This guide is not intended to be a complete list of all available form helpers. Please refer to the Rails API documentation for an exhaustive list of form helpers and their arguments.
1 Working with Basic Forms
The main form helper is form_with
.
<%= form_with do |form| %>
Form contents
<% end %>
When called without arguments, it creates an HTML <form>
tag with the value of the method
attribute set to post
and the value of the action
attribute set to the current page. For example, assuming the current page is a home page at /home
, the generated HTML will look like this:
<form action="/home" accept-charset="UTF-8" method="post">
<input type="hidden" name="authenticity_token" value="Lz6ILqUEs2CGdDa-oz38TqcqQORavGnbGkG0CQA8zc8peOps-K7sHgFSTPSkBx89pQxh3p5zPIkjoOTiA_UWbQ" autocomplete="off">
Form contents
</form>
Notice that the form contains an input
element with type hidden
. This authenticity_token
hidden input is required for non-GET form submissions.
This token is a security feature in Rails used to prevent cross-site request forgery (CSRF) attacks, and form helpers automatically generate it for every non-GET form (assuming the security feature is enabled). You can read more about it in the Securing Rails Applications guide.
1.1 A Generic Search Form
One of the most basic forms on the web is a search form. This form contains:
- a form element with "GET" method,
- a label for the input,
- a text input element, and
- a submit element.
Here is how to create a search form with form_with
:
<%= form_with url: "/search", method: :get do |form| %>
<%= form.label :query, "Search for:" %>
<%= form.search_field :query %>
<%= form.submit "Search" %>
<% end %>
This will generate the following HTML:
<form action="/search" accept-charset="UTF-8" method="get">
<label for="query">Search for:</label>
<input type="search" name="query" id="query">
<input type="submit" name="commit" value="Search" data-disable-with="Search">
</form>
Notice that for the search form we are using the url
option of form_with
. Setting url: "/search"
changes the form action value from the default current page path to action="/search"
.
In general, passing url: my_path
to form_with
tells the form where to make the request. The other option is to pass Active Model objects to the form, as you will learn below. You can also use URL helpers.
The search form example above also shows the form builder object. You will learn about the many helpers provided by the form builder object (likeform.label
and form.text_field
) in the next section.
For every form input
element, an id
attribute is generated from its name ("query"
in above example). These IDs can be very useful for CSS styling or manipulation of form controls with JavaScript.
Use "GET" as the method for search forms. In general, Rails conventions encourage using the right HTTP verb for controller actions. Using "GET" for search allows users to bookmark a specific search.
1.2 Helpers for Generating Form Elements
The form builder object yielded by form_with
provides many helper methods for generating common form elements such as text fields, checkboxes, and radio buttons.
The first argument to these methods is always the name of the input. This is
useful to remember because when the form is submitted, that name will be passed
to the controller along with the form data in the params
hash. The name will be the key in the params
for the value entered by the user for that field.
For example, if the form contains <%= form.text_field :query %>
, then you
would be able to get the value of this field in the controller with
params[:query]
.
When naming inputs, Rails uses certain conventions that make it possible to submit parameters with non-scalar values such as arrays or hashes, which will also be accessible in params
. You can read more about them in the Form Input Naming Conventions and Params Hash section of this guide. For details on the precise usage of these helpers, please refer to the API documentation.
1.2.1 Checkboxes
A Checkbox is a form control that allows for a single value to be selected or deselected. A group of Checkboxes is generally used to allow a user to choose one or more options from the group.
Here's an example with three checkboxes in a form:
<%= form.check_box :biography %>
<%= form.label :biography, "Biography" %>
<%= form.check_box :romance %>
<%= form.label :romance, "Romance" %>
<%= form.check_box :mystery %>
<%= form.label :mystery, "Mystery" %>
The above will generate the following:
<input name="biography" type="hidden" value="0" autocomplete="off"><input type="checkbox" value="1" name="biography" id="biography">
<label for="biography">Biography</label>
<input name="romance" type="hidden" value="0" autocomplete="off"><input type="checkbox" value="1" name="romance" id="romance">
<label for="romance">Romance</label>
<input name="mystery" type="hidden" value="0" autocomplete="off"><input type="checkbox" value="1" name="mystery" id="mystery">
<label for="mystery">Mystery</label>
The first parameter to check_box
is the name of the input which can be found in the params
hash. If the user has checked the "Biography" checkbox only, the params
hash would contain:
{
"biography" => "1",
"romance" => "0",
"mystery" => "0"
}
You can use params[:biography]
to check if that checkbox is selected by the user.
The checkbox's values (the values that will appear in params
) can optionally be specified using the checked_value
and unchecked_value
parameters. See the API documentation for more details.
There is also a collection_check_boxes
, which you can learn about in the Collection Related Helpers section.
1.2.2 Radio Buttons
Radio buttons are form controls that only allow the user to select one option at a time from the list of choices.
