices:
# Give add_invoices an array of 1000 invoices at a time Invoice.find_in_batches(include: :invoice_lines) do |invoices| export.add_invoices(invoices) end
The :include
option allows you to name associations that should be loaded alongside with the models.
1.3.2.1 Options for find_in_batches
The find_in_batches
method accepts the same :batch_size
and :start
options as find_each
, as well as most of the options allowed by the regular find
method, except for :order
and :limit
, which are reserved for internal use by find_in_batches
.
2 Conditions
The where
method allows you to specify conditions to limit the records returned, representing the WHERE
-part of the SQL statement. Conditions can either be specified as a string, array, or hash.
2.1 Pure String Conditions
If you'd like to add conditions to your find, you could just specify them in there, just like Client.where("orders_count = '2'")
. This will find all clients where the orders_count
field's value is 2.
Building your own conditions as pure strings can leave you vulnerable to SQL injection exploits. For example, Client.where("first_name LIKE '%#{params[:first_name]}%'")
is not safe. See the next section for the preferred way to handle conditions using an array.
2.2 Array Conditions
Now what if that number could vary, say as an argument from somewhere? The find would then take the form:
Client.where("orders_count = ?", params[:orders])
Active Record will go through the first element in the conditions value and any additional elements will replace the question marks (?)
in the first element.
If you want to specify multiple conditions:
Client.where("orders_count = ? AND locked = ?", params[:orders], false)
In this example, the first question mark will be replaced with the value in params[:orders]
and the second will be replaced with the SQL representation of false
, which depends on the adapter.
This code is highly preferable:
Client.where("orders_count = ?", params[:orders])
to this code:
Client.where("orders_count = #{params[:orders]}")
because of argument safety. Putting the variable directly into the conditions string will pass the variable to the database as-is. This means that it will be an unescaped variable directly from a user who may have malicious intent. If you do this, you put your entire database at risk because once a user finds out he or she can exploit your database they can do just about anything to it. Never ever put your arguments directly inside the conditions string.
For more information on the dangers of SQL injection, see the Ruby on Rails Security Guide.
2.2.1 Placeholder Conditions
Similar to the (?)
replacement style of params, you can also specify keys/values hash in your array conditions:
Client.where("created_at >= :start_date AND created_at <= :end_date", {start_date: params[:start_date], end_date: params[:end_date]})
This makes for clearer readability if you have a large number of variable conditions.
2.3 Hash Conditions
Active Record also allows you to pass in hash conditions which can increase the readability of your conditions syntax. With hash conditions, you pass in a hash with keys of the fields you want conditionalised and the values of how you want to conditionalise them:
Only equality, range and subset checking are possible with Hash conditions.
2.3.1 Equality Conditions
Client.where(locked: true)
The field name can also be a string:
Client.where('locked' => true)
In the case of a belongs_to relationship, an association key can be used to specify the model if an Active Record object is used as the value. This method works with polymorphic relationships as well.
Post.where(author: author) Author.joins(:posts).where(posts: {author: author})
The values cannot be symbols. For example, you cannot do Client.where(status: :active)
.
2.3.2 Range Conditions
Client.where(created_at: (Time.now.midnight - 1.day)..Time.now.midnight)
This will find all clients created yesterday by using a BETWEEN
SQL statement:
SELECT * FROM clients WHERE (clients.created_at BETWEEN '2008-12-21 00:00:00' AND '2008-12-22 00:00:00')
This demonstrates a shorter syntax for the examples in Array Conditions
2.3.3 Subset Conditions
If you want to find records using the IN
expression you can pass an array to the conditions hash:
Client.where(orders_count: [1,3,5])
This code will generate SQL like this:
SELECT * FROM clients WHERE (clients.orders_count IN (1,3,5))
2.4 NOT Conditions
NOT
SQL queries can be built by where.not
.
Post.where.not(author: author)
In other words, this query can be generated by calling where
with no argument, then immediately chain with not
passing where
conditions.
3 Ordering
To retrieve records from the database in a specific order, you can use the order
method.
