Decouple your models from forms. Reform gives you a form object with validations and nested setup of models. It is completely framework-agnostic and doesn't care about your database.

Although reform can be used in any Ruby framework, it comes with Rails support, works with simple_form and other form gems, allows nesting forms to implement has_one and has_many relationships, can compose a form from multiple objects and gives you coercion.


Add this line to your Gemfile:

gem 'reform'


Reform comes with two base classes.

  • Form is what made you come here - it gives you a form class to handle all validations, wrap models, allow rendering with Rails form helpers, simplifies saving of models, and more.
  • Contract gives you a sub-set of Form: this class is meant for API validation where already populated models get validated without having to maintain validations in the model classes.

Defining Forms

You're working at a famous record label and your job is archiving all the songs, albums and artists. You start with a form to populate your songs table.

class SongForm < Reform::Form
  property :title
  property :length

  validates :title,  presence: true
  validates :length, numericality: true

To add fields to the form use the ::property method. Also, validations no longer go into the model but sit in the form.


Forms have a ridiculously simple API with only a handful of public methods.

  1. #initialize always requires a model that the form represents.
  2. #validate(params) updates the form's fields with the input data (only the form, not the model) and then runs all validations. The return value is the boolean result of the validations.
  3. #errors returns validation messages in a classy ActiveModel style.
  4. #sync writes form data back to the model. This will only use setter methods on the model(s).
  5. #save (optional) will call #save on the model and nested models. Note that this implies a #sync call.

In addition to the main API, forms expose accessors to the defined properties. This is used for rendering or manual operations.


In your controller you'd create a form instance and pass in the models you want to work on.

class SongsController
  def new
    @form =

You can also setup the form for editing existing items.

class SongsController
  def edit
    @form =

Reform will read property values from the model in setup. Given the following form class.

class SongForm < Reform::Form
  property :title

Internally, this form will call song.title to populate the title field.

If you, for whatever reasons, want to use a different public name, use :as.

class SongForm < Reform::Form
  property :name, as: :title

This will still call song.title but expose the attribute as name.

Rendering Forms

Your @form is now ready to be rendered, either do it yourself or use something like Rails' #form_for, simple_form or formtastic.

= form_for @form do |f|

  = f.input :name
  = f.input :title

Nested forms and collections can be easily rendered with fields_for, etc. Just use Reform as if it would be an ActiveModel instance in the view layer.


After a form submission, you want to validate the input.

class SongsController
  def create
    @form =

    #=> params: {song: {title: "Rio", length: "366"}}

    if @form.validate(params[:song])

The #validate method first updates the values of the form - the underlying model is still treated as immutuable and remains unchanged. It then runs all validations you provided in the form.

It's the only entry point for updating the form. This is per design, as separating writing and validation doesn't make sense for a form.

This allows rendering the form after validate with the data that has been submitted. However, don't get confused, the model's values are still the old, original values and are only changed after a #save or #sync operation.

Syncing Back

After validation, you have two choices: either call #save and let Reform sort out the rest. Or call #sync, which will write all the properties back to the model. In a nested form, this works recursively, of course.

It's then up to you what to do with the updated models - they're still unsaved.

Saving Forms

The easiest way to save the data is to call #save on the form.  #=> populates song with incoming data
                #   by calling and

This will sync the data to the model and then call

Sometimes, you need to do stuff manually.

Saving Forms Manually

This is where you call #save with a block. This won't touch the models at all but give you a nice hash, so you can do it yourself.

Note that you can call #sync and then call #save with the block to save models yourself.

  if @form.validate(params[:song]) do |data, nested|
      data.title  #=> "Rio"
      data.length #=> "366"

      nested      #=> {title: "Rio", length: "366"}


While data gives you an object exposing the form property readers, nested is a hash reflecting the nesting structure of your form. Note how you can use arbitrary code to create/update models - in this example, we used Song::create.


Contracts give you a sub-set of the Form API.

  1. #initialize accepts an already populated model.
  2. #validate will run defined validations (without accepting a params hash as in Form).

Contracts can be used to completely remove validation logic from your model classes. Validation should happen in a separate layer - a Contract.

Defining Contracts

A contract looks like a form.

class AlbumContract < Reform::Contract
  property :title
  validates :title, length: {minimum: 9}

  collection :songs do
    property :title
    validates :title, presence: true

It defines the validations and the object graph to be inspected.

In future versions and with the upcoming Trailblazer framework, contracts can be inherited from forms, representers, and cells, and vice-versa. Actually this already works with representer inheritance - let me know if you need help.

Using Contracts

Applying a contract is simple, all you need is a populated object (e.g. an album after #update_attributes).


  raise album.errors.messages.inspect

Contracts help you to make your data layer a dumb persistance tier. My upcoming book discusses that in detail.