For example, radio buttons for choosing your favorite ice cream flavor:
<%= form.radio_button :flavor, "chocolate_chip" %>
<%= form.label :flavor_chocolate_chip, "Chocolate Chip" %>
<%= form.radio_button :flavor, "vanilla" %>
<%= form.label :flavor_vanilla, "Vanilla" %>
<%= form.radio_button :flavor, "hazelnut" %>
<%= form.label :flavor_hazelnut, "Hazelnut" %>
The above will generate the following HTML:
<input type="radio" value="chocolate_chip" name="flavor" id="flavor_chocolate_chip">
<label for="flavor_chocolate_chip">Chocolate Chip</label>
<input type="radio" value="vanilla" name="flavor" id="flavor_vanilla">
<label for="flavor_vanilla">Vanilla</label>
<input type="radio" value="hazelnut" name="flavor" id="flavor_hazelnut">
<label for="flavor_hazelnut">Hazelnut</label>
The second argument to radio_button
is the value of the input. Because these radio buttons share the same name (flavor
), the user will only be able to select one of them, and params[:flavor]
will contain either "chocolate_chip"
, "vanilla"
, or hazelnut
.
Always use labels for checkbox and radio buttons. They associate text with
a specific option using the for
attribute and, by expanding the clickable
region, make it easier for users to click the inputs.
1.3 Other Helpers of Interest
There are many other form controls including text, email, password, date, and time. The below examples show some more helpers and their generated HTML.
Date and time related helpers:
<%= form.date_field :born_on %>
<%= form.time_field :started_at %>
<%= form.datetime_local_field :graduation_day %>
<%= form.month_field :birthday_month %>
<%= form.week_field :birthday_week %>
Output:
<input type="date" name="born_on" id="born_on">
<input type="time" name="started_at" id="started_at">
<input type="datetime-local" name="graduation_day" id="graduation_day">
<input type="month" name="birthday_month" id="birthday_month">
<input type="week" name="birthday_week" id="birthday_week">
Helpers with special formatting:
<%= form.password_field :password %>
<%= form.email_field :address %>
<%= form.telephone_field :phone %>
<%= form.url_field :homepage %>
Output:
<input type="password" name="password" id="password">
<input type="email" name="address" id="address">
<input type="tel" name="phone" id="phone">
<input type="url" name="homepage" id="homepage">
Other common helpers:
<%= form.text_area :message, size: "70x5" %>
<%= form.hidden_field :parent_id, value: "foo" %>
<%= form.number_field :price, in: 1.0..20.0, step: 0.5 %>
<%= form.range_field :discount, in: 1..100 %>
<%= form.search_field :name %>
<%= form.color_field :favorite_color %>
Output:
<textarea name="message" id="message" cols="70" rows="5"></textarea>
<input value="foo" autocomplete="off" type="hidden" name="parent_id" id="parent_id">
<input step="0.5" min="1.0" max="20.0" type="number" name="price" id="price">
<input min="1" max="100" type="range" name="discount" id="discount">
<input type="search" name="name" id="name">
<input value="#000000" type="color" name="favorite_color" id="favorite_color">
Hidden inputs are not shown to the user but instead hold data like any textual input. Values inside them can be changed with JavaScript.
If you're using password input fields, you might want to configure your application to prevent those parameters from being logged. You can learn about how in the Securing Rails Applications guide.
2 Creating Forms with Model Objects
2.1 Binding a Form to an Object
The form_with
helper has a :model
option that allows you to bind the form builder object to a model object. This means that the form will be scoped to that model object, and the form's fields will be populated with values from that model object.
For example, if we have a @book
model object:
@book = Book.find(42)
# => #<Book id: 42, title: "Walden", author: "Henry David Thoreau">
And the following form to create a new book:
<%= form_with model: @book do |form| %>
<div>
<%= form.label :title %>
<%= form.text_field :title %>
</div>
<div>
<%= form.label :author %>
<%= form.text_field :author %>
</div>
<%= form.submit %>
<% end %>
It will generate this HTML:
<form action="/books" accept-charset="UTF-8" method="post">
<input type="hidden" name="authenticity_token" value="ChwHeyegcpAFDdBvXvDuvbfW7yCA3e8gvhyieai7DhG28C3akh-dyuv-IBittsjPrIjETlQQvQJ91T77QQ8xWA" autocomplete="off">
<div>
<label for="book_title">Title</label>
<input type="text" name="book[title]" id="book_title">
</div>
<div>
<label for="book_author">Author</label>
<input type="text" name="book[author]" id="book_author">
</div>
<input type="submit" name="commit" value="Create Book" data-disable-with="Create Book">
</form>
Some important things to notice when using form_with
with a model object:
- The form
action
is automatically filled with an appropriate value,action="/books"
. If you were updating a book, it would beaction="/books/42"
. - The form field names are scoped with
book[...]
. This means thatparams[:book]
will be a hash containing all these field's values. You can read more about the significance of input names in chapter Form Input Naming Conventions and Params Hash of this guide. - The submit button is automatically given an appropriate text value, "Create Book" in this case.
Typically your form inputs will mirror model attributes. However, they don't have to. If there is other information you need you can include a field in your form and access it via params[:book][:my_non_attribute_input]
.
2.1.1 Composite Primary Key Forms
If you have a model with a composite primary key, the form building syntax is the same with slightly different output.
