How to handle Many to Many relationship in mongoDB?

MongoDB is a NoSQL database, typically used for storing JSON-formatted documents. Although it differs from traditional relational databases (such as MySQL or PostgreSQL) in data structure, MongoDB can effectively handle many-to-many relationships. Handling many-to-many relationships primarily involves two strategies: embedded documents and references.

1. Embedded Documents

In MongoDB, we can handle many-to-many relationships by embedding documents. This involves embedding an array of related documents directly within a single document.

Advantages:

• High query efficiency, as all related data resides within the same document.

Disadvantages:

• If embedded data changes frequently, it may cause the document to grow repeatedly.
• It may lead to document size exceeding MongoDB's document size limit (16MB).

Example: Assume a movie database where movies and actors have a many-to-many relationship. A movie can feature multiple actors, and an actor can appear in multiple movies.

json
{
  "title": "The Matrix",
  "year": 1999,
  "actors": [
    { "name": "Keanu Reeves", "role": "Neo" },
    { "name": "Laurence Fishburne", "role": "Morpheus" },
    { "name": "Carrie-Anne Moss", "role": "Trinity" }
  ]
}

2. References

Another approach is to use references, where a document stores a reference (typically an ID) to another document.

Advantages:

• More flexible, allowing for handling complex data relationships and frequently changing data.
• Avoids issues of oversized single documents.

Disadvantages:

• Queries may require multiple operations across different collections, impacting performance.

Example: Continuing with the movie and actor example, we store movies and actors in separate collections.

json
// movies collection
{
  "_id": ObjectId("507f191e810c19729de860ea"),
  "title": "The Matrix",
  "year": 1999
}

// actors collection
{
  "_id": ObjectId("507f191e810c19729de860eb"),
  "name": "Keanu Reeves",
  "movies": [
    ObjectId("507f191e810c19729de860ea")  // Reference to The Matrix
  ]
}

Conclusion

Choosing between embedded documents and references primarily depends on specific application scenarios, data structure, and frequency of data changes. Generally, if query operations far outnumber write operations and data relationships are relatively stable, embedded documents may be preferable. Conversely, if the system handles numerous write operations or data relationships are highly complex and frequently changing, using references is more appropriate.