For example, if you're getting a set of records and want to order them in ascending order by the created_at
field in your table:
Client.order(:created_at) # OR Client.order("created_at")
You could specify ASC
or DESC
as well:
Client.order(created_at: :desc) # OR Client.order(created_at: :asc) # OR Client.order("created_at DESC") # OR Client.order("created_at ASC")
Or ordering by multiple fields:
Client.order(orders_count: :asc, created_at: :desc) # OR Client.order(:orders_count, created_at: :desc) # OR Client.order("orders_count ASC, created_at DESC") # OR Client.order("orders_count ASC", "created_at DESC")
If you want to call order
multiple times e.g. in different context, new order will append previous one
Client.order("orders_count ASC").order("created_at DESC") # SELECT * FROM clients ORDER BY orders_count ASC, created_at DESC
4 Selecting Specific Fields
By default, Model.find
selects all the fields from the result set using select *
.
To select only a subset of fields from the result set, you can specify the subset via the select
method.
For example, to select only viewable_by
and locked
columns:
Client.select("viewable_by, locked")
The SQL query used by this find call will be somewhat like:
SELECT viewable_by, locked FROM clients
Be careful because this also means you're initializing a model object with only the fields that you've selected. If you attempt to access a field that is not in the initialized record you'll receive:
ActiveModel::MissingAttributeError: missing attribute: <attribute>
Where <attribute>
is the attribute you asked for. The id
method will not raise the ActiveRecord::MissingAttributeError
, so just be careful when working with associations because they need the id
method to function properly.
If you would like to only grab a single record per unique value in a certain field, you can use distinct
:
Client.select(:name).distinct
This would generate SQL like:
SELECT DISTINCT name FROM clients
You can also remove the uniqueness constraint:
query = Client.select(:name).distinct # => Returns unique names query.distinct(false) # => Returns all names, even if there are duplicates
5 Limit and Offset
To apply LIMIT
to the SQL fired by the Model.find
, you can specify the LIMIT
using limit
and offset
methods on the relation.
You can use limit
to specify the number of records to be retrieved, and use offset
to specify the number of records to skip before starting to return the records. For example
Client.limit(5)
will return a maximum of 5 clients and because it specifies no offset it will return the first 5 in the table. The SQL it executes looks like this:
SELECT * FROM clients LIMIT 5
Adding offset
to that
Client.limit(5).offset(30)
will return instead a maximum of 5 clients beginning with the 31st. The SQL looks like:
SELECT * FROM clients LIMIT 5 OFFSET 30
6 Group
To apply a GROUP BY
clause to the SQL fired by the finder, you can specify the group
method on the find.
For example, if you want to find a collection of the dates orders were created on:
Order.select("date(created_at) as ordered_date, sum(price) as total_price").group("date(created_at)")
And this will give you a single Order
object for each date where there are orders in the database.
The SQL that would be executed would be something like this:
SELECT date(created_at) as ordered_date, sum(price) as total_price FROM orders GROUP BY date(created_at)
7 Having
SQL uses the HAVING
clause to specify conditions on the GROUP BY
fields. You can add the HAVING
clause to the SQL fired by the Model.find
by adding the :having
option to the find.
For example:
Order.select("date(created_at) as ordered_date, sum(price) as total_price"). group("date(created_at)").having("sum(price) > ?", 100)
The SQL that would be executed would be something like this:
SELECT date(created_at) as ordered_date, sum(price) as total_price FROM orders GROUP BY date(created_at) HAVING sum(price) > 100
This will return single order objects for each day, but only those that are ordered more than $100 in a day.
8 Overriding Conditions
8.1 except
You can specify certain conditions to be excepted by using the except
method. For example:
Post.where('id > 10').limit(20).order('id asc').except(:order)
The SQL that would be executed:
SELECT * FROM posts WHERE id > 10 LIMIT 20
8.2 unscope
The except
method does not work when the relation is merged. For example:
Post.comments.except(:order)
will still have an order if the order comes from a default scope on Comment. In order to remove all ordering, even from relations which are merged in, use unscope as follows:
Post.order('id DESC').limit(20).unscope(:order) = Post.limit(20) Post.order('id DESC').limit(20).unscope(:order, :limit) = Post.all
You can additionally unscope specific where clauses. For example:
Post.where(:id => 10).limit(1).unscope(where: :id, :limit).order('id DESC') = Post.order('id DESC')
8.3 only
You can also override conditions using the only
method. For example:
Post.where('id > 10').limit(20).order('id desc').only(:order, :where)
The SQL that would be executed:
SELECT * FROM posts WHERE id > 10 ORDER BY id DESC
8.4 reorder
The reorder
method overrides the default scope order. For example:
class Post < ActiveRecord::Base .. .. has_many :comments, order: 'posted_at DESC' end Post.find(10).comments.reorder('name')
The SQL that would be executed:
SELECT * FROM posts WHERE id = 10 ORDER BY name
In case the reorder
clause is not used, the SQL executed would be:
SELECT * FROM posts WHERE id = 10 ORDER BY posted_at DESC
8.5 reverse_order
The reverse_order
method reverses the ordering clause if specified.