For example, to update a @book
model object with a composite key [:author_id, :id]
like this:
@book = Book.find([2, 25])
# => #<Book id: 25, title: "Some book", author_id: 2>
The following form:
<%= form_with model: @book do |form| %>
<%= form.text_field :title %>
<%= form.submit %>
<% end %>
Will generate this HTML output:
<form action="/books/2_25" method="post" accept-charset="UTF-8" >
<input name="authenticity_token" type="hidden" value="ChwHeyegcpAFDdBvXvDuvbfW7yCA3e8gvhyieai7DhG28C3akh-dyuv-IBittsjPrIjETlQQvQJ91T77QQ8xWA" />
<input type="text" name="book[title]" id="book_title" value="Some book" />
<input type="submit" name="commit" value="Update Book" data-disable-with="Update Book">
</form>
Note the generated URL contains the author_id
and id
delimited by an
underscore. Once submitted, the controller can extract each primary key
value from the parameters and update the record as it would with a singular
primary key.
2.1.2 The fields_for
Helper
The fields_for
helper is used to render fields for related model objects
within the same form. The associated "inner" model is usually related to the
"main" form model via an Active Record association. For example, if you had a
Person
model with an associated ContactDetail
model, you could create a
single form with inputs for both models like so:
<%= form_with model: @person do |person_form| %>
<%= person_form.text_field :name %>
<%= fields_for :contact_detail, @person.contact_detail do |contact_detail_form| %>
<%= contact_detail_form.text_field :phone_number %>
<% end %>
<% end %>
The above will produce the following output:
<form action="/people" accept-charset="UTF-8" method="post">
<input type="hidden" name="authenticity_token" value="..." autocomplete="off" />
<input type="text" name="person[name]" id="person_name" />
<input type="text" name="contact_detail[phone_number]" id="contact_detail_phone_number" />
</form>
The object yielded by fields_for
is a form builder like the one yielded by
form_with
. The fields_for
helper creates a similar binding but without
rendering a <form>
tag. You can learn more about field_for
in the API
docs.
2.2 Relying on Record Identification
When dealing with RESTful resources, calls to form_with
can be simplified by relying on record identification. This means you pass the model instance and have Rails figure out the model name, method, and other things. In the example below for creating a new record, both calls to form_with
generate the same HTML:
# longer way:
form_with(model: @article, url: articles_path)
# short-hand:
form_with(model: @article)
Similarly, for editing an existing article like below, both the calls to form_with
will also generate the same HTML:
# longer way:
form_with(model: @article, url: article_path(@article), method: "patch")
# short-hand:
form_with(model: @article)
Notice how the short-hand form_with
invocation is conveniently the same, regardless of the record being new or existing. Record identification is smart enough to figure out if the record is new by asking record.persisted?
. It also selects the correct path to submit to, and the name based on the class of the object.
This is assuming that the Article
model is declared with resources :articles
in the routes file.
If you have a singular resource, you will need to call resource
and resolve
for it to work with form_with
:
resource :article
resolve('Article') { [:article] }
Declaring a resource has a number of side effects. See the Rails Routing from the Outside In guide for more information on setting up and using resources.
When you're using single-table inheritance with your models, you can't rely on record identification on a subclass if only their parent class is declared a resource. You will have to specify :url
, and :scope
(the model name) explicitly.
2.3 Working with Namespaces
If you have namespaced routes, form_with
has a shorthand for that. For example, if your application has an admin
namespace:
form_with model: [:admin, @article]
The above will create a form that submits to the Admin::ArticlesController
inside the admin namespace, therefore submitting to admin_article_path(@article)
in the case of an update.
If you have several levels of namespacing then the syntax is similar:
form_with model: [:admin, :management, @article]
For more information on Rails' routing system and the associated conventions, please see the Rails Routing from the Outside In guide.
2.4 Forms with PATCH, PUT, or DELETE Methods
The Rails framework encourages RESTful design, which means forms in your application will make requests where the method
is PATCH
, PUT
, or DELETE
in addition to GET
and POST
. However, HTML forms don't support methods other than GET
and POST
when it comes to submitting forms.
Rails works around this limitation by emulating other methods over POST with a hidden input named "_method"
. For example:
form_with(url: search_path, method: "patch")
The above form Will generate this HTML output:
<form action="/search" accept-charset="UTF-8" method="post">
<input type="hidden" name="_method" value="patch" autocomplete="off">
<input type="hidden" name="authenticity_token" value="R4quRuXQAq75TyWpSf8AwRyLt-R1uMtPP1dHTTWJE5zbukiaY8poSTXxq3Z7uAjXfPHiKQDsWE1i2_-h0HSktQ" autocomplete="off">
<!-- ... -->
</form>
When parsing POSTed data, Rails will take into account the special _method
parameter and proceed as if the request's HTTP method was the one set as the value of _method
(PATCH
in this example).
When rendering a form, submission buttons can override the declared method
attribute through the formmethod:
keyword:
<%= form_with url: "/posts/1", method: :patch do |form| %>
<%= form.button "Delete", formmethod: :delete, data: { confirm: "Are you sure?" } %>
<%= form.button "Update" %>
<% end %>
Similar to <form>
elements, most browsers don't support overriding form methods declared through formmethod other than GET
and POST
.
Rails works around this issue by emulating other methods over POST through a combination of formmethod, value, and name attributes:
<form accept-charset="UTF-8" action="/posts/1" method="post">
<input name="_method" type="hidden" value="patch" />
<input name="authenticity_token" type="hidden" value="f755bb0ed134b76c432144748a6d4b7a7ddf2b71" />
<!-- ... -->
<button type="submit" formmethod="post" name="_method" value="delete" data-confirm="Are you sure?">Delete</button>
<button type="submit" name="button">Update</button>
</form>
In this case, the "Update" button will be treated as PATCH
and the "Delete" button will be treated as DELETE
.
