model-tree-structures-with-child-references.txt

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Model Tree Structures with Child References
===========================================

.. default-domain:: mongodb

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Overview
--------

Data in MongoDB has a *flexible schema*. :term:`Collections
<collection>` do not enforce :term:`document` structure. Decisions
that affect how you model data can affect application performance and
database capacity. See :doc:`/core/data-models` 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 :ref:`references
<data-modeling-referencing>` in the parent-nodes to children nodes.

Pattern
-------

.. start-model-tree-structures-include-here

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:

.. include:: /images/data-model-tree.rst

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

.. code-block:: javascript

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

- The query to retrieve the immediate children of a node is fast and
  straightforward:

  .. code-block:: javascript

     db.categories.findOne( { _id: "Databases" } ).children

- You can create an index on the field ``children`` to enable fast
  search by the child nodes:

  .. code-block:: javascript

     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:

  .. code-block:: javascript

     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.