Nesting Forms: 1-1 Relations

Songs have artists to compose them. Let's say your Song model would implement that as follows.

class Song < ActiveRecord::Base
  has_one :artist

The edit form should allow changing data for artist and song.

class SongForm < Reform::Form
  property :title
  property :length

  property :artist do
    property :name

    validates :name, presence: true

  #validates :title, ...

See how simple nesting forms is? By passing a block to ::property you can define another form nested into your main form.

has_one: Setup

This setup's only requirement is having a working Song#artist reader.

class SongsController
  def edit
    song = Song.find(1)
    song.artist #=> <0x999#Artist title="Duran Duran">

    @form =

has_one: Rendering

When rendering this form you could use the form's accessors manually.

= text_field :title,         @form.title
= text_field "artist[name]",

Or use something like #fields_for in a Rails environment.

= form_for @form |f|
  = f.text_field :title
  = f.text_field :length

  = f.fields_for :artist do |a|
    = a.text_field :name

has_one: Processing

The block form of #save would give you the following data. do |data, nested|
  data.title #=> "Hungry Like The Wolf" #=> "Duran Duran"

  nested #=> {title:  "Hungry Like The Wolf",
         #    artist: {name: "Duran Duran"}}

Supposed you use reform's automatic save without a block, the following assignments would be made.       = "Hungry Like The Wolf" = "Duran Duran"

Nesting Forms: 1-n Relations

Reform also gives you nested collections.

Let's have Albums with songs!

class Album < ActiveRecord::Base
  has_many :songs

The form might look like this.

class AlbumForm < Reform::Form
  property :title

  collection :songs do
    property :title

    validates :title, presence: true

This basically works like a nested property that iterates over a collection of songs.

has_many: Rendering

Reform will expose the collection using the #songs method.

= text_field :title,         @form.title
= text_field "songs[0][title]", @form.songs[0].title

However, #fields_for works just fine, again.

= form_for @form |f|
  = f.text_field :title

  = f.fields_for :songs do |s|
    = s.text_field :title

has_many: Processing

The block form of #save will expose the data structures already discussed. do |data, nested|
  data.title #=> "Rio"
  data.songs.first.title #=> "Hungry Like The Wolf"

  nested #=> {title: "Rio"
         #   songs: [{title: "Hungry Like The Wolf"},
         #          {title: "Last Chance On The Stairways"}]

Nesting Configuration

Turning Off Autosave

You can assign Reform to not call save on a particular nested model (per default, it is called automatically on all nested models).

class AlbumForm < Reform::Form
  # ...

  collection :songs, save: false do
    # ..

The :save options set to false won't save models.

Populating Forms For Validation

With a complex nested setup it can sometimes be painful to setup the model object graph.

Let's assume you rendered the following form.

@form = [,]))

This will render two nested forms to create new songs.

When validating, you're supposed to setup the very same object graph, again. Reform has no way of remembering what the object setup was like a request ago.

So, the following code will fail.

@form =[:album])

However, you can advise Reform to setup the correct objects for you.

class AlbumForm < Reform::Form
  # ...

  collection :songs, populate_if_empty: Song do
    # ..

This works for both property and collection and instantiates Song objects where they're missing when calling #validate.

If you want to create the objects yourself, because you're smarter than Reform, do it with a lambda.

class AlbumForm < Reform::Form
  # ...

  collection :songs, populate_if_empty: lambda { |fragment, args| } do
    # ..


Sometimes you might want to embrace two (or more) unrelated objects with a single form. While you could write a simple delegating composition yourself, reform comes with it built-in.

Say we were to edit a song and the label data the record was released from. Internally, this would imply working on the songs table and the labels table.

class SongWithLabelForm < Reform::Form
  include Composition

  property :title, on: :song
  property :city,  on: :label

  model :song # only needed in ActiveModel context.

  validates :title, :city, presence: true

Note that reform needs to know about the owner objects of properties. You can do so by using the on: option.

Also, the form needs to have a main object configured. This is where ActiveModel-methods like #persisted? or '#id' are delegated to. Use ::model to define the main object.

Composition: Setup

The constructor slightly differs.

@form =, label:

Composition: Rendering

After you configured your composition in the form, reform hides the fact that you're actually showing two different objects.

= form_for @form do |f|

  Song:     = f.input :title

  Label in: = f.input :city

Composition: Processing

When using `#save' without a block reform will use writer methods on the different objects to push validated data to the properties.

Here's how the block parameters look like. do |data, nested|
  data.title #=> "Rio"  #=> "London"

  nested #=> {
         #   song:  {title: "Rio"}
         #   label: {city: "London"}
         #   }


Often you want incoming form data to be converted to a type, like timestamps. Reform uses virtus for coercion, the DSL is seamlessly integrated into Reform with the :type option.