Client.where("orders_count > 10").order(:name).reverse_order
The SQL that would be executed:
SELECT * FROM clients WHERE orders_count > 10 ORDER BY name DESC
If no ordering clause is specified in the query, the reverse_order
orders by the primary key in reverse order.
Client.where("orders_count > 10").reverse_order
The SQL that would be executed:
SELECT * FROM clients WHERE orders_count > 10 ORDER BY clients.id DESC
This method accepts no arguments.
9 Null Relation
The none
method returns a chainable relation with no records. Any subsequent conditions chained to the returned relation will continue generating empty relations. This is useful in scenarios where you need a chainable response to a method or a scope that could return zero results.
Post.none # returns an empty Relation and fires no queries.
# The visible_posts method below is expected to return a Relation. @posts = current_user.visible_posts.where(name: params[:name]) def visible_posts case role when 'Country Manager' Post.where(country: country) when 'Reviewer' Post.published when 'Bad User' Post.none # => returning [] or nil breaks the caller code in this case end end
10 Readonly Objects
Active Record provides readonly
method on a relation to explicitly disallow modification of any of the returned objects. Any attempt to alter a readonly record will not succeed, raising an ActiveRecord::ReadOnlyRecord
exception.
client = Client.readonly.first client.visits += 1 client.save
As client
is explicitly set to be a readonly object, the above code will raise an ActiveRecord::ReadOnlyRecord
exception when calling client.save
with an updated value of visits.
11 Locking Records for Update
Locking is helpful for preventing race conditions when updating records in the database and ensuring atomic updates.
Active Record provides two locking mechanisms:
- Optimistic Locking
- Pessimistic Locking
11.1 Optimistic Locking
Optimistic locking allows multiple users to access the same record for edits, and assumes a minimum of conflicts with the data. It does this by checking whether another process has made changes to a record since it was opened. An ActiveRecord::StaleObjectError
exception is thrown if that has occurred and the update is ignored.
Optimistic locking column
In order to use optimistic locking, the table needs to have a column called lock_version
of type integer. Each time the record is updated, Active Record increments the lock_version
column. If an update request is made with a lower value in the lock_version
field than is currently in the lock_version
column in the database, the update request will fail with an ActiveRecord::StaleObjectError
. Example:
c1 = Client.find(1) c2 = Client.find(1) c1.first_name = "Michael" c1.save c2.name = "should fail" c2.save # Raises an ActiveRecord::StaleObjectError
You're then responsible for dealing with the conflict by rescuing the exception and either rolling back, merging, or otherwise apply the business logic needed to resolve the conflict.
This behavior can be turned off by setting ActiveRecord::Base.lock_optimistically = false
.
To override the name of the lock_version
column, ActiveRecord::Base
provides a class attribute called locking_column
:
class Client < ActiveRecord::Base self.locking_column = :lock_client_column end
11.2 Pessimistic Locking
Pessimistic locking uses a locking mechanism provided by the underlying database. Using lock
when building a relation obtains an exclusive lock on the selected rows. Relations using lock
are usually wrapped inside a transaction for preventing deadlock conditions.
