Guides.rubyonrails.org

Chapters

1. Retrieving Objects from the Database
   • Retrieving a Single Object
   • Retrieving Multiple Objects
   • Retrieving Multiple Objects in Batches
2. Conditions
   • Pure String Conditions
   • Array Conditions
   • Hash Conditions
3. Find Options
   • Ordering
   • Selecting Specific Fields
   • Limit and Offset
   • Group
   • Having
   • Readonly Objects
   • Locking Records for Update
4. Joining Tables
   • Using a String SQL Fragment
   • Using Array/Hash of Named Associations
   • Specifying Conditions on the Joined Tables
5. Eager Loading Associations
   • Eager Loading Multiple Associations
   • Specifying Conditions on Eager Loaded Associations
6. Dynamic Finders
7. Finding by SQL
8. select_all
9. Existence of Objects
10. Calculations
    • Count
    • Average
    • Minimum
    • Maximum
    • Sum
11. Changelog

If you’re used to using raw SQL to find database records then, generally, you will 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:

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 a class method called Model.find. This method allows you to pass arguments into it to perform certain queries on your database without the need of writing raw SQL.

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 lets you retrieve a single object using three different ways.

1.1.1 Using a Primary Key

Using Model.find(primary_key, options = nil), you can retrieve the object corresponding to the supplied primary key and matching the supplied options (if any). For example:

SQL equivalent of the above is:

Model.find(primary_key) will raise an ActiveRecord::RecordNotFound exception if no matching record is found.

1.1.2 first

Model.first(options = nil) finds the first record matched by the supplied options. If no options are supplied, the first matching record is returned. For example:

SQL equivalent of the above is:

Model.first returns nil if no matching record is found. No exception will be raised.

1.1.3 last

Model.last(options = nil) finds the last record matched by the supplied options. If no options are supplied, the last matching record is returned. For example:

SQL equivalent of the above is:

Model.last returns nil if no matching record is found. No exception will be raised.

1.2 Retrieving Multiple Objects

1.2.1 Using Multiple Primary Keys

Model.find(array_of_primary_key, options = nil) also accepts an array of primary keys. An array of all the matching records for the supplied primary keys is returned. For example:

SQL equivalent of the above is:

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 Find all

Model.all(options = nil) finds all the records matching the supplied options. If no options are supplied, all rows from the database are returned.

And the equivalent SQL is:

Model.all returns an empty array [] if no matching record is found. No exception will be raised.

1.3 Retrieving Multiple Objects in Batches

Sometimes you need to iterate over a large set of records. For example to send a newsletter to all users, to export some data, etc.

The following may seem very straight forward at first:

But if the total number of rows in the table is very large, the above approach may vary from being under performant to just plain impossible.

This is because User.all makes Active Record fetch the entire table, build a model object per row, and keep the entire array in the memory. Sometimes that is just too many objects and demands too much memory.

1.3.1 find_each

To efficiently iterate over a large table, Active Record provides a batch finder method called find_each:

Configuring the batch size

Behind the scenes find_each fetches rows in batches of 1000 and yields them one by one. The size of the underlying batches is configurable via the :batch_size option.

To fetch User records in batch size of 5000:

Starting batch find from a specific primary key

Records are fetched in ascending order on the primary key, which must be an integer. The :start option allows you to configure the first ID of the sequence if the lowest is not the one you need. This may be useful for example to be able to resume an interrupted batch process if it saves the last processed ID as a checkpoint.

To send newsletters only to users with the primary key starting from 2000:

Additional options

find_each accepts the same options as the regular find method. However, :order and :limit are needed internally and hence not allowed to be passed explicitly.

1.3.2 find_in_batches

You can also work by chunks instead of row by row using find_in_batches. This method is analogous to find_each, but it yields arrays of models instead:

The above will yield the supplied block with 1000 invoices every time.

2 Conditions

The find 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.first(:conditions => "orders_count = '2'"). This will find all clients where the orders_count field’s value is 2.

2.2 Array Conditions

Now what if that number could vary, say as a argument from somewhere, or perhaps from the user’s level status somewhere? The find then becomes something like:

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.

Or if you want to specify two conditions, you can do it like:

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.

The reason for doing code like:

instead of:

is 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.

