Reform: Options API

Last updated 05 May 2017 reform v2.2

This document describes available options for Reform’s declarative API.

Disposable API

Every Reform form object inherits from Disposable::Twin, making every form a twin and giving each form the entire twin API such as.

  • Defaults using :default.
  • Coercion using :type and :nilify.
  • Nesting
  • Composition
  • Hash fields

If you’re looking for a specific feature, make sure to check the Disposable documentation

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.


Often, fields like password_confirmation should neither be read from nor written back to the model. Reform comes with the :virtual option to handle that case.

class PasswordForm < Reform::Form
  property :password
  property :password_confirmation, virtual: true

Here, the model won’t be queried for a password_confirmation field when creating and rendering the form. 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 manually.

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

form.password_confirmation #=> "321"

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


Use writeable: false to display a value but skip processing it in validate.

property :country, writeable: false
  1. The form will invoke to read the initial value.
  2. It will invoke in validate.
  3. The model’s setter won’t be called in sync.

Non-writeable values are still readable in the nested hash and through the form itself. do |nested|
  nested[:country] #=> "Australia"


Use readable: false to hide a value but still write it to the model.

property :credit_card_number, readable: false
  1. The form won’t invoke model.credit_card_number and will display an empty field.
  2. In validate, the form calls form.credit_card_number=.
  3. In sync, the setter model.credit_card_number= is called and the value written to the database.

Access Protection

Use parse: false to protect the form setters from being called in validate.

property :uuid, parse: false
  1. This will call model.uuid to display the field via the form.
  2. In validate, the form’s setter won’t be called, leaving the value as it is.
  3. In sync, the setter model.uuid is called and restored to the original value.

Note that the :parse option works by leveraging :deserializer.


Incoming form data often needs conversion to a specific type, like timestamps. Reform uses dry-types for coercion. The DSL is seamlessly integrated with the :type option.

Be sure to add dry-types to your Gemfile when requiring coercion.

gem "dry-types"

To use coercion, you need to include the Coercion module into your form class.

require "reform/form/coercion"

class SongForm < Reform::Form
  feature Coercion

  property :written_at, type: Types::Form::DateTime

form.validate("written_at" => "26 September")

Coercion only happens in #validate, not during construction.

form.written_at #=> <DateTime "2014 September 26 00:00">

Available coercion types are documented here.

Manual Coercion

To filter values manually, you can override the setter in the form.

class SongForm < Reform::Form
  property :title

  def title=(value)
    super sanitize(value) # value is raw form input.

Again, setters are only called in validate, not during construction.


A form object is just a twin. In validate, a representer is used to deserialize the incoming hash and populate the form twin graph. This means, you can use any representer you like and process data like JSON or XML, too.


When deserializing the incoming input in validate, advanced logic might be necessary to find nested objects from the database, or populate the form with arbitrary nested objects.

The :populator and its short-hand :populate_if_empty options provide custom deserialization logic and are documented here.


Forms can be derived from other forms and will inherit all properties and validations.

class AlbumForm < Reform::Form
  property :title

  collection :songs do
    property :title

    validates :title, presence: true

Now, a simple inheritance can add fields.

class CompilationForm < AlbumForm
  property :composers do
    property :name

This will add composers to the existing fields.

You can also partially override fields using :inherit.

class CompilationForm < AlbumForm
  property :songs, inherit: true do
    property :band_id
    validates :band_id, presence: true

Using inherit: here will extend the existing songs form with the band_id field. Note that this simply uses representable’s inheritance mechanism.


Use :skip_if to ignore properties in #validate.

property :hit, skip_if: lambda { |fragment, *| fragment["title"].blank? }

This works for both properties and entire nested forms. The property will simply be ignored when deserializing, as if it had never been in the incoming hash/document.

For nested properties you can use :skip_if: :all_blank as a macro to ignore a nested form if all values are blank.

Note that this still runs validations for the property.

Multiparameter Dates

Composed multi-parameter dates as created by the Rails date helper are processed automatically when multi_params: true is set for the date property and the MultiParameterAttributes feature is included. As soon as Reform detects an incoming release_date(i1) or the like it is gonna be converted into a date.

class AlbumForm < Reform::Form
  feature Reform::Form::ActiveModel::FormBuilderMethods
  feature Reform::Form::MultiParameterAttributes

  collection :songs do
    feature Reform::Form::ActiveModel::FormBuilderMethods
    property :title
    property :release_date, :multi_params => true
    validates :title, :presence => true

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