This guide is not intended to be a complete documentation of available form helpers and their arguments. Please visit the Rails API documentation for a complete reference.
1 Dealing with Basic Forms
The main form helper is form_with
.
<%= form_with do %> Form contents <% end %>
When called without arguments like this, it creates a form tag which, when submitted, will POST to the current page. For instance, assuming the current page is a home page, the generated HTML will look like this:
<form accept-charset="UTF-8" action="/" data-remote="true" method="post"> <input name="authenticity_token" type="hidden" value="J7CBxfHalt49OSHp27hblqK20c9PgwJ108nDHX/8Cts=" /> Form contents </form>
You'll notice that the HTML contains an input
element with type hidden
. This input
is important, because non-GET form cannot be successfully submitted without it.
The hidden input element with the name authenticity_token
is a security feature of Rails called cross-site request forgery protection, and form helpers generate it for every non-GET form (provided that this security feature is enabled). You can read more about this in the Securing Rails Applications guide.
1.1 A Generic Search Form
One of the most basic forms you see 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.
To create this form you will use form_with
, label_tag
, text_field_tag
, and submit_tag
, respectively. Like this:
<%= form_with(url: "/search", method: "get") do %> <%= label_tag(:q, "Search for:") %> <%= text_field_tag(:q) %> <%= submit_tag("Search") %> <% end %>
This will generate the following HTML:
<form accept-charset="UTF-8" action="/search" data-remote="true" method="get"> <label for="q">Search for:</label> <input id="q" name="q" type="text" /> <input name="commit" type="submit" value="Search" data-disable-with="Search" /> </form>
Passing url: my_specified_path
to form_with
tells the form where to make the request. However, as explained below, you can also pass ActiveRecord objects to the form.
For every form input, an ID attribute is generated from its name ("q"
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. This allows users to bookmark a specific search and get back to it. More generally Rails encourages you to use the right HTTP verb for an action.
1.2 Helpers for Generating Form Elements
Rails provides a series of helpers for generating form elements such as
checkboxes, text fields, and radio buttons. These basic helpers, with names
ending in _tag
(such as text_field_tag
and check_box_tag
), generate just a
single <input>
element. The first parameter to these is always the name of the
input. When the form is submitted, the name will be passed along with the form
data, and will make its way to the params
in the controller with the
value entered by the user for that field. For example, if the form contains
<%= text_field_tag(: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 chapter Understanding Parameter Naming Conventions of this guide. For details on the precise usage of these helpers, please refer to the API documentation.
1.2.1 Checkboxes
Checkboxes are form controls that give the user a set of options they can enable or disable:
<%= check_box_tag(:pet_dog) %> <%= label_tag(:pet_dog, "I own a dog") %> <%= check_box_tag(:pet_cat) %> <%= label_tag(:pet_cat, "I own a cat") %>
This generates the following:
<input id="pet_dog" name="pet_dog" type="checkbox" value="1" /> <label for="pet_dog">I own a dog</label> <input id="pet_cat" name="pet_cat" type="checkbox" value="1" /> <label for="pet_cat">I own a cat</label>
The first parameter to check_box_tag
, of course, is the name of the input. The second parameter, naturally, is the value of the input. This value will be included in the form data (and be present in params
) when the checkbox is checked.
1.2.2 Radio Buttons
Radio buttons, while similar to checkboxes, are controls that specify a set of options in which they are mutually exclusive (i.e., the user can only pick one):
<%= radio_button_tag(:age, "child") %> <%= label_tag(:age_child, "I am younger than 21") %> <%= radio_button_tag(:age, "adult") %> <%= label_tag(:age_adult, "I am over 21") %>
Output:
<input id="age_child" name="age" type="radio" value="child" /> <label for="age_child">I am younger than 21</label> <input id="age_adult" name="age" type="radio" value="adult" /> <label for="age_adult">I am over 21</label>
As with check_box_tag
, the second parameter to radio_button_tag
is the value of the input. Because these two radio buttons share the same name (age
), the user will only be able to select one of them, and params[:age]
will contain either "child"
or "adult"
.
Always use labels for checkbox and radio buttons. They associate text with a specific option and, by expanding the clickable region, make it easier for users to click the inputs.