For example:
Item.transaction do i = Item.lock.first i.name = 'Jones' i.save end
The above session produces the following SQL for a MySQL backend:
SQL (0.2ms) BEGIN Item Load (0.3ms) SELECT * FROM `items` LIMIT 1 FOR UPDATE Item Update (0.4ms) UPDATE `items` SET `updated_at` = '2009-02-07 18:05:56', `name` = 'Jones' WHERE `id` = 1 SQL (0.8ms) COMMIT
You can also pass raw SQL to the lock
method for allowing different types of locks. For example, MySQL has an expression called LOCK IN SHARE MODE
where you can lock a record but still allow other queries to read it. To specify this expression just pass it in as the lock option:
Item.transaction do i = Item.lock("LOCK IN SHARE MODE").find(1) i.increment!(:views) end
If you already have an instance of your model, you can start a transaction and acquire the lock in one go using the following code:
item = Item.first item.with_lock do # This block is called within a transaction, # item is already locked. item.increment!(:views) end
12 Joining Tables
Active Record provides a finder method called joins
for specifying JOIN
clauses on the resulting SQL. There are multiple ways to use the joins
method.
12.1 Using a String SQL Fragment
You can just supply the raw SQL specifying the JOIN
clause to joins
:
Client.joins('LEFT OUTER JOIN addresses ON addresses.client_id = clients.id')
This will result in the following SQL:
SELECT clients.* FROM clients LEFT OUTER JOIN addresses ON addresses.client_id = clients.id
12.2 Using Array/Hash of Named Associations
This method only works with INNER JOIN
.
Active Record lets you use the names of the associations defined on the model as a shortcut for specifying JOIN
clause for those associations when using the joins
method.
For example, consider the following Category
, Post
, Comments
and Guest
models:
class Category < ActiveRecord::Base has_many :posts end class Post < ActiveRecord::Base belongs_to :category has_many :comments has_many :tags end class Comment < ActiveRecord::Base belongs_to :post has_one :guest end class Guest < ActiveRecord::Base belongs_to :comment end class Tag < ActiveRecord::Base belongs_to :post end
Now all of the following will produce the expected join queries using INNER JOIN
:
12.2.1 Joining a Single Association
Category.joins(:posts)
This produces:
SELECT categories.* FROM categories INNER JOIN posts ON posts.category_id = categories.id
Or, in English: "return a Category object for all categories with posts". Note that you will see duplicate categories if more than one post has the same category. If you want unique categories, you can use Category.joins(:posts).uniq
.
12.2.2 Joining Multiple Associations
Post.joins(:category, :comments)
This produces:
SELECT posts.* FROM posts INNER JOIN categories ON posts.category_id = categories.id INNER JOIN comments ON comments.post_id = posts.id
Or, in English: "return all posts that have a category and at least one comment". Note again that posts with multiple comments will show up multiple times.
12.2.3 Joining Nested Associations (Single Level)
Post.joins(comments: :guest)
This produces:
SELECT posts.* FROM posts INNER JOIN comments ON comments.post_id = posts.id INNER JOIN guests ON guests.comment_id = comments.id
Or, in English: "return all posts that have a comment made by a guest."
12.2.4 Joining Nested Associations (Multiple Level)
Category.joins(posts: [{comments: :guest}, :tags])
This produces:
SELECT categories.* FROM categories INNER JOIN posts ON posts.category_id = categories.id INNER JOIN comments ON comments.post_id = posts.id INNER JOIN guests ON guests.comment_id = comments.id INNER JOIN tags ON tags.post_id = posts.id
12.3 Specifying Conditions on the Joined Tables
You can specify conditions on the joined tables using the regular Array and String conditions. Hash conditions provides a special syntax for specifying conditions for the joined tables:
time_range = (Time.now.midnight - 1.day)..Time.now.midnight Client.joins(:orders).where('orders.created_at' => time_range)
An alternative and cleaner syntax is to nest the hash conditions:
time_range = (Time.now.midnight - 1.day)..Time.now.midnight Client.joins(:orders).where(orders: {created_at: time_range})
This will find all clients who have orders that were created yesterday, again using a BETWEEN
SQL expression.
13 Eager Loading Associations
Eager loading is the mechanism for loading the associated records of the objects returned by Model.find
using as few queries as possible.
N + 1 queries problem
Consider the following code, which finds 10 clients and prints their postcodes:
clients = Client.limit(10) clients.each do |client| puts client.address.postcode end
This code looks fine at the first sight. But the problem lies within the total number of queries executed. The above code executes 1 (to find 10 clients) + 10 (one per each client to load the address) = 11 queries in total.
Solution to N + 1 queries problem
Active Record lets you specify in advance all the associations that are going to be loaded. This is possible by specifying the includes
method of the Model.find
call. With includes
, Active Record ensures that all of the specified associations are loaded using the minimum possible number of queries.
