Model Tree Structures with Parent References

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:

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

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:

  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" } )

• To retrieve subtrees, see $graphLookup.