1.3 Other Helpers of Interest
Other form controls worth mentioning are textareas, password fields, hidden fields, search fields, telephone fields, date fields, time fields, color fields, datetime-local fields, month fields, week fields, URL fields, email fields, number fields, and range fields:
<%= text_area_tag(:message, "Hi, nice site", size: "24x6") %> <%= password_field_tag(:password) %> <%= hidden_field_tag(:parent_id, "5") %> <%= search_field(:user, :name) %> <%= telephone_field(:user, :phone) %> <%= date_field(:user, :born_on) %> <%= datetime_local_field(:user, :graduation_day) %> <%= month_field(:user, :birthday_month) %> <%= week_field(:user, :birthday_week) %> <%= url_field(:user, :homepage) %> <%= email_field(:user, :address) %> <%= color_field(:user, :favorite_color) %> <%= time_field(:task, :started_at) %> <%= number_field(:product, :price, in: 1.0..20.0, step: 0.5) %> <%= range_field(:product, :discount, in: 1..100) %>
Output:
<textarea id="message" name="message" cols="24" rows="6">Hi, nice site</textarea> <input id="password" name="password" type="password" /> <input id="parent_id" name="parent_id" type="hidden" value="5" /> <input id="user_name" name="user[name]" type="search" /> <input id="user_phone" name="user[phone]" type="tel" /> <input id="user_born_on" name="user[born_on]" type="date" /> <input id="user_graduation_day" name="user[graduation_day]" type="datetime-local" /> <input id="user_birthday_month" name="user[birthday_month]" type="month" /> <input id="user_birthday_week" name="user[birthday_week]" type="week" /> <input id="user_homepage" name="user[homepage]" type="url" /> <input id="user_address" name="user[address]" type="email" /> <input id="user_favorite_color" name="user[favorite_color]" type="color" value="#000000" /> <input id="task_started_at" name="task[started_at]" type="time" /> <input id="product_price" max="20.0" min="1.0" name="product[price]" step="0.5" type="number" /> <input id="product_discount" max="100" min="1" name="product[discount]" type="range" />
Hidden inputs are not shown to the user but instead hold data like any textual input. Values inside them can be changed with JavaScript.
The search, telephone, date, time, color, datetime, datetime-local, month, week, URL, email, number, and range inputs are HTML5 controls. If you require your app to have a consistent experience in older browsers, you will need an HTML5 polyfill (provided by CSS and/or JavaScript). There is definitely no shortage of solutions for this, although a popular tool at the moment is Modernizr, which provides a simple way to add functionality based on the presence of detected HTML5 features.
If you're using password input fields (for any purpose), you might want to configure your application to prevent those parameters from being logged. You can learn about this in the Securing Rails Applications guide.
2 Dealing with Model Objects
2.1 Model Object Helpers
A particularly common task for a form is editing or creating a model object. While the *_tag
helpers can certainly be used for this task they are somewhat verbose as for each tag you would have to ensure the correct parameter name is used and set the default value of the input appropriately. Rails provides helpers tailored to this task. These helpers lack the _tag
suffix, for example text_field
, text_area
.
For these helpers the first argument is the name of an instance variable and the second is the name of a method (usually an attribute) to call on that object. Rails will set the value of the input control to the return value of that method for the object and set an appropriate input name. If your controller has defined @person
and that person's name is Henry then a form containing:
<%= text_field(:person, :name) %>
will produce output similar to
<input id="person_name" name="person[name]" type="text" value="Henry" />
Upon form submission the value entered by the user will be stored in params[:person][:name]
.
You must pass the name of an instance variable, i.e. :person
or "person"
, not an actual instance of your model object.
Rails provides helpers for displaying the validation errors associated with a model object. These are covered in detail by the Active Record Validations guide.
2.2 Binding a Form to an Object
While this is an increase in comfort it is far from perfect. If Person
has many attributes to edit then we would be repeating the name of the edited object many times. What we want to do is somehow bind a form to a model object, which is exactly what form_with
with :model
does.
Assume we have a controller for dealing with articles app/controllers/articles_controller.rb
:
def new @article = Article.new end
The corresponding view app/views/articles/new.html.erb
using form_with
looks like this:
<%= form_with model: @article, class: "nifty_form" do |f| %> <%= f.text_field :title %> <%= f.text_area :body, size: "60x12" %> <%= f.submit "Create" %> <% end %>
There are a few things to note here:
-
@article
is the actual object being edited. - There is a single hash of options. HTML options (except
id
andclass
) are passed in the:html
hash. Also you can provide a:namespace
option for your form to ensure uniqueness of id attributes on form elements. The scope attribute will be prefixed with underscore on the generated HTML id. - The
form_with
method yields a form builder object (thef
variable). - If you wish to direct your form request to a particular URL, you would use
form_with url: my_nifty_url_path
instead. To see more in depth options on whatform_with
accepts be sure to check out the API documentation. - Methods to create form controls are called on the form builder object
f
.
