Relation::Merger should not fill values with empty values

activerecord/lib/active_record/relation/merger.rb

@@ -135,31 +135,33 @@ def merge_multi_values
           if other.reordering_value
             # override any order specified in the original relation
             relation.reorder! other.order_values
-          elsif other.order_values
+          elsif other.order_values.any?
             # merge in order_values from relation
             relation.order! other.order_values
           end
 
-          relation.extend(*other.extending_values) unless other.extending_values.blank?
+          extensions = other.extensions - relation.extensions
+          relation.extending!(*extensions) if extensions.any?
         end
 
         def merge_single_values
-          if relation.from_clause.empty?
-            relation.from_clause = other.from_clause
-          end
-          relation.lock_value ||= other.lock_value
+          relation.lock_value ||= other.lock_value if other.lock_value
 
           unless other.create_with_value.blank?
             relation.create_with_value = (relation.create_with_value || {}).merge(other.create_with_value)
           end
         end
 
         def merge_clauses
-          CLAUSE_METHODS.each do |method|
-            clause = relation.get_value(method)
-            other_clause = other.get_value(method)
-            relation.set_value(method, clause.merge(other_clause))
+          if relation.from_clause.empty? && !other.from_clause.empty?
+            relation.from_clause = other.from_clause
           end
+
+          where_clause = relation.where_clause.merge(other.where_clause)
+          relation.where_clause = where_clause unless where_clause.empty?
+
+          having_clause = relation.having_clause.merge(other.having_clause)
+          relation.having_clause = having_clause unless having_clause.empty?
         end
     end
   end

activerecord/lib/active_record/relation/query_methods.rb

@@ -901,16 +901,17 @@ def arel # :nodoc:
       @arel ||= build_arel
     end
 
-    # Returns a relation value with a given name
-    def get_value(name) # :nodoc:
-      @values[name] || default_value_for(name)
-    end
+    protected
+      # Returns a relation value with a given name
+      def get_value(name) # :nodoc:
+        @values[name] || default_value_for(name)
+      end
 
-    # Sets the relation value with the given name
-    def set_value(name, value) # :nodoc:
-      assert_mutability!
-      @values[name] = value
-    end
+      # Sets the relation value with the given name
+      def set_value(name, value) # :nodoc:
+        assert_mutability!
+        @values[name] = value
+      end
 
     private
 

activerecord/test/cases/relation_test.rb

@@ -100,6 +100,14 @@ def test_create_with_value_with_wheres
       assert_equal({ "hello" => "world", "id" => 10 }, relation.scope_for_create)
     end
 
+    def test_empty_scope
+      relation = Relation.new(Post, Post.arel_table, Post.predicate_builder)
+      assert relation.empty_scope?
+
+      relation.merge!(relation)
+      assert relation.empty_scope?
+    end
+
     def test_bad_constants_raise_errors
       assert_raises(NameError) do
         ActiveRecord::Relation::HelloWorld