MongoDB Relationships: Many-to-Many

What is a Many-to-Many Relationship?

A many-to-many relationship means that multiple documents in one collection can be associated with multiple documents in another collection.

For example, in a school database, students can enroll in many courses, and each course can have many students.

• Each student can be linked to multiple courses.
• Each course can be linked to multiple students.

Modeling Many-to-Many Relationships in MongoDB

MongoDB does not support joins like relational databases, so many-to-many relationships are modeled differently.

There are two common approaches: embedding references and using a linking collection.

Example: Students and Courses

Let's see how to implement many-to-many relationships using both approaches with a students and courses example.

Querying Many-to-Many Relationships

Querying many-to-many relationships depends on the chosen modeling approach.

Using references requires multiple queries or aggregation pipelines to fetch related data.

With a linking collection, you query the linking documents and then fetch related entities.

• Use $lookup aggregation stage to perform joins in MongoDB 3.2+.
• Index foreign key fields for better performance.
• Limit the size of embedded arrays to avoid document growth issues.