One-to-One, One-to-Many, Many-to-Many Relationships in MongoDB

One-to-One, One-to-Many, Many-to-Many Relationships in MongoDB

In traditional relational databases, relationships between tables are created using foreign keys. MongoDB, being a NoSQL database, doesn't use foreign keys or joins in the same way, but it still allows you to model relationships using embedded documents or references.

One-to-One Relationship

In a one-to-one relationship, one document is associated with exactly one other document. For example, a user might have one profile.

Example: A user has one profile document containing extra details like bio and location.

Option 1: Embedding

    db.users.insertOne({
      name: "Alice",
      email: "alice@example.com",
      profile: {
        bio: "Web Developer",
        location: "New York"
      }
    });
    

Output:

    {
      acknowledged: true,
      insertedId: ObjectId("...")
    }
    

This approach is ideal when profile data is only used with the user and isn’t large or frequently updated independently.

Option 2: Referencing

    // Insert into profiles
    const profileId = db.profiles.insertOne({
      bio: "Web Developer",
      location: "New York"
    }).insertedId;

    // Insert into users
    db.users.insertOne({
      name: "Alice",
      email: "alice@example.com",
      profileId: profileId
    });
    

Output:

    {
      acknowledged: true,
      insertedId: ObjectId("...")
    }
    

Use referencing when the profile might be reused, updated often, or stored separately due to size.

Q: Should I embed or reference profile info?

A: Embed it if it's tightly coupled and small. Reference it if it's large, reused, or separately managed.

One-to-Many Relationship

In a one-to-many relationship, one document is associated with many others. For example, a blog post can have many comments.

Option 1: Embedding (Good for small, infrequent updates)

    db.posts.insertOne({
      title: "MongoDB Relationships",
      content: "Understanding how to model data...",
      comments: [
        { user: "John", text: "Great post!" },
        { user: "Sarah", text: "Very helpful, thanks!" }
      ]
    });
    

Output:

    {
      acknowledged: true,
      insertedId: ObjectId("...")
    }
    

Embedding comments works well when they are small and usually retrieved together with the post.

Option 2: Referencing (Better for high-volume or independent access)

    const postId = db.posts.insertOne({
      title: "MongoDB Relationships",
      content: "Understanding how to model data..."
    }).insertedId;

    db.comments.insertMany([
      { postId: postId, user: "John", text: "Great post!" },
      { postId: postId, user: "Sarah", text: "Very helpful, thanks!" }
    ]);
    

Output:

    {
      acknowledged: true,
      insertedIds: { "0": ObjectId("..."), "1": ObjectId("...") }
    }
    

Use referencing when there are many comments or they need to be queried or paginated separately.

Q: Why not always embed?

A: Embedding leads to document growth. MongoDB documents have a 16MB size limit, and large embedded arrays can hurt performance.

Many-to-Many Relationship

In a many-to-many relationship, multiple documents relate to multiple others. Example: Students can enroll in multiple courses, and each course can have many students.

Model with Cross-References

    const student1 = db.students.insertOne({ name: "Amit" }).insertedId;
    const student2 = db.students.insertOne({ name: "Priya" }).insertedId;

    const course1 = db.courses.insertOne({ title: "Maths" }).insertedId;
    const course2 = db.courses.insertOne({ title: "Physics" }).insertedId;

    db.enrollments.insertMany([
      { studentId: student1, courseId: course1 },
      { studentId: student1, courseId: course2 },
      { studentId: student2, courseId: course2 }
    ]);
    

Output:

    {
      acknowledged: true,
      insertedIds: {
        "0": ObjectId("..."),
        "1": ObjectId("..."),
        "2": ObjectId("...")
      }
    }
    

This pattern is called a junction collection, where we create an explicit mapping between two collections.

Q: Can we embed courses inside students?

A: Not if the relationship is many-to-many. Embedding would lead to data duplication and inconsistency.

Summary

• One-to-One: Embed if small, reference if reused or large.
• One-to-Many: Embed if small and tightly bound, reference if large or accessed separately.
• Many-to-Many: Always use referencing with a linking/junction collection.

Understanding these relationship patterns helps you design efficient, scalable, and maintainable MongoDB schemas.