allow to autosave associations with polymorphic has many through join…

… model and inverse of

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 wether the association is polymorphic and uses the
correct code to get the inverse association.

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Allow to save records with polymorphic join tables that have inverse of specified.
+
+    *Markus Doits*
+
 *   Make Float distinguish between `float4` and `float8` in PostgreSQL.
 
     Fixes #52742

activerecord/lib/active_record/associations/has_many_through_association.rb

@@ -93,7 +93,13 @@ def build_record(attributes)
           @through_scope = scope
           record = super
 
-          inverse = source_reflection.inverse_of
+          inverse =
+            if source_reflection.polymorphic?
+              source_reflection.polymorphic_inverse_of(record.class)
+            else
+              source_reflection.inverse_of
+            end
+
           if inverse
             if inverse.collection?
               record.send(inverse.name) << build_through_record(record)

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -38,6 +38,9 @@
 require "models/session"
 require "models/sharded"
 require "models/cpk"
+require "models/zine"
+require "models/interest"
+require "models/human"
 
 class HasManyThroughAssociationsTest < ActiveRecord::TestCase
   fixtures :posts, :readers, :people, :comments, :authors, :categories, :taggings, :tags,
@@ -1320,6 +1323,15 @@ def test_has_many_through_with_polymorphic_source
     assert_equal [tags(:general)], post.reload.tags
   end
 
+  def test_has_many_through_with_polymorhic_join_model
+    zine = Zine.create!
+
+    assert_nothing_raised { zine.polymorphic_humans.build.save! }
+
+    assert_equal 1, zine.polymorphic_humans.count
+    assert_equal 1, zine.interests.count
+  end
+
   def test_has_many_through_obeys_order_on_through_association
     owner = owners(:blackbeard)
     assert_includes owner.toys.to_sql, "pets.name desc"

activerecord/test/models/zine.rb

@@ -2,4 +2,5 @@
 
 class Zine < ActiveRecord::Base
   has_many :interests, inverse_of: :zine
+  has_many :polymorphic_humans, through: :interests, source_type: "Human"
 end