Revisiting the above case, we could rewrite Client.limit(10)
to use eager load addresses:
clients = Client.includes(:address).limit(10) clients.each do |client| puts client.address.postcode end
The above code will execute just 2 queries, as opposed to 11 queries in the previous case:
SELECT * FROM clients LIMIT 10 SELECT addresses.* FROM addresses WHERE (addresses.client_id IN (1,2,3,4,5,6,7,8,9,10))
13.1 Eager Loading Multiple Associations
Active Record lets you eager load any number of associations with a single Model.find
call by using an array, hash, or a nested hash of array/hash with the includes
method.
13.1.1 Array of Multiple Associations
Post.includes(:category, :comments)
This loads all the posts and the associated category and comments for each post.
13.1.2 Nested Associations Hash
Category.includes(posts: [{comments: :guest}, :tags]).find(1)
This will find the category with id 1 and eager load all of the associated posts, the associated posts' tags and comments, and every comment's guest association.
13.2 Specifying Conditions on Eager Loaded Associations
Even though Active Record lets you specify conditions on the eager loaded associations just like joins
, the recommended way is to use joins instead.
However if you must do this, you may use where
as you would normally.
Post.includes(:comments).where("comments.visible" => true)
This would generate a query which contains a LEFT OUTER JOIN
whereas the joins
method would generate one using the INNER JOIN
function instead.
SELECT "posts"."id" AS t0_r0, ... "comments"."updated_at" AS t1_r5 FROM "posts" LEFT OUTER JOIN "comments" ON "comments"."post_id" = "posts"."id" WHERE (comments.visible = 1)
If there was no where
condition, this would generate the normal set of two queries.
If, in the case of this includes
query, there were no comments for any posts, all the posts would still be loaded. By using joins
(an INNER JOIN), the join conditions must match, otherwise no records will be returned.
14 Scopes
Scoping allows you to specify commonly-used queries which can be referenced as method calls on the association objects or models. With these scopes, you can use every method previously covered such as where
, joins
and includes
. All scope methods will return an ActiveRecord::Relation
object which will allow for further methods (such as other scopes) to be called on it.
To define a simple scope, we use the scope
method inside the class, passing the query that we'd like to run when this scope is called:
class Post < ActiveRecord::Base scope :published, -> { where(published: true) } end
This is exactly the same as defining a class method, and which you use is a matter of personal preference:
class Post < ActiveRecord::Base def self.published where(published: true) end end
Scopes are also chainable within scopes:
class Post < ActiveRecord::Base scope :published, -> { where(published: true) } scope :published_and_commented, -> { published.where("comments_count > 0") } end
To call this published
scope we can call it on either the class:
Post.published # => [published posts]
Or on an association consisting of Post
objects:
category = Category.first category.posts.published # => [published posts belonging to this category]
14.1 Passing in arguments
Your scope can take arguments:
class Post < ActiveRecord::Base scope :created_before, ->(time) { where("created_at < ?", time) } end
This may then be called using this:
Post.created_before(Time.zone.now)
However, this is just duplicating the functionality that would be provided to you by a class method.
class Post < ActiveRecord::Base def self.created_before(time) where("created_at < ?", time) end end
Using a class method is the preferred way to accept arguments for scopes. These methods will still be accessible on the association objects:
category.posts.created_before(time)
14.2 Merging of scopes
Just like where
clauses scopes are merged using AND
conditions.
class User < ActiveRecord::Base scope :active, -> { where state: 'active' } scope :inactive, -> { where state: 'inactive' } end User.active.inactive # => SELECT "users".* FROM "users" WHERE "users"."state" = 'active' AND "users"."state" = 'inactive'
We can mix and match scope
and where
conditions and the final sql
will have all conditions joined with AND
.
User.active.where(state: 'finished') # => SELECT "users".* FROM "users" WHERE "users"."state" = 'active' AND "users"."state" = 'finished'
If we do want the last where clause
to win then Relation#merge
can
be used .
