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Model Tree Structures with an Array of Ancestors

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Overview

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 Considerations for MongoDB Applications 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 using references to parent nodes and an array that stores all ancestors.

Pattern

The Array of Ancestors 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 ancestors or path.

Consider the following example that models a tree of categories using Array of Ancestors:

db.categories.insert( { _id: "MongoDB", ancestors: [ "Books", "Programming", "Databases" ], parent: "Databases" } )
db.categories.insert( { _id: "Postgres", ancestors: [ "Books", "Programming", "Databases" ], parent: "Databases" } )
db.categories.insert( { _id: "Databases", ancestors: [ "Books", "Programming" ], parent: "Programming" } )
db.categories.insert( { _id: "Languages", ancestors: [ "Books", "Programming" ], parent: "Programming" } )
db.categories.insert( { _id: "Programming", ancestors: [ "Books" ], parent: "Books" } )
db.categories.insert( { _id: "Books", ancestors: [ ], parent: null } )
  • The query to retrieve the ancestors or path of a node is fast and straightforward:

    db.categories.findOne( { _id: "MongoDB" } ).ancestors
    
  • You can create an index on the field ancestors to enable fast search by the ancestors nodes:

    db.categories.ensureIndex( { ancestors: 1 } )
    
  • You can query by the ancestors to find all its descendants:

    db.categories.find( { ancestors: "Programming" } )
    

The Array of Ancestors pattern provides a fast and efficient solution to find the descendants and the ancestors of a node by creating an index on the elements of the ancestors field. This makes Array of Ancestors a good choice for working with subtrees.

The Array of Ancestors pattern is slightly slower than the Materialized Paths pattern but is more straightforward to use.