Model Tree Structures with Child References

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:

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

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:

  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.createIndex( { 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.