Remove :polymorphic from INVALID_AUTOMATIC_INVERSE_OPTIONS

This makes automatic inverse detection possible for polymorphic
:has_one & :has_many possible.

This resolves a number of issues, eg. `touch: true` on polymorphic relationships (#16446) and automatically setting inverse associations on newly built objects (#15028, #21843).

2 files changed, 5 insertions, 10 deletions

diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index 24ca8b0be4..c7472372ab 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -567,7 +567,7 @@ module ActiveRecord
       end
 
       VALID_AUTOMATIC_INVERSE_MACROS = [:has_many, :has_one, :belongs_to]
-      INVALID_AUTOMATIC_INVERSE_OPTIONS = [:conditions, :through, :polymorphic, :foreign_key]
+      INVALID_AUTOMATIC_INVERSE_OPTIONS = [:conditions, :through, :foreign_key]
 
       def add_as_source(seed)
         seed
@@ -640,9 +640,8 @@ module ActiveRecord
         # us from being able to guess the inverse automatically. First, the
         # <tt>inverse_of</tt> option cannot be set to false. Second, we must
         # have <tt>has_many</tt>, <tt>has_one</tt>, <tt>belongs_to</tt> associations.
-        # Third, we must not have options such as <tt>:polymorphic</tt> or
-        # <tt>:foreign_key</tt> which prevent us from correctly guessing the
-        # inverse association.
+        # Third, we must not have options such as <tt>:foreign_key</tt>
+        # which prevent us from correctly guessing the inverse association.
         #
         # Anything with a scope can additionally ruin our attempt at finding an
         # inverse, so we exclude reflections with scopes.
diff --git a/activerecord/test/cases/associations/inverse_associations_test.rb b/activerecord/test/cases/associations/inverse_associations_test.rb
index 287b3e9ebc..cf51f1009f 100644
--- a/activerecord/test/cases/associations/inverse_associations_test.rb
+++ b/activerecord/test/cases/associations/inverse_associations_test.rb
@@ -116,15 +116,11 @@ class AutomaticInverseFindingTests < ActiveRecord::TestCase
     assert !club_reflection.has_inverse?, "A has_many_through association should not find an inverse automatically"
   end
 
-  def test_polymorphic_relationships_should_still_not_have_inverses_when_non_polymorphic_relationship_has_the_same_name
+  def test_polymorphic_has_one_should_find_inverse_automatically
     man_reflection = Man.reflect_on_association(:polymorphic_face_without_inverse)
-    face_reflection = Face.reflect_on_association(:man)
-
-    assert_respond_to face_reflection, :has_inverse?
-    assert face_reflection.has_inverse?, "For this test, the non-polymorphic association must have an inverse"
 
     assert_respond_to man_reflection, :has_inverse?
-    assert !man_reflection.has_inverse?, "The target of a polymorphic association should not find an inverse automatically"
+    assert man_reflection.has_inverse?
   end
 end