具有父级引用的模型树结构

• 父引用模式将每个树节点存储在文档中；除了树节点之外，文档还存储该节点的父节点的ID。

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