Merge pull request rails#47920 from aharpole/fix-none-relation-performing-query

Ensure `none` constraint scopes don’t trigger a query

Author: tenderlove

activerecord/lib/active_record/relation/merger.rb

@@ -69,6 +69,8 @@ def merge
           end
         end
 
+        relation.none! if other.null_relation?
+
         merge_select_values
         merge_multi_values
         merge_single_values

activerecord/test/cases/null_relation_test.rb

@@ -59,6 +59,16 @@ def test_null_relation_content_size_methods
     end
   end
 
+  def test_null_relation_used_with_constraints
+    post = Post.first
+    assert_no_queries do
+      scope = post.comments
+      none = Post.none
+      scope = scope.merge(none)
+      assert_equal 0, scope.size
+    end
+  end
+
   def test_null_relation_metadata_methods
     assert_includes Developer.none.to_sql, " WHERE (1=0)"
     assert_equal({}, Developer.none.where_values_hash)