The resulting HTML is:
<form class="nifty_form" action="/articles" accept-charset="UTF-8" data-remote="true" method="post"> <input type="hidden" name="authenticity_token" value="NRkFyRWxdYNfUg7vYxLOp2SLf93lvnl+QwDWorR42Dp6yZXPhHEb6arhDOIWcqGit8jfnrPwL781/xlrzj63TA==" /> <input type="text" name="article[title]" id="article_title" /> <textarea name="article[body]" id="article_body" cols="60" rows="12"></textarea> <input type="submit" name="commit" value="Create" data-disable-with="Create" /> </form>
The object passed as :model
in form_with
controls the key used in params
to access the form's values. Here the name is article
and so all the inputs have names of the form article[attribute_name]
. Accordingly, in the create
action params[:article]
will be a hash with keys :title
and :body
. You can read more about the significance of input names in chapter Understanding Parameter Naming Conventions of this guide.
Conventionally your inputs will mirror model attributes. However, they don't have to! If there is other information you need you can include it in your form just as with attributes and access it via params[:article][:my_nifty_non_attribute_input]
.
The helper methods called on the form builder are identical to the model object helpers except that it is not necessary to specify which object is being edited since this is already managed by the form builder.
You can create a similar binding without actually creating <form>
tags with the fields_for
helper. This is useful for editing additional model objects with the same form. For example, if you had a Person
model with an associated ContactDetail
model, you could create a form for creating both 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 %>
which produces the following output:
<form action="/people" accept-charset="UTF-8" data-remote="true" method="post"> <input type="hidden" name="authenticity_token" value="bL13x72pldyDD8bgtkjKQakJCpd4A8JdXGbfksxBDHdf1uC0kCMqe2tvVdUYfidJt0fj3ihC4NxiVHv8GVYxJA==" /> <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
.
2.3 Relying on Record Identification
The Article model is directly available to users of the application, so - following the best practices for developing with Rails - you should declare it a resource:
resources :articles
Declaring a resource has a number of side effects. See Rails Routing from the Outside In guide for more information on setting up and using resources.
When dealing with RESTful resources, calls to form_with
can get significantly easier if you rely on record identification. In short, you can just pass the model instance and have Rails figure out model name and the rest:
## Creating a new article # long-style: form_with(model: @article, url: articles_path) # short-style: form_with(model: @article) ## Editing an existing article # long-style: form_with(model: @article, url: article_path(@article), method: "patch") # short-style: form_with(model: @article)
Notice how the short-style 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.
When you're using STI (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.
2.3.1 Dealing with Namespaces
If you have created namespaced routes, form_with
has a nifty shorthand for that too. If your application has an admin namespace then
form_with model: [:admin, @article]
will create a form that submits to the ArticlesController
inside the admin namespace (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 Rails Routing from the Outside In guide.
2.4 How do forms with PATCH, PUT, or DELETE methods work?
The Rails framework encourages RESTful design of your applications, which means you'll be making a lot of "PATCH", "PUT", and "DELETE" requests (besides "GET" and "POST"). However, most browsers don't support methods other than "GET" and "POST" when it comes to submitting forms.
Rails works around this issue by emulating other methods over POST with a hidden input named "_method"
, which is set to reflect the desired method:
form_with(url: search_path, method: "patch")
Output:
<form accept-charset="UTF-8" action="/search" data-remote="true" method="post"> <input name="_method" type="hidden" value="patch" /> <input name="authenticity_token" type="hidden" value="f755bb0ed134b76c432144748a6d4b7a7ddf2b71" /> ... </form>
When parsing POSTed data, Rails will take into account the special _method
parameter and act as if the HTTP method was the one specified inside it ("PATCH" in this example).
All forms using form_with
implement remote: true
by default. These forms will submit data using an XHR (Ajax) request. To disable this include local: true
. To dive deeper see Working with JavaScript in Rails guide.
3 Making Select Boxes with Ease
Select boxes in HTML require a significant amount of markup (one OPTION
element for each option to choose from), therefore it makes the most sense for them to be dynamically generated.