2.2.1 Placeholder Conditions

Similar to the (?) replacement style of params, you can also specify keys/values hash in your array conditions:

This makes for clearer readability if you have a large number of variable conditions.

2.2.2 Range Conditions

If you’re looking for a range inside of a table (for example, users created in a certain timeframe) you can use the conditions option coupled with the IN SQL statement for this. If you had two dates coming in from a controller you could do something like this to look for a range:

This would generate the proper query which is great for small ranges but not so good for larger ranges. For example if you pass in a range of date objects spanning a year that’s 365 (or possibly 366, depending on the year) strings it will attempt to match your field against.

2.2.3 Time and Date Conditions

Things can get really messy if you pass in Time objects as it will attempt to compare your field to every second in that range:

This could possibly cause your database server to raise an unexpected error, for example MySQL will throw back this error:

Where query is the actual query used to get that error.

In this example it would be better to use greater-than and less-than operators in SQL, like so:

You can also use the greater-than-or-equal-to and less-than-or-equal-to like this:

Just like in Ruby. If you want a shorter syntax be sure to check out the Hash Conditions section later on in the guide.

2.3 Hash Conditions

Active Record also allows you to pass in a 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:

2.3.1 Equality Conditions

The field name does not have to be a symbol it can also be a string:

2.3.2 Range Conditions

The good thing about this is that we can pass in a range for our fields without it generating a large query as shown in the preamble of this section.

This will find all clients created yesterday by using a BETWEEN SQL statement:

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:

This code will generate SQL like this:

3 Find Options

Apart from :conditions, Model.find takes a variety of other options via the options hash for customizing the resulting record set.

The following sections give a top level overview of all the possible keys for the options_hash.

3.1 Ordering

To retrieve records from the database in a specific order, you can specify the :order option to the find call.

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:

You could specify ASC or DESC as well:

Or ordering by multiple fields:

3.2 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 :select option on the find.

For example, to select only viewable_by and locked columns:

The SQL query used by this find call will be somewhat like:

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:

Where 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.

You can also call SQL functions within the select option. For example, if you would like to only grab a single record per unique value in a certain field by using the DISTINCT function you can do it like this:

3.3 Limit and Offset

To apply LIMIT to the SQL fired by the Model.find, you can specify the LIMIT using :limit and :offset options on the find.

If you want to limit the amount of records to a certain subset of all the records retrieved you usually use :limit for this, sometimes coupled with :offset. Limit is the maximum number of records that will be retrieved from a query, and offset is the number of records it will start reading from from the first record of the set. For example:

This code will return a maximum of 5 clients and because it specifies no offset it will return the first 5 clients in the table. The SQL it executes will look like this:

Or specifying both :limit and :offset:

This code will return a maximum of 5 clients and because it specifies an offset this time, it will return these records starting from the 5th client in the clients table. The SQL looks like:

3.4 Group

To apply GROUP BY clause to the SQL fired by the Model.find, you can specify the :group option on the find.

For example, if you want to find a collection of the dates orders were created on:

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:

3.5 Having

SQL uses HAVING clause to specify conditions on the GROUP BY fields. You can specify the HAVING clause to the SQL fired by the Model.find using :having option on the find.

For example:

The SQL that would be executed would be something like this:

This will return single order objects for each day, but only for the last month.

3.6 Readonly Objects

To explicitly disallow modification/destroyal of the matching records returned by Model.find, you could specify the :readonly option as true to the find call.

Any attempt to alter or destroy the readonly records will not succeed, raising an ActiveRecord::ReadOnlyRecord exception. To set this option, specify it like this:

If you assign this record to a variable client, calling the following code will raise an ActiveRecord::ReadOnlyRecord exception:

3.7 Locking Records for Update

Locking is helpful for preventing the race conditions when updating records in the database and ensuring atomic updated. Active Record provides two locking mechanism:

• Optimistic Locking
• Pessimistic Locking

3.7.1 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. Each time the record is updated, Active Record increments the lock_version column and the locking facilities ensure that records instantiated twice will let the last one saved raise an ActiveRecord::StaleObjectError exception if the first was also updated. Example:

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 method called set_locking_column:

3.7.2 Pessimistic Locking

Pessimistic locking uses locking mechanism provided by the underlying database. Passing :lock => true to Model.find obtains an exclusive lock on the selected rows. Model.find using :lock are usually wrapped inside a transaction for preventing deadlock conditions.

