文档模型关系
Model One-to-One Relationships with Embedded Documents
Overview
Data in MongoDB has a flexible schema. Collections do not enfoce document structure.
Pattern
In the normalized data model, the address document contains a reference to the patron document.
If the address data is frequently retrivevd with name information, then with referencing, your application needs to issue multiple queries to resolve the reference. The better data model would to be embed the address data in the patron data, as in the following document:
Model One-to-Many Relationships with Embedded Documents
Pattern
Consider the following example that map patron and multiple address relationships. In this one-to-many relationships between patron and address data, the patron has multiple address entities.
In the normalized data model, the address documents contain a reference to the parton document:
If your application frequently retrieves the address data with name information, then your application needs to issue multiple queries to resolve the references. A more schema would to embed the address data entities in the patron data, as in the following document:
Model one-to-Many Relationships with Document References
Pattern
Consider the following example that maps publisher and book relationships. The example illustrates the advantage of referencing over embedding to avoid repetition of publisher information.
Embeddding the publisher document inside the book document would lead to repetion of publisher data as the following documents show:
To avoid repetion of the publisher data, use references and keep the publisher information in separate collection from the book collection.
When using references, the growth of the relationships determine where to store the reference. If the number of books per publisher is small with limited growth, storing the book references inside the publisher document may sometimes be useful. Otherwise, if the number of books per publihser is unbounded, this data model would lead to mutable, growing arrays, as in the following example:
To avoid mutable, growing arrays, store the publisher reference inside the book document:
