The most basic form helper is form_tag.

When called without arguments like this, it creates a form element that has the current page for action attribute and "POST" as method (some line breaks added for readability):

If you carefully observe this output, you can see that the helper generated something you didn’t specify: a div element with a hidden input inside. This is a security feature of Rails called cross-site request forgery protection and form helpers generate it for every form which action isn’t "GET" (provided that this security feature is enabled).

Probably the most minimal form often seen on the web is a search form with a single text input for search terms. This form consists of:

To create that, you will use form_tag, label_tag, text_field_tag and submit_tag, respectively.

The above view code will result in the following markup:

Besides text_field_tag and submit_tag, there is a similar helper for every form control in HTML.

By now you’ve seen that the form_tag helper accepts 2 arguments: the path for the action and an options hash. This hash specifies the method of form submission and HTML options such as the form element’s class.

As with the ‘link_to` helper, the path argument doesn’t have to be given a string. It can be a hash of URL parameters that Rails’ routing mechanism will turn into a valid URL. Still, you cannot simply write this:

Here you wanted to pass two hashes, but the Ruby interpreter sees only one hash, so Rails will construct a URL with extraneous parameters. The solution is to delimit the first hash (or both hashes) with curly brackets:

This is a common pitfall when using form helpers, since many of them accept multiple hashes. So in future, if a helper produces unexpected output, make sure that you have delimited the hash parameters properly.

Checkboxes are form controls that give the user a set of options they can enable or disable:

Radio buttons, while similar to checkboxes, are controls that specify a set of options in which they are mutually exclusive (user can only pick one):

Other form controls worth mentioning are the text area, password input and hidden input:

Hidden inputs are not shown to the user, but they hold data same as any textual input. Values inside them can be changed with JavaScript.

Rails framework encourages RESTful design of your applications, which means you’ll be making a lot of "PUT" and "DELETE" requests (besides "GET" and "POST"). Still, most browsers don’t support methods other than "GET" and "POST" when it comes to submitting forms. How does this work, then?

Rails works around this issue by emulating other methods over POST with a hidden input named "_method" that is set to reflect the desired method:

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 ("PUT" in this example).

Most of Rails' form helpers are available in two forms.

These just generate the appropriate markup. These have names ending in _tag such as text_field_tag, check_box_tag. The first parameter to these is always the name of the input. This is the name under which value will appear in the params hash in the controller. For example if the form contains

then the controller code should use

to retrieve the value entered by the user. When naming inputs be aware that Rails uses certain conventions that control whether values appear at the top level of the params hash, inside an array or a nested hash and so on. You can read more about them in the parameter names section. For details on the precise usage of these helpers, please refer to the API documentation.

These are designed to work with a model object (commonly an Active Record object but this need not be the case). These lack the _tag suffix, for example text_field, text_area.

For these helpers the first arguement is the name of an instance variable and the second is the name 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:

will produce output similar to

Upon form submission the value entered by the user will be stored in params[:person][:name]. The params[:person] hash is suitable for passing to Person.new or, if @person is an instance of Person, @person.update_attributes.

While the helpers seen so far are handy Rails can save you some work. For example typically a form is used to edit multiple attributes of a single object, so having to repeat the name of the object being edited is clumsy. The following examples will handle an Article model. First, have the controller create one:

Now switch to the view. The first thing to remember is to use the form_for helper instead of form_tag, and that you should pass the model name and object as arguments:

There are a few things to note here:

The resulting HTML is:

The name passed to form_for controls where in the params hash the form values will appear. 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 the parameter names section.

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.

In the previous chapter you handled the Article model. This model is directly available to users of our application, so — following the best practices for developing with Rails — you should declare it a resource.

When dealing with RESTful resources, calls to form_for 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:

Notice how the short-style form_for 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.new_record?. It also selects the correct path to submit to and the name based on the class of the object.

Rails will also automatically set the class and id of the form appropriately: a form creating an article would have id and class new_article. If you were editing the article with id 23 the class would be set to edit_article and the id to edit_article_23. The attributes will be omitted or brevity in the rest of this guide.

If you have created namespaced routes form_for has a nifty shorthand for that too. If your application has an admin namespace then

will create a form that submits to the articles controller 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:

For more information on Rails' routing system and the associated conventions, please see the routing guide.

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 from data stored in arrays or hashes.

Here is what our wanted markup might look like:

Here you have a list of cities where their names are presented to the user, but internally the application only wants to handle their IDs so they are used as the options' value attributes. Let’s see how Rails can help out here.

The most generic helper is select_tag, which — as the name implies — simply generates the SELECT tag that encapsulates an options string:

This is a start, but it doesn’t dynamically create our option tags. You can generate option tags with the options_for_select helper:

For input data you use a nested array where each element has two elements: option text (city name) and option value (city id). The option value is what will get submitted to your controller. It is often true that the option value is 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:

Sometimes, depending on an application’s needs, you also wish a specific option to be pre-selected. The options_for_select helper supports this with an optional second argument:

So whenever Rails sees that the internal value of an option being generated matches this value, it will add the selected attribute to that option.

Until now you’ve seen how to make generic select boxes, but in most cases our form controls will be tied to a specific database model. So, to continue from our previous examples, let’s assume that you have a "Person" model with a city_id attribute.

Consistent with other form helpers, when dealing with models you drop the _tag suffix from select_tag.

