Add Relation#merge!

activerecord/lib/active_record/relation/spawn_methods.rb

@@ -4,20 +4,23 @@ require 'active_record/relation/merger'
 module ActiveRecord
   module SpawnMethods
     def merge(other)
-      if other
-        case other
-        when Array
-          to_a & other
-        when Hash
-          Relation::HashMerger.new(clone, other).merge
-        else
-          Relation::Merger.new(clone, other).merge
-        end
+      if other.is_a?(Array)
+        to_a & other
+      elsif other
+        clone.merge!(other)
       else
         self
       end
     end
 
+    def merge!(other)
+      if other.is_a?(Hash)
+        Relation::HashMerger.new(self, other).merge
+      else
+        Relation::Merger.new(self, other).merge
+      end
+    end
+
     # Removes from the query the condition(s) specified in +skips+.
     #
     # Example:

activerecord/test/cases/relation_test.rb

@@ -206,5 +206,10 @@ module ActiveRecord
       assert relation.create_with!(foo: 'bar').equal?(relation)
       assert_equal({foo: 'bar'}, relation.create_with_value)
     end
+
+    test 'merge!' do
+      assert relation.merge!(where: ['foo']).equal?(relation)
+      assert_equal ['foo'], relation.where_values
+    end
   end
 end