Here is what the markup might look like:
<select name="city_id" id="city_id"> <option value="1">Lisbon</option> <option value="2">Madrid</option> <option value="3">Berlin</option> </select>
Here you have a list of cities whose names are presented to the user. Internally the application only wants to handle their IDs so they are used as the options' value attribute. Let's see how Rails can help out here.
3.1 The Select and Option Tags
The most generic helper is select_tag
, which - as the name implies - simply generates the SELECT
tag that encapsulates an options string:
<%= select_tag(:city_id, raw('<option value="1">Lisbon</option><option value="2">Madrid</option><option value="3">Berlin</option>')) %>
This is a start, but it doesn't dynamically create the option tags. You can generate option tags with the options_for_select
helper:
<%= options_for_select([['Lisbon', 1], ['Madrid', 2], ['Berlin', 3]]) %>
Output:
<option value="1">Lisbon</option> <option value="2">Madrid</option> <option value="3">Berlin</option>
The first argument to options_for_select
is a nested array where each element has two elements: option text (city name) and option value (city id). The option value is what will be submitted to your controller. Often this will be the id of a corresponding database object but this does not have to be the case.
Knowing this, you can combine select_tag
and options_for_select
to achieve the desired, complete markup:
<%= select_tag(:city_id, options_for_select(...)) %>
options_for_select
allows you to pre-select an option by passing its value.
<%= options_for_select([['Lisbon', 1], ['Madrid', 2], ['Berlin', 3]], 2) %>
Output:
<option value="1">Lisbon</option> <option value="2" selected="selected">Madrid</option> <option value="3">Berlin</option>
Whenever Rails sees that the internal value of an option being generated matches this value, it will add the selected
attribute to that option.
You can add arbitrary attributes to the options using hashes:
<%= options_for_select( [ ['Lisbon', 1, { 'data-size' => '2.8 million' }], ['Madrid', 2, { 'data-size' => '3.2 million' }], ['Berlin', 3, { 'data-size' => '3.4 million' }] ], 2 ) %>
Output:
<option value="1" data-size="2.8 million">Lisbon</option> <option value="2" selected="selected" data-size="3.2 million">Madrid</option> <option value="3" data-size="3.4 million">Berlin</option>
3.2 Select Boxes for Dealing with Model Objects
In most cases form controls will be tied to a specific model and as you might expect Rails provides helpers tailored for that purpose. Consistent with other form helpers, when dealing with a model object drop the _tag
suffix from select_tag
:
If your controller has defined @person
and that person's city_id is 2:
@person = Person.new(city_id: 2)
<%= select(:person, :city_id, [['Lisbon', 1], ['Madrid', 2], ['Berlin', 3]]) %>
will produce output similar to
<select name="person[city_id]" id="person_city_id"> <option value="1">Lisbon</option> <option value="2" selected="selected">Madrid</option> <option value="3">Berlin</option> </select>
Notice that the third parameter, the options array, is the same kind of argument you pass to options_for_select
. One advantage here is that you don't have to worry about pre-selecting the correct city if the user already has one - Rails will do this for you by reading from the @person.city_id
attribute.
As with other helpers, if you were to use the select
helper on a form builder scoped to the @person
object, the syntax would be:
<%= form_with model: @person do |person_form| %> <%= person_form.select(:city_id, [['Lisbon', 1], ['Madrid', 2], ['Berlin', 3]]) %> <% end %>
You can also pass a block to select
helper:
<%= form_with model: @person do |person_form| %> <%= person_form.select(:city_id) do %> <% [['Lisbon', 1], ['Madrid', 2], ['Berlin', 3]].each do |c| %> <%= content_tag(:option, c.first, value: c.last) %> <% end %> <% end %> <% end %>
If you are using select
or similar helpers to set a belongs_to
association you must pass the name of the foreign key (in the example above city_id
), not the name of association itself.
When :include_blank
or :prompt
are not present, :include_blank
is forced true if the select attribute required
is true, display size
is one, and multiple
is not true.
