class Person < ActiveRecord::Base
  has_many :parent_children
  has_many :children, :through => :parent_children
  has_many :parents, :through => :parent_children


end

class ParentChild < ActiveRecord::Base
  belongs_to :parent, :class_name => "Person"
  belongs_to :child, :class_name => "Person"
  scope :mother, lambda {
    joins(:parent).where("parent.gender = 'F'")
  }

  scope :father, lambda {
    joins(:parent).where("parent.gender ='M'")
  }
end