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 before, if you were to use select helper on a form builder scoped to @person object, the syntax would be:

Until now you were generating option tags from nested arrays with the help of options_for_select method. Data in our array were raw values:

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:

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:

As the name implies, this only generates option tags. To generate a working select box you would need to use it in conjunction with select_tag, just as you would with options_for_select. A method to go along with it is collection_select:

To recap, options_from_collection_for_select is to collection_select what options_for_select is to select.

To leverage time zone support in Rails, you have to ask our users what time zone they are in. Doing so would require generating select options from a list of pre-defined TimeZone objects using collection_select, but you can simply use the time_zone_select helper that already wraps this:

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. When using this do be aware that the exclusion or inclusion of certain names from the list can be somewhat controversial (and was the reason this functionality was extracted from rails)

The date and time helpers differ from all the other form helpers in two important respects:

Both of these families of helpers will create a series of select boxes for the different components (year, month, day etc...).

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

outputs (with the actual option values omitted for brevity)

The above inputs would result in params[:start_date] being a hash with keys :year, :month, :day. To get an actual Time or Date object you would have to extract these values and pass them to the appropriate constructor, for example

The :prefix option controls where in the params hash the date components will be placed. Here it was set to start_date, if omitted it will default to date.

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

outputs (with the actual option values omitted for brevity)

which results in a params hash like

When this is passed to Person.new, 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.

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 others cases, such as a search form which filters results by date.

As mentioned previously the object yielded by form_for and fields_for is an instance of FormBuilder (or a subclass thereof). Form builders encapsulate the notion of displaying a form elements for a single object. While you can of course write helpers for your forms in the usual way you can also subclass FormBuilder and add the helpers there. For example

can be replaced with

by defining a LabellingFormBuilder class similar to the following:

If you reuse this frequently you could define a labeled_form_for helper that automatically applies the :builder => LabellingFormBuilder option.

The form builder used also determines what happens when you do

If f is an instance of 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.

fields_for creates a form builder in exactly the same way as form_for but doesn’t create the actual <form> tags. It creates a scope around a specific model object like form_for, which is useful for specifying additional model objects in the same form. For example if you had a Person model with an associated ContactDetail model you could create a form for editing both like so:

which produces the following output:

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 form’s encoding MUST be set to multipart/form-data. If you forget to do this the file will not be uploaded. This can be done by passing :multi_part => true as an HTML option. This means that in the case of form_tag it must be passed in the second options hash and in the case of form_for inside the :html hash.

The following two forms both upload a file.

Rails provides the usual pair of helpers: the barebones file_field_tag and the model oriented file_field. The only difference with other helpers is that you cannot set a default value for file inputs as this would have no meaning. As you would expect in the first case the uploaded file is in params[:picture] and in the second case in params[:person][:picture].

The object in the params hash is an instance of a subclass of IO. Depending on the size of the uploaded file it may in fact be a StringIO or an instance of File backed by a temporary file. In both cases the object will have an original_filename attribute containing the name the file had on the user’s computer and a content_type attribute containing the MIME type of the uploaded file. The following snippet saves the uploaded content in #{RAILS_ROOT}/public/uploads under the same name as the original file (assuming the form was the one in the previous example).

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) and associating them with models to resizing image files and generating thumbnails. The intricacies of this are beyond the scope of this guide, but there are several plugins designed to assist with these. Two of the better known ones are Attachment-Fu and Paperclip.

Unlike other forms making an asynchronous file upload form is not as simple as replacing form_for with remote_form_for. With an AJAX form the serialization is done by javascript running inside the browser and since javascript cannot read files from your hard drive the file cannot be uploaded. The most common workaround is to use an invisible iframe that serves as the target for the form submission.

As you’ve seen in the previous sections values from forms can appear either at the top level of the params hash or may appear nested in another hash. For example in a standard create action for a Person model, params[:model] 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 know about is name-value pairs. Rails tacks some conventions onto parameter names which it uses to express some structure.

The two basic structures are arrays and hashes. Hashes mirror the syntax used for accessing the value in the params. For example if a form contains

the params hash will contain

and params["name"] will retrieve the submitted value in the controller.

Hashes can be nested as many levels as required, for example

will result in the params hash being

Normally Rails ignores duplicate parameter names. If the parameter name contains [] then they will be accumulated in an array. If you wanted people to be able to input multiple phone numbers, your could place this in the form:

This would result in params[:person][:phone_number] being an array.

We can mix and match these two concepts. For example, 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

This would result in params[:addresses] being an array of hashes with keys line1, line2 and city. Rails decides to start accumulating values in a new hash whenever it encounters a input name that already exists in the current hash.

The one restriction is that although hashes can be nested arbitrarily deep then can be only one level of "arrayness". Frequently arrays can be usually 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.

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_for/fields_for and the :index option.

You might want to render a form with a set of edit fields for each of a person’s addresses. Something a little like this will do the trick

Assuming our person had two addresses, with ids 23 and 45 this would create output similar to this:

This will result in a params hash that looks like

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. If you pass an Active Record object as we did then Rails will call to_param on it, which by default returns the database id. This is often useful it is then easy to locate which Address record should be modified but you could 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, for example

will create inputs like

As a general rule the final input name is the concatenation of the name given to fields_for/form_for, the index value and the name of the attribute. You can also pass an :index option directly to helpers such as text_field, but usually it is 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 specifing :index => address so

produces exactly the same output as the previous example.

Many apps grow beyond simple forms editing a single object. For example when creating a Person instance 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. While this guide has shown you all the pieces necessary to handle this, Rails does not yet have a standard end-to-end way of accomplishing this, but many have come up with viable approaches. These include:

Lighthouse ticket