User.active.merge(User.inactive) # => SELECT "users".* FROM "users" WHERE "users"."state" = 'inactive'
One important caveat is that default_scope
will be overridden by
scope
and where
conditions.
class User < ActiveRecord::Base default_scope { where state: 'pending' } scope :active, -> { where state: 'active' } scope :inactive, -> { where state: 'inactive' } end User.all # => SELECT "users".* FROM "users" WHERE "users"."state" = 'pending' User.active # => SELECT "users".* FROM "users" WHERE "users"."state" = 'active' User.where(state: 'inactive') # => SELECT "users".* FROM "users" WHERE "users"."state" = 'inactive'
As you can see above the default_scope
is being overridden by both
scope
and where
conditions.
14.3 Applying a default scope
If we wish for a scope to be applied across all queries to the model we can use the
default_scope
method within the model itself.
class Client < ActiveRecord::Base default_scope { where("removed_at IS NULL") } end
When queries are executed on this model, the SQL query will now look something like this:
SELECT * FROM clients WHERE removed_at IS NULL
If you need to do more complex things with a default scope, you can alternatively define it as a class method:
class Client < ActiveRecord::Base def self.default_scope # Should return an ActiveRecord::Relation. end end
14.4 Removing All Scoping
If we wish to remove scoping for any reason we can use the unscoped
method. This is
especially useful if a default_scope
is specified in the model and should not be
applied for this particular query.
Client.unscoped.all
This method removes all scoping and will do a normal query on the table.
Note that chaining unscoped
with a scope
does not work. In these cases, it is
recommended that you use the block form of unscoped
:
Client.unscoped { Client.created_before(Time.zone.now) }
15 Dynamic Finders
Dynamic finders have been deprecated in Rails 4.0 and will be removed in Rails 4.1. The best practice is to use Active Record scopes instead. You can find the deprecation gem at https://github.com/rails/activerecord-deprecated_finders
For every field (also known as an attribute) you define in your table, Active Record provides a finder method. If you have a field called first_name
on your Client
model for example, you get find_by_first_name
for free from Active Record. If you have a locked
field on the Client
model, you also get find_by_locked
and methods.
You can specify an exclamation point (!
) on the end of the dynamic finders to get them to raise an ActiveRecord::RecordNotFound
error if they do not return any records, like Client.find_by_name!("Ryan")
If you want to find both by name and locked, you can chain these finders together by simply typing "and
" between the fields. For example, Client.find_by_first_name_and_locked("Ryan", true)
.
16 Find or Build a New Object
It's common that you need to find a record or create it if it doesn't exist. You can do that with the find_or_create_by
and find_or_create_by!
methods.
16.1 find_or_create_by
The find_or_create_by
method checks whether a record with the attributes exists. If it doesn't, then create
is called. Let's see an example.
Suppose you want to find a client named 'Andy', and if there's none, create one. You can do so by running:
Client.find_or_create_by(first_name: 'Andy') # => #<Client id: 1, first_name: "Andy", orders_count: 0, locked: true, created_at: "2011-08-30 06:09:27", updated_at: "2011-08-30 06:09:27">
The SQL generated by this method looks like this:
SELECT * FROM clients WHERE (clients.first_name = 'Andy') LIMIT 1 BEGIN INSERT INTO clients (created_at, first_name, locked, orders_count, updated_at) VALUES ('2011-08-30 05:22:57', 'Andy', 1, NULL, '2011-08-30 05:22:57') COMMIT
find_or_create_by
returns either the record that already exists or the new record. In our case, we didn't already have a client named Andy so the record is created and returned.
The new record might not be saved to the database; that depends on whether validations passed or not (just like create
).
Suppose we want to set the 'locked' attribute to true if we're creating a new record, but we don't want to include it in the query. So we want to find the client named "Andy", or if that client doesn't exist, create a client named "Andy" which is not locked.
We can achieve this in two ways. The first is to use create_with
:
Client.create_with(locked: false).find_or_create_by(first_name: 'Andy')
The second way is using a block:
Client.find_or_create_by(first_name: 'Andy') do |c| c.locked = false end
The block will only be executed if the client is being created. The second time we run this code, the block will be ignored.
16.2 find_or_create_by!
