test.rb

require "active_record"
require "logger"

puts ActiveRecord::VERSION::STRING

$rails4 = ActiveRecord::VERSION::MAJOR > 3

# ActiveRecord::Base.logger = Logger.new(STDERR)
ActiveRecord::Base.establish_connection adapter: 'sqlite3', database: ':memory:'

ActiveRecord::Schema.define do
  create_table :projects
  create_table :tasks
end

class Project < ActiveRecord::Base
  if $rails4
    has_many :tasks, -> { where foo: 'a' }
  else
    has_many :tasks, conditions: { foo: 'a' }
  end
end

class Task < ActiveRecord::Base
  default_scope -> { where foo: 'b' }
end

p = Project.create
puts p.tasks.to_sql