具有子引级用的模型树结构

• 子引用模式将每个树节点存储在文档中。除树节点外，文档还以数组的形式存储该节点的子节点的ID

db.categories.insertMany( [
   { _id: "MongoDB", children: [] },
   { _id: "dbm", children: [] },
   { _id: "Databases", children: [ "MongoDB", "dbm" ] },
   { _id: "Languages", children: [] },
   { _id: "Programming", children: [ "Databases", "Languages" ] },
   { _id: "Books", children: [ "Programming" ] }
] )