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 in the parent-nodes to children nodes.

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.