Forms in web applications are an essential interface for user input. However, form markup can quickly become tedious to write and maintain because of the need to handle form control naming and its numerous attributes. Rails does away with this complexity by providing view helpers for generating form markup. However, since these helpers have different use cases, developers need to know the differences between the helper methods before putting them to use.
NOTE: This guide is not intended to be a complete documentation of available form helpers and their arguments. Please visit [the Rails API documentation](https://api.rubyonrails.org/) for a complete reference.
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:
You'll notice that the HTML contains an `input` element with type `hidden`. This `input` is important, because non-GET forms 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](security.html#cross-site-request-forgery-csrf) guide.
TIP: 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.
TIP: For every form input, 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.
IMPORTANT: 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.
The form builder object yielded by `form_with` provides numerous helper methods for generating form elements such as text fields, checkboxes, and radio buttons. The first parameter to these methods is always the name of the
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](#understanding-parameter-naming-conventions) of this guide. For details on the precise usage of these helpers, please refer to the [API documentation](https://api.rubyonrails.org/classes/ActionView/Helpers/FormTagHelper.html).
The first parameter to [`check_box`](https://api.rubyonrails.org/classes/ActionView/Helpers/FormBuilder.html#method-i-check_box) is the name of the input. The second parameter 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.
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):
As with `check_box`, the second parameter to [`radio_button`](https://api.rubyonrails.org/classes/ActionView/Helpers/FormBuilder.html#method-i-radio_button) 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"`.
There is definitely [no shortage of solutions for this](https://github.com/Modernizr/Modernizr/wiki/HTML5-Cross-Browser-Polyfills), although a popular tool at the moment is
TIP: 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](security.html#logging) guide.
The `:model` argument of `form_with` allows us 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.
* The form `action` is automatically filled with an appropriate value for `@article`.
* The form fields are automatically filled with the corresponding values from `@article`.
* The form field names are scoped with `article[...]`. This means that `params[:article]` will be a hash containing all these field's values. You can read more about the significance of input names in chapter [Understanding Parameter Naming Conventions](#understanding-parameter-naming-conventions) of this guide.
* The submit button is automatically given an appropriate text value.
TIP: 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]`.
You can create a similar binding without actually creating `<form>` tags with the [`fields_for`](https://api.rubyonrails.org/classes/ActionView/Helpers/FormBuilder.html#method-i-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:
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**:
TIP: Declaring a resource has a number of side effects. See [Rails Routing from the Outside In](routing.html#resource-routing-the-rails-default) 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. In both of these examples, the long and short style have the same outcome:
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.
WARNING: 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.
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:
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.
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).
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][button-value], and [name][button-name] attributes:
IMPORTANT: In Rails 6.0 and 5.2, 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](working_with_javascript_in_rails.html#remote-elements) guide.
Select boxes in HTML require a significant amount of markup - one `<option>` element for each option to choose from. So Rails provides helper methods to reduce this burden.
For example, let's say we have a list of cities for the user to choose from. We can use the [`select`](https://api.rubyonrails.org/classes/ActionView/Helpers/FormBuilder.html#method-i-select) helper like so:
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`:
Notice that the appropriate option was automatically marked `selected="selected"`. Since this select box was bound to a model, we didn't need to specify a `:selected` argument!
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`](https://api.rubyonrails.org/classes/ActiveSupport/TimeZone.html) objects, but you can simply use the [`time_zone_select`](https://api.rubyonrails.org/classes/ActionView/Helpers/FormBuilder.html#method-i-time_zone_select) helper that already wraps this:
Rails _used_ to have a `country_select` helper for choosing countries, but this has been extracted to the [country_select plugin](https://github.com/stefanpenner/country_select).
If you do not wish to use HTML5 date and time inputs, Rails provides alternative date and time form helpers that render plain select boxes. These helpers render a select box for each temporal component (e.g. year, month, day, etc). For example, if we have a `@person` model object like:
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)"`. Active Record knows how to assemble these specially-named values into a full date or time, based on the declared type of the model attribute. So we can pass `params[:person]` to e.g. `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`](https://api.rubyonrails.org/classes/ActionView/Helpers/FormBuilder.html#method-i-date_select) helper, Rails provides [`time_select`](https://api.rubyonrails.org/classes/ActionView/Helpers/FormBuilder.html#method-i-time_select) and [`datetime_select`](https://api.rubyonrails.org/classes/ActionView/Helpers/FormBuilder.html#method-i-datetime_select).