3.3 Option Tags from a Collection of Arbitrary Objects
Generating options tags with options_for_select
requires that you create an array containing the text and value for each option. But what if you had a City
model (perhaps an Active Record one) and you wanted to generate option tags from a collection of those objects? One solution would be to make a nested array by iterating over them:
<% cities_array = City.all.map { |city| [city.name, city.id] } %> <%= options_for_select(cities_array) %>
This is a perfectly valid solution, but Rails provides a less verbose alternative: options_from_collection_for_select
. This helper expects a collection of arbitrary objects and two additional arguments: the names of the methods to read the option value and text from, respectively:
<%= options_from_collection_for_select(City.all, :id, :name) %>
As the name implies, this only generates option tags. To generate a working select box you would need to use collection_select
:
<%= collection_select(:person, :city_id, City.all, :id, :name) %>
As with other helpers, if you were to use the collection_select
helper on a form builder scoped to the @person
object, the syntax would be:
<%= form_with model: @person do |person_form| %> <%= person_form.collection_select(:city_id, City.all, :id, :name) %> <% end %>
Pairs passed to options_for_select
should have the text first and the value second, however with options_from_collection_for_select
should have the value method first and the text method second.
3.4 Time Zone and Country Select
To leverage time zone support in Rails, you have to ask your users what time zone they are in. Doing so would require generating select options from a list of pre-defined ActiveSupport::TimeZone
objects using collection_select
, but you can simply use the time_zone_select
helper that already wraps this:
<%= time_zone_select(:person, :time_zone) %>
There is also time_zone_options_for_select
helper for a more manual (therefore more customizable) way of doing this. Read the API documentation to learn about the possible arguments for these two methods.
Rails used to have a country_select
helper for choosing countries, but this has been extracted to the country_select plugin.
4 Using Date and Time Form Helpers
You can choose not to use the form helpers generating HTML5 date and time input fields and use the alternative date and time helpers. These date and time helpers differ from all the other form helpers in two important respects:
- Dates and times are not representable by a single input element. Instead, you have several, one for each component (year, month, day etc.) and so there is no single value in your
params
hash with your date or time. - Other helpers use the
_tag
suffix to indicate whether a helper is a barebones helper or one that operates on model objects. With dates and times,select_date
,select_time
andselect_datetime
are the barebones helpers,date_select
,time_select
anddatetime_select
are the equivalent model object helpers.
Both of these families of helpers will create a series of select boxes for the different components (year, month, day etc.).
4.1 Barebones Helpers
The select_*
family of helpers take as their first argument an instance of Date
, Time
, or DateTime
that is used as the currently selected value. You may omit this parameter, in which case the current date is used. For example:
<%= select_date Date.today, prefix: :start_date %>
outputs (with actual option values omitted for brevity)
<select id="start_date_year" name="start_date[year]"> </select> <select id="start_date_month" name="start_date[month]"> </select> <select id="start_date_day" name="start_date[day]"> </select>
The above inputs would result in params[:start_date]
being a hash with keys :year
, :month
, :day
. To get an actual Date
, Time
, or DateTime
object you would have to extract these values and pass them to the appropriate constructor, for example:
Date.civil(params[:start_date][:year].to_i, params[:start_date][:month].to_i, params[:start_date][:day].to_i)
The :prefix
option is the key used to retrieve the hash of date components from the params
hash. Here it was set to start_date
, if omitted it will default to date
.
4.2 Model Object Helpers
select_date
does not work well with forms that update or create Active Record objects as Active Record expects each element of the params
hash to correspond to one attribute.
The model object helpers for dates and times submit parameters with special names; when Active Record sees parameters with such names it knows they must be combined with the other parameters and given to a constructor appropriate to the column type. For example:
<%= date_select :person, :birth_date %>
outputs (with actual option values omitted for brevity)
<select id="person_birth_date_1i" name="person[birth_date(1i)]"> </select> <select id="person_birth_date_2i" name="person[birth_date(2i)]"> </select> <select id="person_birth_date_3i" name="person[birth_date(3i)]"> </select>
which results in a params
hash like
{'person' => {'birth_date(1i)' => '2008', 'birth_date(2i)' => '11', 'birth_date(3i)' => '22'}}
When this is passed to Person.new
(or update
), Active Record spots that these parameters should all be used to construct the birth_date
attribute and uses the suffixed information to determine in which order it should pass these parameters to functions such as Date.civil
.
4.3 Common Options
Both families of helpers use the same core set of functions to generate the individual select tags and so both accept largely the same options. In particular, by default Rails will generate year options 5 years either side of the current year. If this is not an appropriate range, the :start_year
and :end_year
options override this. For an exhaustive list of the available options, refer to the API documentation.
As a rule of thumb you should be using date_select
when working with model objects and select_date
in other cases, such as a search form which filters results by date.
