Question

class A
 has_many :c

class B
 has_many :c

class C
 belongs_to :a
 belongs_to :b

When I have an instance of C, its parent can be either A or B but I don’t know ahead of time which it is. Is there a simple way I can use Rails association to access the parent whichever it happens to be?

Answer 1

There are several ways you could do this, including polymorphism, but perhaps the most straightforward way is this:

class C
  belongs_to :a
  belongs_to :b

  def parent
    a || b
  end
end

# Usage:
C.new.parent # => nil

a1 = A.new
c1 = a1.c.create
c1.parent    # => #<A:...>

b1 = B.new
c2 = b1.c.create
c2.parent    # => #<B:...>