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.

3 files changed, 26 insertions, 3 deletions

diff --git a/activerecord/lib/active_record/relation/where_clause.rb b/activerecord/lib/active_record/relation/where_clause.rb
index 752bb38481..a502713e56 100644
--- a/activerecord/lib/active_record/relation/where_clause.rb
+++ b/activerecord/lib/active_record/relation/where_clause.rb
@@ -47,7 +47,7 @@ module ActiveRecord
       end
 
       def to_h(table_name = nil)
-        equalities = predicates.grep(Arel::Nodes::Equality)
+        equalities = equalities(predicates)
         if table_name
           equalities = equalities.select do |node|
             node.left.relation.name == table_name
@@ -90,6 +90,20 @@ module ActiveRecord
         end
 
       private
+        def equalities(predicates)
+          equalities = []
+
+          predicates.each do |node|
+            case node
+            when Arel::Nodes::Equality
+              equalities << node
+            when Arel::Nodes::And
+              equalities.concat equalities(node.children)
+            end
+          end
+
+          equalities
+        end
 
         def predicates_unreferenced_by(other)
           predicates.reject do |n|
@@ -121,7 +135,7 @@ module ActiveRecord
         end
 
         def except_predicates(columns)
-          self.predicates.reject do |node|
+          predicates.reject do |node|
             case node
             when Arel::Nodes::Between, Arel::Nodes::In, Arel::Nodes::NotIn, Arel::Nodes::Equality, Arel::Nodes::NotEqual, Arel::Nodes::LessThan, Arel::Nodes::LessThanOrEqual, Arel::Nodes::GreaterThan, Arel::Nodes::GreaterThanOrEqual
               subrelation = (node.left.kind_of?(Arel::Attributes::Attribute) ? node.left : node.right)
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