特定の条件下でのカラムの値を宣言的に定義する

force_attribute.rb

# app/models/concerns/forced_attribute.rb
module ForcedAttribute
  extend ActiveSupport::Concern

  CONDITIONAL_OPTIONS = %i[if unless on].freeze

  class ForcedAttributeCallbacks
    # @param attributes [Hash<symbol, any>]
    def initialize(attributes = {})
      @attributes = attributes.freeze
    end

    # 指定された値を強制的に代入する
    #
    # @param record [ActiveModel::Model, ActiveRecord::Base]
    def before_validation(record)
      record.assign_attributes(forced_attributes(record))
    end

    # 指定された値が強制的に代入した値か検証する
    #
    # @param record [ActiveModel::Model, ActiveRecord::Base]
    def validate(record)
      forced_attributes(record).each do |attribute, value|
        validate_forced_attribute(record, attribute, value)
      end
    end

    private

    def validate_forced_attribute(record, attribute, expected)
      actual = record.public_send(attribute)
      return if actual == expected

      record.errors.add(attribute, :not_forced_attribute, expected: expected, actual: actual)
    end

    def forced_attributes(record)
      caster = record.class.new

      @attributes.map { |attribute, value|
        original_value = evaluate_force_attribute(record, value)

        # 型変換をする
        caster.public_send("#{attribute}=", original_value)
        casted_value = caster.public_send(attribute)

        [attribute, casted_value]
      }.to_h
    end

    def evaluate_force_attribute(record, value)
      if value.respond_to?(:call)
        record.instance_exec(&value)
      else
        value
      end
    end
  end

  private_constant(:ForcedAttributeCallbacks)

  class_methods do
    # 指定された条件下で値を強制する
    #
    # @param [Hash<String, Object>] { カラム名 => 強制する値or強制する値を返すProc }
    #
    # @example
    #   forced_attribute(disabled: false)
    #   forced_attribute(published_at: nil, on: :create, if: :state_draft?)
    #
    # @return void
    def forced_attribute(attributes)
      options = attributes.extract!(*CONDITIONAL_OPTIONS)
      callbacks = ForceAttributeCallbacks.new(attributes)

      before_validation(callbacks, options)
      validate(callbacks, options)
    end
  end
end