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- Model Tree Structures with Parent References
Model Tree Structures with Parent References¶
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 Data Modeling Concepts 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 hierarchy of categories:
The following example models the tree using Parent References, storing the reference to the parent category in the field parent:
db.categories.insert( { _id: "MongoDB", parent: "Databases" } )
db.categories.insert( { _id: "dbm", 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.createIndex( { 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.
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