How would you model a one-to-many relationship in MongoDB

Question

Can you tell me How would you model a one-to-many relationship in MongoDB?

Answer 1

In MongoDB, a one-to-many relationship can be modeled in multiple ways, depending on the use case and access patterns. The two most common approaches are embedding and referencing.

1. Embedding (Denormalization) - Best for Read-Heavy Operations

In this approach, related documents are stored within the parent document as an array.

Example: One author has many books (Embedded Approach)

{

  "_id": ObjectId("60f7c0b0c25f6d4d3c4b1e1f"),

  "name": "J.K. Rowling",

  "books": [

    { "title": "Harry Potter and the Sorcerer’s Stone", "year": 1997 },

    { "title": "Harry Potter and the Chamber of Secrets", "year": 1998 }

  ]

}

2. Referencing (Normalization) - Best for Write-Heavy & Large Collections

In this approach, related documents are stored separately, and the relationship is maintained using references (_id).

Example: Separate authors and books collections with references

// Authors Collection

{

  "_id": ObjectId("60f7c0b0c25f6d4d3c4b1e1f"),

  "name": "J.K. Rowling"

}


// Books Collection

{

  "_id": ObjectId("60f7c0b0c25f6d4d3c4b1e2a"),

  "title": "Harry Potter and the Sorcerer’s Stone",

  "year": 1997,

  "author_id": ObjectId("60f7c0b0c25f6d4d3c4b1e1f")

}

Query Example (Joining Data)

db.books.aggregate([

  {

    $lookup: {

      from: "authors",

      localField: "author_id",

      foreignField: "_id",

      as: "author_details"

    }

  }

])