4.4 Individual Components
Occasionally you need to display just a single date component such as a year or a month. Rails provides a series of helpers for this, one for each component select_year
, select_month
, select_day
, select_hour
, select_minute
, select_second
. These helpers are fairly straightforward. By default they will generate an input field named after the time component (for example, "year" for select_year
, "month" for select_month
etc.) although this can be overridden with the :field_name
option. The :prefix
option works in the same way that it does for select_date
and select_time
and has the same default value.
The first parameter specifies which value should be selected and can either be an instance of a Date
, Time
, or DateTime
, in which case the relevant component will be extracted, or a numerical value. For example:
<%= select_year(2009) %> <%= select_year(Time.new(2009)) %>
will produce the same output and the value chosen by the user can be retrieved by params[:date][:year]
.
5 Uploading Files
A common task is uploading some sort of file, whether it's a picture of a person or a CSV file containing data to process. The most important thing to remember with file uploads is that the rendered form's enctype attribute must be set to "multipart/form-data". If you use form_with
with :model
, this is done automatically. If you use form_with
without :model
, you must set it yourself, as per the following example.
The following two forms both upload a file.
<%= form_with(url: {action: :upload}, multipart: true) do %> <%= file_field_tag 'picture' %> <% end %> <%= form_with model: @person do |f| %> <%= f.file_field :picture %> <% end %>
Rails provides the usual pair of helpers: the barebones file_field_tag
and the model oriented file_field
. As you would expect in the first case the uploaded file is in params[:picture]
and in the second case in params[:person][:picture]
.
5.1 What Gets Uploaded
The object in the params
hash is an instance of ActionDispatch::Http::UploadedFile
. The following snippet saves the uploaded file in #{Rails.root}/public/uploads
under the same name as the original file.
def upload uploaded_file = params[:picture] File.open(Rails.root.join('public', 'uploads', uploaded_file.original_filename), 'wb') do |file| file.write(uploaded_file.read) end end
Once a file has been uploaded, there are a multitude of potential tasks, ranging from where to store the files (on Disk, Amazon S3, etc), associating them with models, resizing image files, and generating thumbnails, etc. Active Storage is designed to assist with these tasks.
6 Customizing Form Builders
The object yielded by form_with
and fields_for
is an instance of ActionView::Helpers::FormBuilder
. Form builders encapsulate the notion of displaying form elements for a single object. While you can write helpers for your forms in the usual way, you can also create subclass ActionView::Helpers::FormBuilder
and add the helpers there. For example:
<%= form_with model: @person do |f| %> <%= text_field_with_label f, :first_name %> <% end %>
can be replaced with
<%= form_with model: @person, builder: LabellingFormBuilder do |f| %> <%= f.text_field :first_name %> <% end %>
by defining a LabellingFormBuilder
class similar to the following:
class LabellingFormBuilder < ActionView::Helpers::FormBuilder def text_field(attribute, options={}) label(attribute) + super end end
If you reuse this frequently you could define a labeled_form_with
helper that automatically applies the builder: LabellingFormBuilder
option:
def labeled_form_with(model: nil, scope: nil, url: nil, format: nil, **options, &block) options.merge! builder: LabellingFormBuilder form_with model: model, scope: scope, url: url, format: format, **options, &block end
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.
7 Understanding Parameter Naming Conventions
Values from forms 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]
would usually be a hash of all the attributes for the person to create. The params
hash can also contain arrays, arrays of hashes, and so on.
Fundamentally HTML forms don't know about any sort of structured data, all they generate is name-value pairs, where pairs are just plain strings. The arrays and hashes you see in your application are the result of some parameter naming conventions that Rails uses.
7.1 Basic Structures
The two basic structures 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"/>
will result in the params
hash being
{'person' => {'address' => {'city' => 'New York'}}}
Normally Rails ignores duplicate parameter names. If the parameter name contains an empty set of square brackets []
then they will be accumulated in an array. If you wanted 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 inputted phone numbers.
7.2 Combining Them
We can mix and match these two concepts. One element of a hash might be an array as in the previous example, or you can have an array of hashes. For example, a form might let you 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 with keys line1
, line2
, and city
.
There's a restriction, however, while hashes can be nested arbitrarily, only one level of "arrayness" is allowed. Arrays can usually be replaced by hashes; for example, instead of having an array of model objects, one can have a hash of model objects keyed by their id, an array index, or some other parameter.
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 and if it is checked then both are submitted but the value submitted by the checkbox takes precedence.
