A `NullRelation` should represent nothing. Closes #15176.

[Matthew Draper & Yves Senn]

Closes #16860. (pull request to discuss the implementation)

2 files changed, 9 insertions, 0 deletions

diff --git a/activerecord/test/cases/relation/mutation_test.rb b/activerecord/test/cases/relation/mutation_test.rb
index 1da5c36e1c..4c94c2fd0d 100644
--- a/activerecord/test/cases/relation/mutation_test.rb
+++ b/activerecord/test/cases/relation/mutation_test.rb
@@ -18,6 +18,10 @@ module ActiveRecord
       def attribute_alias?(name)
         false
       end
+
+      def sanitize_sql(sql)
+        sql
+      end
     end
 
     def relation
diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index 7163697a68..3bfbf95b0a 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -420,6 +420,11 @@ class RelationTest < ActiveRecord::TestCase
     assert_equal        nil, ac.engines.maximum(:id)
   end
 
+  def test_null_relation_in_where_condition
+    assert_operator 0, :<, Comment.count # precondition, make sure there are comments.
+    assert_equal 0, Comment.where(post_id: Post.none).to_a.size
+  end
+
   def test_joins_with_nil_argument
     assert_nothing_raised { DependentFirm.joins(nil).first }
   end