3 Making Select Boxes with Ease
Select boxes, also known as drop-down list, allow users to select from a list of options. The HTML for select boxes requires a decent amount of markup - one <option>
element for each option to choose from. Rails provides helper methods to help generate that markup.
For example, let's say we have a list of cities for the user to choose from. We can use the select
helper:
<%= form.select :city, ["Berlin", "Chicago", "Madrid"] %>
The above will generate this HTML output:
<select name="city" id="city">
<option value="Berlin">Berlin</option>
<option value="Chicago">Chicago</option>
<option value="Madrid">Madrid</option>
</select>
And the selection will be available in params[:city]
as usual.
We can also specify <option>
values that differ from their labels:
<%= form.select :city, [["Berlin", "BE"], ["Chicago", "CHI"], ["Madrid", "MD"]] %>
Output:
<select name="city" id="city">
<option value="BE">Berlin</option>
<option value="CHI">Chicago</option>
<option value="MD">Madrid</option>
</select>
This way, the user will see the full city name, but params[:city]
will be one of "BE"
, "CHI"
, or "MD"
.
Lastly, we can specify a default choice for the select box with the :selected
argument:
<%= form.select :city, [["Berlin", "BE"], ["Chicago", "CHI"], ["Madrid", "MD"]], selected: "CHI" %>
Output:
<select name="city" id="city">
<option value="BE">Berlin</option>
<option value="CHI" selected="selected">Chicago</option>
<option value="MD">Madrid</option>
</select>
3.1 Option Groups for Select Boxes
In some cases we may want to improve the user experience by grouping related options together. We can do so by passing a Hash
(or comparable Array
) to select
:
<%= form.select :city,
{
"Europe" => [ ["Berlin", "BE"], ["Madrid", "MD"] ],
"North America" => [ ["Chicago", "CHI"] ],
},
selected: "CHI" %>
Output:
<select name="city" id="city">
<optgroup label="Europe">
<option value="BE">Berlin</option>
<option value="MD">Madrid</option>
</optgroup>
<optgroup label="North America">
<option value="CHI" selected="selected">Chicago</option>
</optgroup>
</select>
3.2 Binding Select Boxes to Model Objects
Like other form controls, a select box can be bound to a model attribute. For example, if we have a @person
model object like:
@person = Person.new(city: "MD")
The following form:
<%= form_with model: @person do |form| %>
<%= form.select :city, [["Berlin", "BE"], ["Chicago", "CHI"], ["Madrid", "MD"]] %>
<% end %>
Will output this select box:
<select name="person[city]" id="person_city">
<option value="BE">Berlin</option>
<option value="CHI">Chicago</option>
<option value="MD" selected="selected">Madrid</option>
</select>
The only difference is that the selected option will be found in params[:person][:city]
instead of params[:city]
.
Notice that the appropriate option was automatically marked selected="selected"
. Since this select box was bound to an existing @person
record, we didn't need to specify a :selected
argument.
4 Using Date and Time Form Helpers
In addition to the date_field
and time_field
helpers mentioned earlier, Rails provides alternative date and time form helpers that render plain select boxes. The date_select
helper renders a separate select box for year, month, and day.
For example, if we have a @person
model object like:
@person = Person.new(birth_date: Date.new(1995, 12, 21))
The following form:
<%= form_with model: @person do |form| %>
<%= form.date_select :birth_date %>
<% end %>
Will output select boxes like:
<select name="person[birth_date(1i)]" id="person_birth_date_1i">
<option value="1990">1990</option>
<option value="1991">1991</option>
<option value="1992">1992</option>
<option value="1993">1993</option>
<option value="1994">1994</option>
<option value="1995" selected="selected">1995</option>
<option value="1996">1996</option>
<option value="1997">1997</option>
<option value="1998">1998</option>
<option value="1999">1999</option>
<option value="2000">2000</option>
</select>
<select name="person[birth_date(2i)]" id="person_birth_date_2i">
<option value="1">January</option>
<option value="2">February</option>
<option value="3">March</option>
<option value="4">April</option>
<option value="5">May</option>
<option value="6">June</option>
<option value="7">July</option>
<option value="8">August</option>
<option value="9">September</option>
<option value="10">October</option>
<option value="11">November</option>
<option value="12" selected="selected">December</option>
</select>
<select name="person[birth_date(3i)]" id="person_birth_date_3i">
<option value="1">1</option>
...
<option value="21" selected="selected">21</option>
...
<option value="31">31</option>
</select>
Notice that, when the form is submitted, there will be no single value in the params
hash that contains the full date. Instead, there will be several values with special names like "birth_date(1i)"
. However, Active Model knows how to assemble these values into a full date, based on the declared type of the model attribute. So we can pass params[:person]
to Person.new
or Person#update
just like we would if the form used a single field to represent the full date.
In addition to the date_select
helper, Rails provides time_select
which outputs select boxes for the hour and minute. There is datetime_select
as well which combines both date and time select boxes.
