模型树结构: 父引用

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 to “parent” nodes in children nodes.

Pattern¶

The Parent References pattern stores each tree node in a document; in addition to the tree node, the document stores the id of the node’s parent.

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

db.categories.insert( { _id: "MongoDB", parent: "Databases" } )
db.categories.insert( { _id: "Postgres", parent: "Databases" } )
db.categories.insert( { _id: "Databases", parent: "Programming" } )
db.categories.insert( { _id: "Languages", parent: "Programming" } )
db.categories.insert( { _id: "Programming", parent: "Books" } )
db.categories.insert( { _id: "Books", parent: null } )

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

  db.categories.findOne( { _id: "MongoDB" } ).parent
  

• You can create an index on the field parent to enable fast search by the parent node:

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

• You can query by the parent field to find its immediate children nodes:

  db.categories.find( { parent: "Databases" } )
  

The Parent Links pattern provides a simple solution to tree storage, but requires multiple queries to retrieve subtrees.