Fix `scope_for_create` to do not lose polymorphic associations

This regression was caused at 213796fb due to polymorphic predicates are
combined by `Arel::Nodes::And`. But I'd like to keep that combined
because it would help inverting polymorphic predicates correctly
(e9ba12f7), and we can collect equality nodes regardless of combined by
`Arel::Nodes::And` (`a AND (b AND c) AND d` == `a AND b AND c AND d`).
This change fixes the regression to collect equality nodes in
`Arel::Nodes::And` as well.

Fixes #31338.

2 files changed, 10 insertions, 1 deletions

diff --git a/activerecord/test/cases/relation_test.rb b/activerecord/test/cases/relation_test.rb
index a71d8de521..b424ca91de 100644
--- a/activerecord/test/cases/relation_test.rb
+++ b/activerecord/test/cases/relation_test.rb
@@ -68,7 +68,7 @@ module ActiveRecord
       relation = Relation.new(Post, Post.arel_table, Post.predicate_builder)
       left     = relation.table[:id].eq(10)
       right    = relation.table[:id].eq(10)
-      combine  = left.and right
+      combine  = left.or(right)
       relation.where! combine
       assert_equal({}, relation.where_values_hash)
     end
diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index 50ad1d5b26..675aafabda 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -1189,6 +1189,15 @@ class RelationTest < ActiveRecord::TestCase
     assert_equal "hen", hen.name
   end
 
+  def test_create_with_polymorphic_association
+    author = authors(:david)
+    post = posts(:welcome)
+    comment = Comment.where(post: post, author: author).create!(body: "hello")
+
+    assert_equal author, comment.author
+    assert_equal post, comment.post
+  end
+
   def test_first_or_create
     parrot = Bird.where(color: "green").first_or_create(name: "parrot")
     assert_kind_of Bird, parrot