Be sure to add virtus to your Gemfile.

require 'reform/form/coercion'

class SongForm < Reform::Form
  include Coercion

  property :written_at, type: DateTime
end do |data, nested|
  data.written_at #=> <DateTime XXX>

Virtual Attributes

Virtual fields come in handy when there's no direct mapping to a model attribute or when you plan on displaying but not processing a value.

Empty Fields

Often, fields like password_confirmation shouldn't be retrieved from the model. Reform comes with the :empty option for that.

class PasswordForm < Reform::Form
  property :password
  property :password_confirmation, :empty => true

Here, the model won't be queried for a password_confirmation field when creating and rendering the form. Likewise, when saving the form, the input value is not written to the decorated model. It is only readable in validations and when saving the form.

form.validate("password" => "123", "password_confirmation" => "321")

form.password_confirmation #=> "321"

The nested hash in the block-#save provides the same value. do |f, nested|
  nested[:password_confirmation] #=> "321"

Read-Only Fields

Almost identical, the :virtual option makes fields read-only. Say you want to show a value, but not process it after submission, this option is your friend.

class ProfileForm < Reform::Form
  property :country, :virtual => true

This time reform will query the model for the value by calling

You want to use this to display an initial value or to further process this field with JavaScript. However, after submission, the field is no longer considered: it won't be written to the model when saving.

It is still readable in the nested hash and through the form itself. do |f, nested|
  nested[:country] #=> "Australia" #=> "Australia"

Agnosticism: Mapping Data

Reform doesn't really know whether it's working with a PORO, an ActiveRecord instance or a Sequel row.

When rendering the form, reform calls readers on the decorated model to retrieve the field data (Song#title, Song#length).

When syncing a submitted form, the same happens using writers. Reform simply calls Song#title=(value). No knowledge is required about the underlying database layer.

The same applies to saving: Reform will call #save on the main model and nested models.

Nesting forms only requires readers for the nested properties as Album#songs.

Rails Integration

Check out @gogogarret's sample Rails app using Reform.

Rails and Reform work together out-of-the-box.

However, you should know about two things.

  1. In case you explicitely don't want to have automatic support for ActiveRecord and form builder: require reform/form, only.
  2. In some setups around Rails 4 the Form::ActiveRecord module is not loaded properly, usually triggering a NoMethodError saying undefined method 'model'. If that happened to you, require 'reform/rails' manually at the bottom of your config/application.rb.

ActiveRecord Compatibility

Reform provides the following ActiveRecord specific features. They're mixed in automatically in a Rails/AR setup.

  • Uniqueness validations. Use validates_uniqueness_of in your form.

As mentioned in the Rails Integration section some Rails 4 setups do not properly load.

You may want to include the module manually then.

class SongForm < Reform::Form
  include Reform::Form::ActiveRecord

ActiveModel Compliance

Forms in Reform can easily be made ActiveModel-compliant.

Note that this step is not necessary in a Rails environment.

class SongForm < Reform::Form
  include Reform::Form::ActiveModel

If you're not happy with the model_name result, configure it manually.

class CoverSongForm < Reform::Form
  include Reform::Form::ActiveModel

  model :song

This is especially helpful when your framework tries to render cover_song_path although you want to go with song_path.

FormBuilder Support

To make your forms work with all the form gems like simple_form or Rails form_for you need to include another module.

Again, this step is implicit in Rails and you don't need to do it manually.

class SongForm < Reform::Form
  include Reform::Form::ActiveModel
  include Reform::Form::ActiveModel::FormBuilderMethods

Multiparameter Dates

Composed multi-parameter dates as created by the Rails date helper are processed automatically. As soon as Reform detects an incoming release_date(i1) or the like it is gonna be converted into a date.

Note that the date will be nil when one of the components (year/month/day) is missing.


By explicitely defining the form layout using ::property there is no more need for protecting from unwanted input. strong_parameter or attr_accessible become obsolete. Reform will simply ignore undefined incoming parameters.

Additional Features

Nesting Without Inline Representers

When nesting form, you usually use a so-called inline form doing property :song do .. end.

Sometimes you want to specify an explicit form rather than using an inline form. Use the form: option here.

property :song, form: SongForm

The nested SongForm is a stand-alone form class you have to provide.

Overriding Accessors

When "real" coercion is too much and you simply want to convert incoming data yourself, override the setter.

class SongForm < Reform::Form
  property :title

  def title=(v)

This will capitalize the title after calling form.validate but before validation happens. Note that you can use super to call the original setter.

Undocumented Features

(Please don't read this section!)


You can run your very own populator logic if you're keen (and you know what you're doing).

class AlbumForm < Reform::Form
  # ...

  collection :songs, populator: lambda { |fragment, args| args.binding[:form].new(Song.find fragment[:id]) } do
    # ..


If you run into any trouble chat with us on


Nick Sutterer

Garrett Heinlen


Great thanks to Blake Education for giving us the freedom and time to develop this project in 2013 while working on their project.