subdocument

Subdocument refers to a document that is embedded within another document, forming a hierarchical data structure. A subdocument is a discrete unit with its own content and possibly metadata, yet it exists as part of a larger parent document. In many formats, subdocuments are serialized as a nested structure that can be extracted, validated, or transmitted independently of the parent.

In document-oriented data models, subdocuments appear as embedded objects within a parent document. For example, in JSON or BSON, an object value can itself contain another object; in XML the equivalent is a nested element. This arrangement supports representing complex, modular data without requiring separate, cross-document references for every component.

Common use cases include storing related information together—such as a user profile with embedded address and preferences, or a product entry with embedded specifications. Subdocuments can be manipulated with path-like addressing (for example, addressing a field inside the subdocument). Validation rules and schemas can be defined for the subdocument separately from the parent.

Advantages include locality of data, reduced need for joins or multiple fetches, and easier transactional updates for closely related fields. Drawbacks include potential inefficiencies from very deep nesting, larger document size, and complexity in validation, indexing, and schema evolution.

Subdocuments are often contrasted with references to separate documents. Related concepts include embedded documents, nested structures, fragments, and subfiles in certain document-generation systems.