4.1 Select Boxes for Time or Date Components
Rails also provides helpers to render select boxes for individual date and time components: select_year
, select_month
, select_day
, select_hour
, select_minute
, and select_second
. These helpers are "bare" methods, meaning they are not called on a form builder instance. For example:
<%= select_year 2024, prefix: "party" %>
The above outputs a select box like:
<select id="party_year" name="party[year]">
<option value="2019">2019</option>
<option value="2020">2020</option>
<option value="2021">2021</option>
<option value="2022">2022</option>
<option value="2023">2023</option>
<option value="2024" selected="selected">2024</option>
<option value="2025">2025</option>
<option value="2026">2026</option>
<option value="2027">2027</option>
<option value="2028">2028</option>
<option value="2029">2029</option>
</select>
For each of these helpers, you may specify a Date
or Time
object instead of a number as the default value (for example <%= select_year Date.today, prefix: "party" %>
instead of the above), and the appropriate date and time parts will be extracted and used.
4.2 Selecting Time Zone
When you need to ask users what time zone they are in, there is a very convenient time_zone_select
helper to use.
Typically, you would have to provide a list of time zone options for users to select from. This can get tedious if not for the list of pre-defined ActiveSupport::TimeZone
objects. The time_with_zone
helper wraps this and can be used as follows:
<%= form.time_zone_select :time_zone %>
Output:
<select name="time_zone" id="time_zone">
<option value="International Date Line West">(GMT-12:00) International Date Line West</option>
<option value="American Samoa">(GMT-11:00) American Samoa</option>
<option value="Midway Island">(GMT-11:00) Midway Island</option>
<option value="Hawaii">(GMT-10:00) Hawaii</option>
<option value="Alaska">(GMT-09:00) Alaska</option>
...
<option value="Samoa">(GMT+13:00) Samoa</option>
<option value="Tokelau Is.">(GMT+13:00) Tokelau Is.</option>
</select>
5 Collection Related Helpers
If you need to generate a set of choices from a collection of arbitrary objects, Rails has collection_select
, collection_radio_button
, and collection_check_boxes
helpers.
To see when these helpers are useful, suppose you have a City
model and corresponding belongs_to :city
association with Person
:
class City < ApplicationRecord
end
class Person < ApplicationRecord
belongs_to :city
end
Assuming we have the following cities stored in the database:
City.order(:name).map { |city| [city.name, city.id] }
# => [["Berlin", 1], ["Chicago", 3], ["Madrid", 2]]
We can allow the user to choose from the cities with the following form:
<%= form_with model: @person do |form| %>
<%= form.select :city_id, City.order(:name).map { |city| [city.name, city.id] } %>
<% end %>
The above will generate this HTML:
<select name="person[city_id]" id="person_city_id">
<option value="1">Berlin</option>
<option value="3">Chicago</option>
<option value="2">Madrid</option>
</select>
The above example shows how you'd generate the choices manually. However, Rails has helpers that generate choices from a collection without having to explicitly iterate over it. These helpers determine the value and text label of each choice by calling specified methods on each object in the collection.
When rendering a field for a belongs_to
association, you must specify the name of the foreign key (city_id
in the above example), rather than the name of the association itself.
5.1 The collection_select
Helper
To generate a select box, we can use collection_select
:
<%= form.collection_select :city_id, City.order(:name), :id, :name %>
The above outputs the same HTML as the manual iteration above:
<select name="person[city_id]" id="person_city_id">
<option value="1">Berlin</option>
<option value="3">Chicago</option>
<option value="2">Madrid</option>
</select>
The order of arguments for collection_select
is different from the order for select
. With collection_select
we specify the value method first (:id
in the example above), and the text label method second (:name
in the example above). This is opposite of the order used when specifying choices for the select
helper, where the text label comes first and the value second (["Berlin", 1]
in the previous example).
5.2 The collection_radio_buttons
Helper
To generate a set of radio buttons, we can use collection_radio_buttons
:
<%= form.collection_radio_buttons :city_id, City.order(:name), :id, :name %>
Output:
<input type="radio" value="1" name="person[city_id]" id="person_city_id_1">
<label for="person_city_id_1">Berlin</label>
<input type="radio" value="3" name="person[city_id]" id="person_city_id_3">
<label for="person_city_id_3">Chicago</label>
<input type="radio" value="2" name="person[city_id]" id="person_city_id_2">
<label for="person_city_id_2">Madrid</label>
5.3 The collection_check_boxes
Helper
To generate a set of check boxes — for example, to support a has_and_belongs_to_many
association — we can use collection_check_boxes
:
<%= form.collection_check_boxes :interest_ids, Interest.order(:name), :id, :name %>
Output:
<input type="checkbox" name="person[interest_id][]" value="3" id="person_interest_id_3">
<label for="person_interest_id_3">Engineering</label>
<input type="checkbox" name="person[interest_id][]" value="4" id="person_interest_id_4">
<label for="person_interest_id_4">Math</label>
<input type="checkbox" name="person[interest_id][]" value="1" id="person_interest_id_1">
<label for="person_interest_id_1">Science</label>
<input type="checkbox" name="person[interest_id][]" value="2" id="person_interest_id_2">
<label for="person_interest_id_2">Technology</label>
6 Uploading Files
A common task with forms is allowing users to upload a file. It could be an avatar image or a CSV file with data to process. File upload fields can be rendered with the file_field
helper.