7.3 Using Form Helpers
The previous sections did not use the Rails form helpers at all. While you can craft the input names yourself and pass them directly to helpers such as text_field_tag
Rails also provides higher level support. The two tools at your disposal here are the name parameter to form_with
and fields_for
and the :index
option that helpers take.
You might want to render a form with a set of edit fields for each of a person's addresses. For example:
<%= 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 had two addresses, with ids 23 and 45 this would create output similar to this:
<form accept-charset="UTF-8" action="/people/1" data-remote="true" 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>
This will result in a params
hash that looks like
{'person' => {'name' => 'Bob', 'address' => {'23' => {'city' => 'Paris'}, '45' => {'city' => 'London'}}}}
Rails knows that all these inputs should be part of the person hash because you
called fields_for
on the first form builder. By specifying an :index
option
you're telling Rails that instead of naming the inputs person[address][city]
it should insert that index surrounded by [] between the address and the city.
This is often useful as it is then easy to locate which Address record
should be modified. You can pass numbers with some other significance,
strings or even nil
(which will result in an array parameter being created).
To create more intricate nestings, you can specify the first part of the input
name (person[address]
in the previous example) explicitly:
<%= fields_for 'person[address][primary]', address, index: address.id do |address_form| %> <%= address_form.text_field :city %> <% end %>
will create inputs like
<input id="person_address_primary_1_city" name="person[address][primary][1][city]" type="text" value="Bologna" />
As a general rule the final input name is the concatenation of the name given to fields_for
/form_with
, the index value, and the name of the attribute. You can also pass an :index
option directly to helpers such as text_field
, but it is usually less repetitive to specify this at the form builder level rather than on individual input controls.
As a shortcut you can append [] to the name and omit the :index
option. This is the same as specifying index: address.id
so
<%= fields_for 'person[address][primary][]', address do |address_form| %> <%= address_form.text_field :city %> <% end %>
produces exactly the same output as the previous example.
8 Forms to External Resources
Rails' form helpers can also be used to build a form for posting data to an external resource. However, at times it can be necessary to set an authenticity_token
for the resource; this can be done by passing an authenticity_token: 'your_external_token'
parameter to the form_with
options:
<%= form_with url: 'http://farfar.away/form', authenticity_token: 'external_token' do %> Form contents <% end %>
Sometimes when submitting data to an external resource, like a payment gateway, 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, simply pass false
to the :authenticity_token
option:
<%= form_with url: 'http://farfar.away/form', authenticity_token: false do %> Form contents <% end %>
9 Building Complex Forms
Many apps grow beyond simple forms editing a single object. For example, when creating a Person
you might want to allow the user to (on the same form) create multiple address records (home, work, etc.). When later editing that person the user should be able to add, remove, or amend addresses as necessary.
9.1 Configuring the Model
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 (optionally) destroy addresses.
9.2 Nested Forms
The following form allows a user to create a Person
and its associated addresses.
<%= form_with model: @person do |f| %> Addresses: <ul> <%= f.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.
def new @person = Person.new 2.times { @person.addresses.build } end
The fields_for
yields a form builder. The parameters' name will be what
accepts_nested_attributes_for
expects. For example, when creating a user with
2 addresses, the submitted parameters would look like:
{ 'person' => { 'name' => 'John Doe', 'addresses_attributes' => { '0' => { 'kind' => 'Home', 'street' => '221b Baker Street' }, '1' => { 'kind' => 'Office', 'street' => '31 Spooner Street' } } } }
The keys of the :addresses_attributes
hash are unimportant, they need merely be 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
.
9.3 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 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
(eg. 1, '1', true, or 'true') then the object will be destroyed.
This form allows users to remove addresses:
<%= form_with model: @person do |f| %> Addresses: <ul> <%= f.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 %>
Don't forget to update the permitted params in your controller to also 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 false
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
.
9.6 Adding Fields on the Fly
Rather than rendering multiple sets of fields ahead of time you may wish to add them only when a user clicks on an 'Add new address' button. Rails does not provide any built-in support for this. When generating new sets of fields you must ensure the key of the associated array is unique - the current JavaScript date (milliseconds since the epoch) is a common choice.
10 Using form_for and form_tag
Before form_with
was introduced in Rails 5.1 its functionality used to be split between form_tag
and form_for
. Both are now soft-deprecated. Documentation on their usage can be found in older versions of this guide.
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