module ActiveRecord::Associations::ThroughAssociation

def ensure_mutable
  unless source_reflection.belongs_to?
    if reflection.has_one?
      raise HasOneThroughCantAssociateThroughHasOneOrManyReflection.new(owner, reflection)
    else
      raise HasManyThroughCantAssociateThroughHasOneOrManyReflection.new(owner, reflection)
    end
  end
end