You can also use find_or_create_by!
to raise an exception if the new record is invalid. Validations are not covered on this guide, but let's assume for a moment that you temporarily add
validates :orders_count, presence: true
to your Client
model. If you try to create a new Client
without passing an orders_count
, the record will be invalid and an exception will be raised:
Client.find_or_create_by!(first_name: 'Andy') # => ActiveRecord::RecordInvalid: Validation failed: Orders count can't be blank
16.3 find_or_initialize_by
The find_or_initialize_by
method will work just like
find_or_create_by
but it will call new
instead of create
. This
means that a new model instance will be created in memory but won't be
saved to the database. Continuing with the find_or_create_by
example, we
now want the client named 'Nick':
nick = Client.find_or_initialize_by(first_name: 'Nick') # => <Client id: nil, first_name: "Nick", orders_count: 0, locked: true, created_at: "2011-08-30 06:09:27", updated_at: "2011-08-30 06:09:27"> nick.persisted? # => false nick.new_record? # => true
Because the object is not yet stored in the database, the SQL generated looks like this:
SELECT * FROM clients WHERE (clients.first_name = 'Nick') LIMIT 1
When you want to save it to the database, just call save
:
nick.save # => true
17 Finding by SQL
If you'd like to use your own SQL to find records in a table you can use find_by_sql
. The find_by_sql
method will return an array of objects even if the underlying query returns just a single record. For example you could run this query:
Client.find_by_sql("SELECT * FROM clients INNER JOIN orders ON clients.id = orders.client_id ORDER clients.created_at desc")
find_by_sql
provides you with a simple way of making custom calls to the database and retrieving instantiated objects.
17.1 select_all
find_by_sql
has a close relative called connection#select_all
. select_all
will retrieve objects from the database using custom SQL just like find_by_sql
but will not instantiate them. Instead, you will get an array of hashes where each hash indicates a record.
Client.connection.select_all("SELECT * FROM clients WHERE id = '1'")
17.2 pluck
pluck
can be used to query a single or multiple columns from the underlying table of a model. It accepts a list of column names as argument and returns an array of values of the specified columns with the corresponding data type.
Client.where(active: true).pluck(:id) # SELECT id FROM clients WHERE active = 1 # => [1, 2, 3] Client.distinct.pluck(:role) # SELECT DISTINCT role FROM clients # => ['admin', 'member', 'guest'] Client.pluck(:id, :name) # SELECT clients.id, clients.name FROM clients # => [[1, 'David'], [2, 'Jeremy'], [3, 'Jose']]
pluck
makes it possible to replace code like:
Client.select(:id).map { |c| c.id } # or Client.select(:id).map(&:id) # or Client.select(:id, :name).map { |c| [c.id, c.name] }
with:
Client.pluck(:id) # or Client.pluck(:id, :name)
Unlike select
, pluck
directly converts a database result into a Ruby Array
,
without constructing ActiveRecord
objects. This can mean better performance for
a large or often-running query. However, any model method overrides will
not be available. For example:
class Client < ActiveRecord::Base def name "I am #{super}" end end Client.select(:name).map &:name # => ["I am David", "I am Jeremy", "I am Jose"] Client.pluck(:name) # => ["David", "Jeremy", "Jose"]
Furthermore, unlike select
and other Relation
scopes, pluck
triggers an immediate
query, and thus cannot be chained with any further scopes, although it can work with
scopes already constructed earlier:
Client.pluck(:name).limit(1) # => NoMethodError: undefined method `limit' for #<Array:0x007ff34d3ad6d8> Client.limit(1).pluck(:name) # => ["David"]
17.3 ids
ids
can be used to pluck all the IDs for the relation using the table's primary key.
Person.ids # SELECT id FROM people
class Person < ActiveRecord::Base self.primary_key = "person_id" end Person.ids # SELECT person_id FROM people
18 Existence of Objects
If you simply want to check for the existence of the object there's a method called exists?
. This method will query the database using the same query as find
, but instead of returning an object or collection of objects it will return either true
or false
.
Client.exists?(1)
The exists?
method also takes multiple ids, but the catch is that it will return true if any one of those records exists.
Client.exists?(1,2,3) # or Client.exists?([1,2,3])
It's even possible to use exists?
without any arguments on a model or a relation.
Client.where(first_name: 'Ryan').exists?
The above returns true
if there is at least one client with the first_name
'Ryan' and false
otherwise.
Client.exists?
The above returns false
if the clients
table is empty and true
otherwise.
You can also use any?
and many?
to check for existence on a model or relation.
