模型树结构: 子引用

Overview¶

Data in MongoDB has a flexible schema. Collections do not enforce document structure. Decisions that affect how you model data can affect application performance and database capacity. See 数据建模 for a full high level overview of data modeling in MongoDB.

This document describes a data model that describes a tree-like structure in MongoDB documents by storing references in the parent-nodes to children nodes.

Pattern¶

The Child References pattern stores each tree node in a document; in addition to the tree node, document stores in an array the id(s) of the node’s children.

Consider the following example that models a tree of categories using Child References:

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

• The query to retrieve the immediate children of a node is fast and straightforward:

  db.categories.findOne( { _id: "Databases" } ).children
  

• You can create an index on the field children to enable fast search by the child nodes:

  db.categories.ensureIndex( { children: 1 } )
  

• You can query for a node in the children field to find its parent node as well as its siblings:

  db.categories.find( { children: "MongoDB" } )
  

The Child References pattern provides a suitable solution to tree storage as long as no operations on subtrees are necessary. This pattern may also provide a suitable solution for storing graphs where a node may have multiple parents.