form-objects-query-objects-presenters

Form Objects, Query Objects, Presenters

Three patterns for code that doesn't fit on the model and doesn't justify a service object. AI agents either over-extract (every form is a form object) or under-extract (1000-line model has its own form binding inline). This skill maps each pattern to the right trigger.

The opinion

Form objects when a single form spans multiple models or has non-AR validations. Query objects when a query is 10+ lines or reused across actions. Presenters / decorators when view helpers grow procedural and need state. None of these for simple cases — keep the logic on the model or in the controller.

Decision matrix

Core patterns

Pattern 1: Form objects

The case: signup creates a User + Account + Subscription.

# app/forms/signup_form.rb
class SignupForm
  include ActiveModel::Model
  include ActiveModel::Attributes

  attribute :email, :string
  attribute :password, :string
  attribute :password_confirmation, :string
  attribute :company_name, :string
  attribute :plan_id, :string
  attribute :terms_accepted, :boolean

  validates :email, :password, :company_name, :plan_id, presence: true
  validates :password, length: { minimum: 12 }
  validate :password_matches
  validate :terms_must_be_accepted

  attr_reader :user

  def save
    return false unless valid?
    ActiveRecord::Base.transaction do
      account = Account.create!(name: company_name)
      @user = User.create!(email: email, password: password, account: account)
      account.subscriptions.create!(plan: Plan.find(plan_id))
    end
    true
  rescue ActiveRecord::RecordInvalid => e
    errors.add(:base, e.message)
    false
  end

  private

  def password_matches
    errors.add(:password_confirmation, "doesn't match") unless password == password_confirmation
  end

  def terms_must_be_accepted
    errors.add(:terms_accepted, "must be accepted") unless terms_accepted
  end
end