allow merging a single where value

activerecord/lib/active_record/relation/merger.rb

@@ -85,7 +85,7 @@ def merged_binds
 
       def merged_wheres
         if values[:where]
-          merged_wheres = relation.where_values + values[:where]
+          merged_wheres = relation.where_values + Array(values[:where])
 
           unless relation.where_values.empty?
             # Remove duplicates, last one wins.

activerecord/test/cases/relation_test.rb

@@ -156,6 +156,12 @@ def test_references_values_dont_duplicate
       relation = Relation.new(:a, :b, where: [:foo])
       assert_equal [:foo], relation.where_values
     end
+
+    test 'merging a single where value' do
+      relation = Relation.new(:a, :b)
+      relation.merge!(where: :foo)
+      assert_equal [:foo], relation.where_values
+    end
   end
 
   class RelationMutationTest < ActiveSupport::TestCase