fix merging error in reverse order of object extension

lib/schema.go

@@ -5,6 +5,7 @@ import (
 	"os"
 	"path/filepath"
 	"reflect"
+	"sort"
 	"sync"
 )
 
@@ -538,6 +539,9 @@ func (s *Schema) MergeTypeName(wg *sync.WaitGroup) {
 	defer wg.Done()
 	j := 0
 	seen := make(map[string]struct{}, len(s.Types))
+	sort.SliceStable(s.Types, func(i, j int) bool {
+		return !s.Types[i].Extend && s.Types[j].Extend
+	})
 	for _, v := range s.Types {
 		if _, ok := seen[v.Name]; ok {
 			for i := 0; i < j; i++ {

test/object_extension/schema/a.graphql

@@ -1,6 +1,3 @@
-type Person implements Node @talkable{
-  id: ID!
-  createTime: Time!
-  updateTime: Time!
-  name: String!
+extend type Person @walkable {
+  hasCar: Boolean!
 }

test/object_extension/schema/b.graphql

@@ -1,3 +1,6 @@
-extend type Person @walkable {
-  hasCar: Boolean!
+type Person implements Node @talkable{
+  id: ID!
+  createTime: Time!
+  updateTime: Time!
+  name: String!
 }