# via a model Post.any? Post.many? # via a named scope Post.recent.any? Post.recent.many? # via a relation Post.where(published: true).any? Post.where(published: true).many? # via an association Post.first.categories.any? Post.first.categories.many?
19 Calculations
This section uses count as an example method in this preamble, but the options described apply to all sub-sections.
All calculation methods work directly on a model:
Client.count # SELECT count(*) AS count_all FROM clients
Or on a relation:
Client.where(first_name: 'Ryan').count # SELECT count(*) AS count_all FROM clients WHERE (first_name = 'Ryan')
You can also use various finder methods on a relation for performing complex calculations:
Client.includes("orders").where(first_name: 'Ryan', orders: {status: 'received'}).count
Which will execute:
SELECT count(DISTINCT clients.id) AS count_all FROM clients LEFT OUTER JOIN orders ON orders.client_id = client.id WHERE (clients.first_name = 'Ryan' AND orders.status = 'received')
19.1 Count
If you want to see how many records are in your model's table you could call Client.count
and that will return the number. If you want to be more specific and find all the clients with their age present in the database you can use Client.count(:age)
.
For options, please see the parent section, Calculations.
19.2 Average
If you want to see the average of a certain number in one of your tables you can call the average
method on the class that relates to the table. This method call will look something like this:
Client.average("orders_count")
This will return a number (possibly a floating point number such as 3.14159265) representing the average value in the field.
For options, please see the parent section, Calculations.
19.3 Minimum
If you want to find the minimum value of a field in your table you can call the minimum
method on the class that relates to the table. This method call will look something like this:
Client.minimum("age")
For options, please see the parent section, Calculations.
19.4 Maximum
If you want to find the maximum value of a field in your table you can call the maximum
method on the class that relates to the table. This method call will look something like this:
Client.maximum("age")
For options, please see the parent section, Calculations.
19.5 Sum
If you want to find the sum of a field for all records in your table you can call the sum
method on the class that relates to the table. This method call will look something like this:
Client.sum("orders_count")
For options, please see the parent section, Calculations.
20 Running EXPLAIN
You can run EXPLAIN on the queries triggered by relations. For example,
User.where(id: 1).joins(:posts).explain
may yield
EXPLAIN for: SELECT `users`.* FROM `users` INNER JOIN `posts` ON `posts`.`user_id` = `users`.`id` WHERE `users`.`id` = 1 +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+ | 1 | SIMPLE | users | const | PRIMARY | PRIMARY | 4 | const | 1 | | | 1 | SIMPLE | posts | ALL | NULL | NULL | NULL | NULL | 1 | Using where | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+ 2 rows in set (0.00 sec)
under MySQL.
Active Record performs a pretty printing that emulates the one of the database shells. So, the same query running with the PostgreSQL adapter would yield instead
EXPLAIN for: SELECT "users".* FROM "users" INNER JOIN "posts" ON "posts"."user_id" = "users"."id" WHERE "users"."id" = 1 QUERY PLAN ------------------------------------------------------------------------------ Nested Loop Left Join (cost=0.00..37.24 rows=8 width=0) Join Filter: (posts.user_id = users.id) -> Index Scan using users_pkey on users (cost=0.00..8.27 rows=1 width=4) Index Cond: (id = 1) -> Seq Scan on posts (cost=0.00..28.88 rows=8 width=4) Filter: (posts.user_id = 1) (6 rows)
Eager loading may trigger more than one query under the hood, and some queries
may need the results of previous ones. Because of that, explain
actually
executes the query, and then asks for the query plans. For example,
User.where(id: 1).includes(:posts).explain
yields
EXPLAIN for: SELECT `users`.* FROM `users` WHERE `users`.`id` = 1 +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ | 1 | SIMPLE | users | const | PRIMARY | PRIMARY | 4 | const | 1 | | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ 1 row in set (0.00 sec) EXPLAIN for: SELECT `posts`.* FROM `posts` WHERE `posts`.`user_id` IN (1) +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | posts | ALL | NULL | NULL | NULL | NULL | 1 | Using where | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec)
under MySQL.
20.1 Interpreting EXPLAIN
Interpretation of the output of EXPLAIN is beyond the scope of this guide. The following pointers may be helpful:
SQLite3: EXPLAIN QUERY PLAN
MySQL: EXPLAIN Output Format
PostgreSQL: Using EXPLAIN
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