Rails also provides helpers to render select boxes for individual temporal components: [`select_year`](https://api.rubyonrails.org/classes/ActionView/Helpers/DateHelper.html#method-i-select_year), [`select_month`](https://api.rubyonrails.org/classes/ActionView/Helpers/DateHelper.html#method-i-select_month), [`select_day`](https://api.rubyonrails.org/classes/ActionView/Helpers/DateHelper.html#method-i-select_day), [`select_hour`](https://api.rubyonrails.org/classes/ActionView/Helpers/DateHelper.html#method-i-select_hour), [`select_minute`](https://api.rubyonrails.org/classes/ActionView/Helpers/DateHelper.html#method-i-select_minute), and [`select_second`](https://api.rubyonrails.org/classes/ActionView/Helpers/DateHelper.html#method-i-select_second). These helpers are "bare" methods, meaning they are not called on a form builder instance. For example:
For each of these helpers, you may specify a date or time object instead of a number as the default value, and the appropriate temporal component will be extracted and used.
Often, we want to generate a set of choices in a form from a collection of objects. For example, when we want the user to choose from cities in our database, and we have a `City` model like:
Rails provides 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.
To generate a select box for our cities, we can use [`collection_select`](https://api.rubyonrails.org/classes/ActionView/Helpers/FormBuilder.html#method-i-collection_select):
NOTE: 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.
### The `collection_radio_buttons` Helper
To generate a set of radio buttons for our cities, we can use [`collection_radio_buttons`](https://api.rubyonrails.org/classes/ActionView/Helpers/FormBuilder.html#method-i-collection_radio_buttons):
To generate a set of check boxes for our cities (which allows users to choose more than one), we can use [`collection_check_boxes`](https://api.rubyonrails.org/classes/ActionView/Helpers/FormBuilder.html#method-i-collection_check_boxes):
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. File upload fields can be rendered with the [`file_field`](https://api.rubyonrails.org/classes/ActionView/Helpers/FormBuilder.html#method-i-file_field) helper.
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:
Note that, in accordance with `form_with` conventions, the field names in the two forms above will also differ. That is, the field name in the first form will be `person[picture]` (accessible via `params[:person][:picture]`), and the field name in the second form will be just `picture` (accessible via `params[:picture]`).
The object in the `params` hash is an instance of [`ActionDispatch::Http::UploadedFile`](https://api.rubyonrails.org/classes/ActionDispatch/Http/UploadedFile.html). The following snippet saves the uploaded file in `#{Rails.root}/public/uploads` under the same name as the original file.
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](active_storage_overview.html) is designed to assist with these tasks.
The object yielded by `form_with` and `fields_for` is an instance of [`ActionView::Helpers::FormBuilder`](https://api.rubyonrails.org/classes/ActionView/Helpers/FormBuilder.html). 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 a subclass of `ActionView::Helpers::FormBuilder`, and add the helpers there. For example,
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.
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.
Normally Rails ignores duplicate parameter names. If the parameter name ends with 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:
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
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.
WARNING: 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.
Let's say we want to render a form with a set of fields for each of a person's addresses. The `fields_for` helper and its `:index` argument can assist with this:
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.
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:
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:
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.
Active Record provides model level support via the [`accepts_nested_attributes_for`](https://api.rubyonrails.org/classes/ActiveRecord/NestedAttributes/ClassMethods.html#method-i-accepts_nested_attributes_for) method:
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.
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`.
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.
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`.
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](https://en.wikipedia.org/wiki/Unix_time)) is a common choice.
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`](https://api.rubyonrails.org/classes/ActionView/Helpers/FormTagHelper.html#method-i-check_box_tag):
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](https://api.rubyonrails.org/classes/ActionView/Helpers/FormTagHelper.html).
Before `form_with` was introduced in Rails 5.1 its functionality used to be split between [`form_tag`](https://api.rubyonrails.org/v5.2/classes/ActionView/Helpers/FormTagHelper.html#method-i-form_tag) and [`form_for`](https://api.rubyonrails.org/v5.2/classes/ActionView/Helpers/FormHelper.html#method-i-form_for). Both are now soft-deprecated. Documentation on their usage can be found in [older versions of this guide](https://guides.rubyonrails.org/v5.2/form_helpers.html).