<%= form_with model: @person do |form| %>
<%= form.file_field :csv_file %>
<% end %>
The most important thing to remember with file uploads is that the rendered form's enctype
attribute must be set to multipart/form-data
. This is done automatically if you use a file_field
inside a form_with
. You can also set the attribute manually:
<%= form_with url: "/uploads", multipart: true do |form| %>
<%= file_field_tag :csv_file %>
<% end %>
Both of which, output the following HTML form:
<form enctype="multipart/form-data" action="/people" accept-charset="UTF-8" method="post">
<!-- ... -->
</form>
Note that, per form_with
conventions, the field names in the two forms above will be different. In the first form, it will be person[csv_file]
(accessible via params[:person][:csv_file]
), and in the second form it will be just csv_file
(accessible via params[:csv_file]
).
6.1 CSV File Upload Example
When using file_field
, the object in the params
hash is an instance of ActionDispatch::Http::UploadedFile
. Here is an example of how to save data in an uploaded CSV file to records in your application:
require 'csv'
def upload
uploaded_file = params[:csv_file]
if uploaded_file.present?
csv_data = CSV.parse(uploaded_file.read, headers: true)
csv_data.each do |row|
# Process each row of the CSV file
# SomeInvoiceModel.create(amount: row['Amount'], status: row['Status'])
Rails.logger.info row.inspect
#<CSV::Row "id":"po_1KE3FRDSYPMwkcNz9SFKuaYd" "Amount":"96.22" "Created (UTC)":"2022-01-04 02:59" "Arrival Date (UTC)":"2022-01-05 00:00" "Status":"paid">
end
end
# ...
end
If the file is an image that needs to be stored with a model (e.g. user's profile picture), there are a number of tasks to consider, like where to store the file (on Disk, Amazon S3, etc), resizing image files, and generating thumbnails, etc. Active Storage is designed to assist with these tasks.
7 Customizing Form Builders
We call the objects yielded by form_with
or fields_for
Form Builders. Form builders allow you to generate form elements associated with a model object
and are an instance of
ActionView::Helpers::FormBuilder
. This class can be extended to add custom helpers for your application.
For example, if you want to display a text_field
along with a label
across your application, you could add the following helper method to application_helper.rb
:
module ApplicationHelper
def text_field_with_label(form, attribute)
form.label(attribute) + form.text_field(attribute)
end
end
And use it in a form as usual:
<%= form_with model: @person do |form| %>
<%= text_field_with_label form, :first_name %>
<% end %>
But you can also create a subclass of ActionView::Helpers::FormBuilder
, and
add the helpers there. After defining this LabellingFormBuilder
subclass:
class LabellingFormBuilder < ActionView::Helpers::FormBuilder
def text_field(attribute, options = {})
# super will call the original text_field method
label(attribute) + super
end
end
The above form can be replaced with:
<%= form_with model: @person, builder: LabellingFormBuilder do |form| %>
<%= form.text_field :first_name %>
<% end %>
If you reuse this frequently you could define a labeled_form_with
helper that automatically applies the builder: LabellingFormBuilder
option:
module ApplicationHelper
def labeled_form_with(**options, &block)
options[:builder] = LabellingFormBuilder
form_with(**options, &block)
end
end
The above can be used instead of form_with
:
<%= labeled_form_with model: @person do |form| %>
<%= form.text_field :first_name %>
<% end %>
All three cases above (the text_field_with_label
helper, the LabellingFormBuilder
subclass, and the labeled_form_with
helper) will generate the same HTML output:
<form action="/people" accept-charset="UTF-8" method="post">
<!-- ... -->
<label for="person_first_name">First name</label>
<input type="text" name="person[first_name]" id="person_first_name">
</form>
The form builder used also determines what happens when you do:
<%= render partial: f %>
If f
is an instance of ActionView::Helpers::FormBuilder
, then this will render the form
partial, setting the partial's object to the form builder. If the form builder is of class LabellingFormBuilder
, then the labelling_form
partial would be rendered instead.
Form builder customizations, such as LabellingFormBuilder
, do hide the implementation details (and may seem like an overkill for the simple example above). Choose between different customizations, extending FormBuilder
class or creating helpers, based on how frequently your forms use the custom elements.
8 Form Input Naming Conventions and params
Hash
All of the form helpers described above help with generating the HTML for form elements so that the user can enter various types of input. How do you access the user input values in the Controller? The params
hash is the answer. You've already seen the params
hash in the above example. This section will more explicitly go over naming conventions around how form input is structured in the params
hash.
The params
hash can contain arrays and arrays of hashes. Values can be at the top level of the params
hash or nested in another hash. For example, in a standard create
action for a Person model, params[:person]
will be a hash of all the attributes for the Person
object.
Note that HTML forms don't have an inherent structure to the user input data, all they generate is name-value string pairs. The arrays and hashes you see in your application are the result of parameter naming conventions that Rails uses.
The fields in the params
hash need to be permitted in the controller.
8.1 Basic Structure
The two basic structures for user input form data are arrays and hashes.
Hashes mirror the syntax used for accessing the value in params
. For example, if a form contains:
<input id="person_name" name="person[name]" type="text" value="Henry"/>
the params
hash will contain
{ 'person' => { 'name' => 'Henry' } }
and params[:person][:name]
will retrieve the submitted value in the controller.
