Model Tree Structures with Parent References¶

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Overview
Pattern

Overview¶

This page 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:

Tree data model for a sample hierarchy of categories.

The following example models the tree using Parent References, storing the reference to the parent category in the field parent:

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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 }
] )

The query to retrieve the parent of a node is fast and straightforward:
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```
db.categories.findOne( { _id: "MongoDB" } ).parent
```
You can create an index on the field parent to enable fast search by the parent node:
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```
db.categories.createIndex( { parent: 1 } )
```
You can query by the parent field to find its immediate children nodes:
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```
db.categories.find( { parent: "Databases" } )
```
To retrieve subtrees, see $graphLookup.

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