use statement cache for belongs_to relations

activerecord/lib/active_record/associations/collection_association.rb

@@ -416,7 +416,7 @@ module ActiveRecord
         return scope.to_a if reflection.scope_chain.any?(&:any?)
 
         conn = klass.connection
-        sc = reflection.association_scope_cache(conn) do
+        sc = reflection.association_scope_cache(conn, owner) do
           StatementCache.create(conn) { |params|
             as = AssociationScope.create { params.bind }
             target_scope.merge as.scope(self, conn)

activerecord/lib/active_record/associations/singular_association.rb

@@ -39,7 +39,18 @@ module ActiveRecord
         end
 
         def get_records
-          scope.limit(1).to_a
+          return scope.limit(1).to_a if reflection.scope_chain.any?(&:any?)
+
+          conn = klass.connection
+          sc = reflection.association_scope_cache(conn, owner) do
+            StatementCache.create(conn) { |params|
+              as = AssociationScope.create { params.bind }
+              target_scope.merge(as.scope(self, conn)).limit(1)
+            }
+          end
+
+          binds = AssociationScope.get_bind_values(owner, reflection.chain)
+          sc.execute binds, klass, klass.connection
         end
 
         def find_target

activerecord/lib/active_record/reflection.rb

@@ -205,8 +205,11 @@ module ActiveRecord
         @scope_lock = Mutex.new
       end
 
-      def association_scope_cache(conn)
+      def association_scope_cache(conn, owner)
         key = conn.prepared_statements
+        if options[:polymorphic]
+          key = [key, owner.read_attribute(@foreign_type)]
+        end
         @association_scope_cache[key] ||= @scope_lock.synchronize {
           @association_scope_cache[key] ||= yield
         }