For example:

The above session produces the following SQL for a MySQL backend:

You can also pass raw SQL to the :lock option to allow 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:

4 Joining Tables

Model.find provides a :joins option for specifying JOIN clauses on the resulting SQL. There multiple different ways to specify the :joins option:

4.1 Using a String SQL Fragment

You can just supply the raw SQL specifying the JOIN clause to the :joins option. For example:

This will result in the following SQL:

4.2 Using Array/Hash of Named Associations

Active Record lets you use the names of the associations defined on the model as a shortcut for specifying the :joins option.

For example, consider the following Category, Post, Comments and Guest models:

Now all of the following will produce the expected join queries using INNER JOIN:

4.2.1 Joining a Single Association

This produces:

4.2.2 Joining Multiple Associations

This produces:

4.2.3 Joining Nested Associations (Single Level)

4.2.4 Joining Nested Associations (Multiple Level)

4.3 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:

An alternative and cleaner syntax to this is to nest the hash conditions:

This will find all clients who have orders that were created yesterday, again using a BETWEEN SQL expression.

5 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:

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 all the associations in advanced that are going to be loaded. This is possible by specifying the :include option of the Model.find call. By :include, Active Record ensures that all the specified associations are loaded using minimum possible number of queries.

Revisiting the above case, we could rewrite Client.all to use eager load addresses:

The above code will execute just 2 queries, as opposed to 11 queries in the previous case:

5.1 Eager Loading Multiple Associations

Active Record lets you eager load any possible number of associations with a single Model.find call by using an array, hash, or a nested hash of array/hash with the :include option.

5.1.1 Array of Multiple Associations

This loads all the posts and the associated category and comments for each post.

5.1.2 Nested Associations Hash

The above code finds the category with id 1 and eager loads all the posts associated with the found category. Additionally, it will also eager load every posts’ tags and comments. Every comment’s guest association will get eager loaded as well.

5.2 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.

6 Dynamic 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 name on your Client model for example, you get find_by_name and find_all_by_name for free from Active Record. If you have also have a locked field on the Client model, you also get find_by_locked and find_all_by_locked.

You can do find_last_by_* methods too which will find the last record matching your argument.

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_name_and_locked("Ryan", true).

There’s another set of dynamic finders that let you find or create/initialize objects if they aren’t found. These work in a similar fashion to the other finders and can be used like find_or_create_by_name(params[:name]). Using this will firstly perform a find and then create if the find returns nil. The SQL looks like this for Client.find_or_create_by_name("Ryan"):

find_or_create’s sibling, find_or_initialize, will find an object and if it does not exist will act similar to calling new with the arguments you passed in. For example:

will either assign an existing client object with the name “Ryan” to the client local variable, or initialize a new object similar to calling Client.new(:name => 'Ryan'). From here, you can modify other fields in client by calling the attribute setters on it: client.locked = true and when you want to write it to the database just call save on it.

7 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 the underlying query returns just a single record. For example you could run this query:

find_by_sql provides you with a simple way of making custom calls to the database and retrieving instantiated objects.

8 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.

9 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.

The exists? method also takes multiple ids, but the catch is that it will return true if any one of those records exists.

Further more, exists takes a conditions option much like find:

It’s even possible to use exists? without any arguments:

The above returns false if the clients table is empty and true otherwise.

10 Calculations

This section uses count as an example method in this preamble, but the options described apply to all sub-sections.

count takes conditions much in the same way exists? does:

Which will execute:

You can also use :include or :joins for this to do something a little more complex:

Which will execute:

This code specifies clients.first_name just in case one of the join tables has a field also called first_name and it uses orders.status because that’s the name of our join table.

10.1 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.

10.2 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:

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.

10.3 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:

For options, please see the parent section, Calculations.

10.4 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:

For options, please see the parent section, Calculations.

10.5 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:

For options, please see the parent section, Calculations.

11 Changelog

Lighthouse ticket

• February 7, 2009: Second version by Pratik
• December 29 2008: Initial version by Ryan Bigg