Hashes can be nested as many levels as required, for example:
<input id="person_address_city" name="person[address][city]" type="text" value="New York"/>
The above will result in the params
hash being
{ 'person' => { 'address' => { 'city' => 'New York' } } }
The other structure is an Array. Normally Rails ignores duplicate parameter names, but if the parameter name ends with an empty set of square brackets []
then the parameters will be accumulated in an Array.
For example, if you want users to be able to input multiple phone numbers, you could place this in the form:
<input name="person[phone_number][]" type="text"/>
<input name="person[phone_number][]" type="text"/>
<input name="person[phone_number][]" type="text"/>
This would result in params[:person][:phone_number]
being an array containing the submitted phone numbers:
{ 'person' => { 'phone_number' => ['555-0123', '555-0124', '555-0125'] } }
8.2 Combining Arrays and Hashes
You can mix and match these two concepts. One element of a hash might be an array as in the previous example params[:person]
hash has a key called [:phone_number]
whose value is an array.
You also can have an array of hashes. For example, you can create any number of addresses by repeating the following form fragment:
<input name="person[addresses][][line1]" type="text"/>
<input name="person[addresses][][line2]" type="text"/>
<input name="person[addresses][][city]" type="text"/>
<input name="person[addresses][][line1]" type="text"/>
<input name="person[addresses][][line2]" type="text"/>
<input name="person[addresses][][city]" type="text"/>
This would result in params[:person][:addresses]
being an array of hashes. Each hash in the array will have the keys line1
, line2
, and city
, something like this:
{ 'person' =>
{ 'addresses' => [
{ 'line1' => '1000 Fifth Avenue',
'line2' => '',
'city' => 'New York'
},
{ 'line1' => 'Calle de Ruiz de Alarcón',
'line2' => '',
'city' => 'Madrid'
}
]
}
}
It's important to note that while hashes can be nested arbitrarily, only one level of "arrayness" is allowed. Arrays can usually be replaced by hashes. For example, instead of an array of model objects, you can have a hash of model objects keyed by their id or similar.
Array parameters do not play well with the check_box
helper. According to the HTML specification unchecked checkboxes submit no value. However it is often convenient for a checkbox to always submit a value. The check_box
helper fakes this by creating an auxiliary hidden input with the same name. If the checkbox is unchecked only the hidden input is submitted. If it is checked then both are submitted but the value submitted by the checkbox takes precedence. There is a include_hidden
option that can be set to false
if you want to omit this hidden field. By default, this option is true
.
8.3 Hashes with an Index
Let's say you want to render a form with a set of fields for each of a person's
addresses. The [fields_for
][] helper with its :index
option can assist:
<%= form_with model: @person do |person_form| %>
<%= person_form.text_field :name %>
<% @person.addresses.each do |address| %>
<%= person_form.fields_for address, index: address.id do |address_form| %>
<%= address_form.text_field :city %>
<% end %>
<% end %>
<% end %>
Assuming the person has two addresses with IDs 23 and 45, the above form would render this output:
<form accept-charset="UTF-8" action="/people/1" method="post">
<input name="_method" type="hidden" value="patch" />
<input id="person_name" name="person[name]" type="text" />
<input id="person_address_23_city" name="person[address][23][city]" type="text" />
<input id="person_address_45_city" name="person[address][45][city]" type="text" />
</form>
Which will result in a params
hash that looks like:
{
"person" => {
"name" => "Bob",
"address" => {
"23" => {
"city" => "Paris"
},
"45" => {
"city" => "London"
}
}
}
}
All of the form inputs map to the "person"
hash because we called fields_for
on the person_form
form builder. Also, by specifying index: address.id
, we
rendered the name
attribute of each city input as
person[address][#{address.id}][city]
instead of person[address][city]
. This
way you can tell which Address
records should be modified when processing the
params
hash.
You can find more details about fields_for
index option in the API docs.
9 Building Complex Forms
As your application grows, you may need to create more complex forms, beyond editing a single object. For example, when creating a Person
you can allow the user to create multiple Address
records (home, work, etc.) within the same form. When editing a Person
record later, the user should be able to add, remove, or update addresses as well.
9.1 Configuring the Model for Nested Attributes
For editing an associated record for a given model (Person
in this case), Active Record provides model level support via the accepts_nested_attributes_for
method:
class Person < ApplicationRecord
has_many :addresses, inverse_of: :person
accepts_nested_attributes_for :addresses
end
class Address < ApplicationRecord
belongs_to :person
end
This creates an addresses_attributes=
method on Person
that allows you to create, update, and destroy addresses.
9.2 Nested Forms in the View
The following form allows a user to create a Person
and its associated addresses.
<%= form_with model: @person do |form| %>
Addresses:
<ul>
<%= form.fields_for :addresses do |addresses_form| %>
<li>
<%= addresses_form.label :kind %>
<%= addresses_form.text_field :kind %>
<%= addresses_form.label :street %>
<%= addresses_form.text_field :street %>
...
</li>
<% end %>
</ul>
<% end %>
When an association accepts nested attributes, fields_for
renders its block once for every element of the association. In particular, if a person has no addresses, it renders nothing.
A common pattern is for the controller to build one or more empty children so that at least one set of fields is shown to the user. The example below would result in 2 sets of address fields being rendered on the new person form.
