Model Tree Structures with Child 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 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 hierarchy of categories:

Tree data model for a sample hierarchy of categories.

The following example models the tree using Child References, storing the reference to the node’s children in the field children:

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

The query to retrieve the immediate children of a node is fast and straightforward:
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```
db.categories.findOne( { _id: "Databases" } ).children
```
You can create an index on the field children to enable fast search by the child nodes:
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```
db.categories.createIndex( { children: 1 } )
```
You can query for a node in the children field to find its parent node as well as its siblings:
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```
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.

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