• 物化路径模式将每个树节点存储在文档中。 除树节点外，文档还以字符串形式存储该节点的祖先或路径的ID。 尽管“物化路径”模式需要使用字符串和正则表达式的其他步骤，但该模式在使用路径时也提供了更大的灵活性，例如通过部分路径查找节点。

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    1. db.categories.insertMany( [
    2.    { _id: "Books", path: null },
    3.    { _id: "Programming", path: ",Books," },
    4.    { _id: "Databases", path: ",Books,Programming," },
    5.    { _id: "Languages", path: ",Books,Programming," },
    6.    { _id: "MongoDB", path: ",Books,Programming,Databases," },
    7.    { _id: "dbm", path: ",Books,Programming,Databases," }
    8. ] )