http://stackoverflow.com/questions/28711074/undefined-method-error-for-scope-on-sti-subclass

sti_scope.rb

# http://stackoverflow.com/questions/28711074/undefined-method-error-for-scope-on-sti-subclass

unless File.exist?('Gemfile')
File.write('Gemfile', <<-GEMFILE)
source 'https://rubygems.org'
gem 'rails'
gem 'arel'
gem 'sqlite3'
GEMFILE

system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

require 'active_record'
require 'minitest/autorun'
require 'logger'

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :transactions, force: true do |t|
    t.string :type
    t.references :account
  end

  create_table :accounts, force: true do |t|
  end
end

class Transaction < ActiveRecord::Base
  belongs_to :account
  scope :deposits, -> { where type: Deposit }
end

class Deposit < Transaction
  scope :pending, -> { where state: :pending }
end

class Account < ActiveRecord::Base
  has_many :transactions
end

class StiTest < Minitest::Test
  def test_sti_scope
    a = Account.create
    a.transactions.create
    Deposit.create account: a

    assert_equal Deposit::ActiveRecord_AssociationRelation, a.transactions.deposits.class
  end
end

sti_scope2.rb

unless File.exist?('Gemfile')
File.write('Gemfile', <<-GEMFILE)
source 'https://rubygems.org'
gem 'rails'
gem 'arel'
gem 'sqlite3'
GEMFILE

system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

require 'active_record'
require 'minitest/autorun'
require 'logger'
require 'pry'

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :transactions, force: true do |t|
    t.string :type
    t.string :state

    t.references :account
  end

  create_table :accounts, force: true do |t|
  end
end

class Transaction < ActiveRecord::Base
  belongs_to :account

  def self.deposits
    conditions = where(nil).where_values.reduce(&:and)
    Deposit.unscoped.where(conditions)
  end
end

class Deposit < Transaction
  scope :pending, -> { where state: :pending }
end

class Account < ActiveRecord::Base
  has_many :transactions
end

class StiTest < Minitest::Test
  def test_sti_scope
    a = Account.create
    a.transactions.create
    Deposit.create account: a, state: :pending

    assert_equal a.transactions.deposits.class, Deposit::ActiveRecord_Relation
  end
end