Before this was not possible:

```ruby
class Zine < ActiveRecord::Base
  has_many :interests, inverse_of: :zine
  has_many :humans, through: :interests, source: :interestable, source_type: 'Human'
end

class Interest < ActiveRecord::Base
  belongs_to :zine, inverse_of: :interests
  belongs_to :interestable, polymorphic: true, inverse_of: :interests
end

class Human < ActiveRecord::Base
  has_many :interests, as: :interestable, inverse_of: :interestable
end

zine = Zine.create!
zine.humans.create!

# => ArgumentError: Polymorphic associations do not support computing the class.
```

Now the code checks whether the association is polymorphic and uses the
correct code to get the inverse association.