Attempt to mock basic features of ActiveRecord.I'm willing to include ActiveModel::Dirty, but I do not want to specify my model attributes for every models.

mock_active_record.rb

require 'ostruct'
require 'forwardable'

class MockActiveRecord < OpenStruct

  def save(opts); end
  alias_method :save!, :save

  def attributes
    to_h.symbolize_keys
  end

  def attributes=(values)
    values.each do |k,v|
      send("#{k}=",v)
    end
  end

  class << self
    extend Forwardable
    def_delegators :@collection, :count, :first, :last

    def transaction
      yield if block_given?
    end

    def create(attributes = {}, &blk)
      object = new(attributes)
      if block_given?
        if blk.arity == 1
          yield object
        else
          object.instance_eval(&blk)
        end
      end
      collection << object
      return object
    end
    alias_method :create!, :create

    def pluck(attribute)
      collection.map { |o| o.send(attribute) }
    end

    def find(id)
      collection.each do |object|
        return object if object.id == id
      end
      return nil
    end

    def destroy_all
      @collection = []
    end
  private
    def collection
      @collection ||= []
    end
  end
end

mock_active_record_spec.rb

require 'support/models/mock_active_record'

describe MockActiveRecord do
  describe '::create' do
    context 'given a block with arity 1' do
      it 'should create an object' do
        object = MockActiveRecord.create do
          self.title = 'Hello World'
        end
        expect(object.title).to eq 'Hello World'
        expect(MockActiveRecord.last).to eq object
      end
    end
    context 'given a block with arity 0' do
      it 'should create an object' do
        object = MockActiveRecord.create do |t|
          t.title = 'Hello World 2'
        end
        expect(object.title).to eq 'Hello World 2'
        expect(MockActiveRecord.last).to eq object
      end
    end
    context 'given a hash of attributes' do
      it 'should create an object' do
        object = MockActiveRecord.create(title: 'Hello World 3')
        expect(object.title).to eq 'Hello World 3'
        expect(MockActiveRecord.last).to eq object
      end
    end
  end

  describe '::pluck' do
    it 'should return an array of value' do
      MockActiveRecord.create(title: 'Hello World 1')
      MockActiveRecord.create(title: 'Hello World 2')
      expect(MockActiveRecord.pluck(:title)).to include *[
        'Hello World 1', 'Hello World 2'
      ]
    end
  end

  describe '::destroy_all' do
    it 'should reset the collection' do
      MockActiveRecord.create(title: 'Hello World 1')
      expect(MockActiveRecord.count).to be > 1
      MockActiveRecord.destroy_all
      expect(MockActiveRecord.count).to eq 0
    end
  end

  describe "::find" do
    it 'should return nil when no id found' do
      expect(MockActiveRecord.find(3)).to be_nil
    end
    it 'should return the correct object' do
      id_1 = MockActiveRecord.create(id: 1)
      id_2 = MockActiveRecord.create(id: 2)
      expect(MockActiveRecord.find(1)).to eq id_1
      expect(MockActiveRecord.find(2)).to eq id_2
    end
  end

  describe 'class collection' do
    it 'should not collide' do
      UserMock = Class.new(MockActiveRecord)
      ClientMock = Class.new(MockActiveRecord)
      UserMock.create(name: "Foo")
      ClientMock.create(name: "Bar")

      expect(MockActiveRecord.pluck(:name)).to_not include *['Foo', 'Bar']
      expect(UserMock.pluck(:name)).to eq ['Foo']
      expect(ClientMock.pluck(:name)).to eq ['Bar']
    end
  end
end