add export missed

data/2.PostgreSQL中阶/1.PostgreSQL数据库的基本结构/6.数据库扩展/config.json

 {
   "node_id": "pg-d2b6e840cfad44a2aa357cfee56d437d",
-  "keywords": ["数据库扩展", "数据库插件", "extension"],
+  "keywords": [
+    "数据库扩展",
+    "数据库插件",
+    "extension"
+  ],
   "children": [],
-  "export": []
+  "export": [
+    "language.json",
+    "extension.json"
+  ]
 }
\ No newline at end of file

data/2.PostgreSQL中阶/2.服务端编程/1.聚合和分组/config.json

 {
   "node_id": "pg-3a87b0dbead643e58f1ff1d93acd80cc",
-  "keywords": ["group by", "分组"],
+  "keywords": [
+    "group by",
+    "分组"
+  ],
   "children": [],
-  "export": []
+  "export": [
+    "salary.json"
+  ]
 }
\ No newline at end of file

data/2.PostgreSQL中阶/2.服务端编程/2.连接查询/config.json

 {
   "node_id": "pg-458d5d590ea64188847b45d8e6343f83",
-  "keywords": ["join", "连接查询"],
+  "keywords": [
+    "join",
+    "连接查询"
+  ],
   "children": [],
-  "export": []
+  "export": [
+    "customer_order.json"
+  ]
 }
\ No newline at end of file

data/2.PostgreSQL中阶/2.服务端编程/5.CTE/config.json

 {
   "node_id": "pg-6beb084543204f4f9d87930c0a6c5707",
-  "keywords": ["common table expression", "cte", "recursive"],
+  "keywords": [
+    "common table expression",
+    "cte",
+    "recursive"
+  ],
   "children": [],
-  "export": []
+  "export": [
+    "to_root.json"
+  ]
 }
\ No newline at end of file

data/3.PostgreSQL高阶/1.集群和复制/3.fdw/config.json

 {
   "node_id": "pg-20a433f6ae65429f9b1a7f17c751ac01",
-  "keywords": ["外部数据连接", "fdw"],
+  "keywords": [
+    "外部数据连接",
+    "fdw"
+  ],
   "children": [],
-  "export": []
+  "export": [
+    "fdw.json"
+  ]
 }
\ No newline at end of file

src/tree.py

@@ -306,13 +306,23 @@ class TreeWalker:
 
     def ensure_exercises(self, section_path):
         config = self.ensure_section_config(section_path)
+        flag = False
         for e in os.listdir(section_path):
             base, ext = os.path.splitext(e)
             _, source = os.path.split(e)
             if ext != ".md":
                 continue
-            meta_path = os.path.join(section_path, base + ".json")
+            mfile = base + ".json"
+            meta_path = os.path.join(section_path, mfile)
             self.ensure_exercises_meta(meta_path, source)
+            export = config.get("export", [])
+            if mfile not in export:
+                export.append(mfile)
+                flag = True
+                config["export"] = export
+
+        if flag:
+            dump_json(os.path.join(section_path, "config.json"), config, True, True)
 
         for e in config.get("export", []):
             full_name = os.path.join(section_path, e)