Form Objects in Rails

For a while now I have been using form objects instead of nested attributes for complex forms, and the experience has been pleasant. A form object is an object designed explicitly to back a given form. It handles validation, defaults, casting, and translation of attributes to the persistence layer. A basic example:

class Form::NewRegistration
  include ActiveModel::Validations

  def self.scalar_attributes
    [:name, :age]
  end

  attr_accessor *scalar_attributes
  attr_reader :event

  validates_presence_of :name

  def initialize(event, params = {})
    self.class.scalar_attributes.each do |attr|
      self.send("%s=" % attr, params[attr]) if params.has_key?(attr)
    end
  end

  def create
    return unless valid?

    registration = Registration.create!(
      event: event,
      data_json: {
        name: name,
        age:  age.to_i,
      }.to_json
    )

    registration
  end

  # ActiveModel support
  def self.name; "Registration"; end
  def persisted?; false; end
  def to_key; nil; end
end

Note how this allows an easy mapping from form fields to a serialized JSON blob.

I have found this more explicit and flexible than tying forms directly to nested attributes. It allows more fine tuned control of the form behaviour, is easier to reason about and test, and enables you to refactor your data model with minimal other changes. (In fact, if you are planning on refactoring your data model, adding in a form object as a “shim” to protect other parts of the system from change before you refactor is usually desirable.) It even works well with nested attributes, using the form object to build up the required nested hash in the #create method.

Relationships

A benefit of this approach, albeit still a little clunky, is having accessors map one to one with form fields even for one to many associations. My approach takes advantages of Ruby’s flexible object model to define accessors on the fly. For example, say a registration has multiple custom answer fields, as defined on the event, I would call the following method on initialisation:

def add_answer_accessors!
  event.questions.each do |q|
    attr = :"answer_#{q.id}"
    instance_eval <<-RUBY
      def #{attr};     answers[#{q.id}]; end
      def #{attr}=(x); answers[#{q.id}] = x; end
    RUBY
  end
end

With the exception of the above code (which isn’t too bad), this greatly simplifies typical code for handling one to many relationships: it avoids fields_for, index, and is easier to set up sane defaults for.

Casting

I use a small supporting module to handle casting of attributes to certain types.

module TypedWriter
  def typed_writer(type, attribute)
    class_eval <<-EOS
      def #{attribute}=(x)
        @#{attribute} = type_cast(x, :#{type})
      end
    EOS
  end

  def type_cast(x, type)
    case type
    when :integer
      x.to_s.length > 0 ? x.to_i : nil
    when :boolean
      x.to_s.length > 0 ? x == true || x == "true" : nil
    when :boolean_with_nil
      if x.to_s == 'on' || x.nil?
        nil
      else
        x.to_s.length > 0 ? x == true || x == "true" : nil
      end
    when :int_array
      [*x].map(&:to_i).select {|x| x > 0 }
    else
      raise "Unknown type #{type}"
    end
  end

  def self.included(klass)
    # Make methods available both as class and instance methods.
    klass.extend(self)
  end
end

It is used like so:

class Form::NewRegistration
  # ...

  include TypedWriter

  typed_writer :age, :integer
end

Testing

I don’t load Rails for my form tests, so an explicit require of active model is necessary. I do this in my form code since I like explicitly requiring third-party dependencies everywhere they are used.

require 'unit_helper'

require 'form/new_registration'

describe Form::NewRegistration do
  include RSpec::Fire

  let(:event) { fire_double('Event') }

  subject { described_class.new(event) }

  def valid_attributes
    {
      name: 'don',
      age:  25
    }
  end

  def form(extra = {})
    described_class.new(event, valid_attributes.merge(extra))
  end

  describe 'validations' do
    it 'is valid for default attributes' do
      form.should be_valid
    end

    it { form(name: '').should have_error_on(:name) }
  end

  describe 'type-casting' do
    let(:f) { form } # Memoize the form

    # This pattern is overkill in this example, but useful when you have many
    # typed attributes.
    let(:typecasts) {{
      int: {
        nil  => nil,
        ""   => nil,
        23   => 23,
        "23" => 23,
      }
    }}

    it 'casts age to an int' do
      typecasts[:int].each do |value, expected|
        f.age = value
        f.age.should == expected
      end
    end
  end

  describe '#create' do
    it 'returns false when not valid' do
      subject.create.should_not be
    end

    it 'creates a new registration' do
      f = form
      dao = fire_replaced_class_double("Registration")
      dao.should_receive(:create).with {|x|
        x[:event].should == event

        data = JSON.parse(x[:data_json])

        data['name'].should == valid_attributes[:name]
        data['age'].should == valid_attributes[:age]
      }
      f.create.should new_rego
    end
  end

  it { should_not be_persisted }
end

Code Sharing

I tend to have a parent object Form::Registration, with subclasses for Form::{New,Update,View}Registration. A common mixin would also work. For testing, I use a shared spec that is run by the specs for each of the three subclasses.

Conclusion

There are other solutions to this problem (such as separating validations completely) which I haven’t tried yet, and I haven’t used this approach on a team yet. It has worked well for my solo projects though, and I’m just about confident enough to recommend it for production use.

Posted on December 03, 2012
Tagged code, forms, rails, ruby

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