For example, the above form_with
with this change:
def new
@person = Person.new
2.times { @person.addresses.build }
end
Will output the following HTML:
<form action="/people" accept-charset="UTF-8" method="post"><input type="hidden" name="authenticity_token" value="lWTbg-4_5i4rNe6ygRFowjDfTj7uf-6UPFQnsL7H9U9Fe2GGUho5PuOxfcohgm2Z-By3veuXwcwDIl-MLdwFRg" autocomplete="off">
Addresses:
<ul>
<li>
<label for="person_addresses_attributes_0_kind">Kind</label>
<input type="text" name="person[addresses_attributes][0][kind]" id="person_addresses_attributes_0_kind">
<label for="person_addresses_attributes_0_street">Street</label>
<input type="text" name="person[addresses_attributes][0][street]" id="person_addresses_attributes_0_street">
...
</li>
<li>
<label for="person_addresses_attributes_1_kind">Kind</label>
<input type="text" name="person[addresses_attributes][1][kind]" id="person_addresses_attributes_1_kind">
<label for="person_addresses_attributes_1_street">Street</label>
<input type="text" name="person[addresses_attributes][1][street]" id="person_addresses_attributes_1_street">
...
</li>
</ul>
</form>
The fields_for
yields a form builder. The parameter names will be what
accepts_nested_attributes_for
expects. For example, when creating a person
with 2 addresses, the submitted parameters in params
would look like this:
{
'person' => {
'name' => 'John Doe',
'addresses_attributes' => {
'0' => {
'kind' => 'Home',
'street' => '221b Baker Street'
},
'1' => {
'kind' => 'Office',
'street' => '31 Spooner Street'
}
}
}
}
The actual value of the keys in the :addresses_attributes
hash is not important. But they need to be strings of integers and different for each address.
If the associated object is already saved, fields_for
autogenerates a hidden input with the id
of the saved record. You can disable this by passing include_id: false
to fields_for
.
{
'person' => {
'name' => 'John Doe',
'addresses_attributes' => {
'0' => {
'id' => 1,
'kind' => 'Home',
'street' => '221b Baker Street'
},
'1' => {
'id' => '2',
'kind' => 'Office',
'street' => '31 Spooner Street'
}
}
}
}
9.3 Permitting Parameters in the Controller
As usual you need to declare the permitted parameters in the controller before you pass them to the model:
def create
@person = Person.new(person_params)
# ...
end
private
def person_params
params.require(:person).permit(:name, addresses_attributes: [:id, :kind, :street])
end
9.4 Removing Associated Objects
You can allow users to delete associated objects by passing allow_destroy: true
to accepts_nested_attributes_for
class Person < ApplicationRecord
has_many :addresses
accepts_nested_attributes_for :addresses, allow_destroy: true
end
If the hash of attributes for an object contains the key _destroy
with a value
that evaluates to true
(e.g. 1
, '1'
, true
, or 'true'
) then the object will be
destroyed. This form allows users to remove addresses:
<%= form_with model: @person do |form| %>
Addresses:
<ul>
<%= form.fields_for :addresses do |addresses_form| %>
<li>
<%= addresses_form.check_box :_destroy %>
<%= addresses_form.label :kind %>
<%= addresses_form.text_field :kind %>
...
</li>
<% end %>
</ul>
<% end %>
The HTML for the _destroy
field:
<input type="checkbox" value="1" name="person[addresses_attributes][0][_destroy]" id="person_addresses_attributes_0__destroy">
You also need to update the permitted params in your controller to include
the _destroy
field:
def person_params
params.require(:person).
permit(:name, addresses_attributes: [:id, :kind, :street, :_destroy])
end
9.5 Preventing Empty Records
It is often useful to ignore sets of fields that the user has not filled in. You can control this by passing a :reject_if
proc to accepts_nested_attributes_for
. This proc will be called with each hash of attributes submitted by the form. If the proc returns true
then Active Record will not build an associated object for that hash. The example below only tries to build an address if the kind
attribute is set.
class Person < ApplicationRecord
has_many :addresses
accepts_nested_attributes_for :addresses, reject_if: lambda { |attributes| attributes['kind'].blank? }
end
As a convenience you can instead pass the symbol :all_blank
which will create a proc that will reject records where all the attributes are blank excluding any value for _destroy
.
10 Forms to External Resources
Rails form helpers can be used to build a form for posting data to an external resource. If the external API expects an authenticity_token
for the resource, this can be passed as an authenticity_token: 'your_external_token'
parameter to form_with
:
<%= form_with url: 'http://farfar.away/form', authenticity_token: 'external_token' do %>
Form contents
<% end %>
At other times, the fields that can be used in the form are limited by an external API and it may be undesirable to generate an authenticity_token
. To not send a token, you can pass false
to the :authenticity_token
option:
<%= form_with url: 'http://farfar.away/form', authenticity_token: false do %>
Form contents
<% end %>
11 Using Tag Helpers without a Form Builder
In case you need to render form fields outside of the context of a form builder, Rails provides tag helpers for common form elements. For example, check_box_tag
:
<%= check_box_tag "accept" %>
Output:
<input type="checkbox" name="accept" id="accept" value="1" />
Generally, these helpers have the same name as their form builder counterparts plus a _tag
suffix. For a complete list, see the FormTagHelper
API documentation.
12 Using form_tag
and form_for
Before form_with
was introduced in Rails 5.1 its functionality was split between form_tag
and form_for
. Both are now discouraged in favor of form_with
, but you can still find being used in some codebases.
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