- In MongoDB, relationships between documents are not as strictly defined as in SQL-based databases. Instead of using joins or foreign keys, MongoDB handles relationships by embedding documents or referencing documents. Here’s a detailed explanation of both types of relationships, with examples.
- One-to-One Relationship
- One-to-Many Relationship
- Many-to-Many Relationship
- A one-to-one relationship exists when one document in a collection is related to one document in another collection. This can be handled by embedding one document inside another or referencing the related document.
- Suppose you have a Users collection, and each user has a unique profile stored in the Profiles collection. You can embed the Profile document directly inside the User document.
{
"_id": 1,
"name": "John Doe",
"email": "john@example.com",
"profile": {
"age": 29,
"gender": "Male",
"address": "1234 Elm Street"
}
}
Example (Referencing):
- You could also keep the Profiles in a separate collection and reference the Profile document in the Users collection using the profile's ID.
// Users Collection:
{
"_id": 1,
"name": "John Doe",
"email": "john@example.com",
"profile_id": 101
}
// Profiles Collection:
{
"_id": 101,
"age": 29,
"gender": "Male",
"address": "1234 Elm Street"
}
- Here, the profile_id is used to reference the corresponding profile in the Profiles collection.
- In a one-to-many relationship, a single document is related to many documents in another collection. This is commonly used when a single entity has multiple associated entities.
- Suppose you have an Authors collection, and each author has multiple books. You can embed the books inside the author document.
{
"_id": 1,
"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
}
]
}
- Alternatively, you can store the books in a separate collection and reference them in the author document.
// Authors Collection:
{
"_id": 1,
"name": "J.K. Rowling",
"book_ids": [
101,
102
]
}
// Books Collection:
{
"_id": 101,
"title": "Harry Potter and the Sorcerer's Stone",
"year": 1997,
"author_id": 1
}
{
"_id": 102,
"title": "Harry Potter and the Chamber of Secrets",
"year": 1998,
"author_id": 1
}
- This creates a one-to-many relationship where an author can have multiple books.
- In a many-to-many relationship, multiple documents in one collection can relate to multiple documents in another collection. This is typically managed using references.
- Suppose you have a Students collection and a Courses collection, where students can enroll in multiple courses, and each course can have multiple students.
// Students Collection:
{
"_id": 1,
"name": "Alice",
"course_ids": [
101,
102
]
}
{
"_id": 2,
"name": "Bob",
"course_ids": [
101
]
}
// Courses Collection:
{
"_id": 101,
"course_name": "Math 101",
"student_ids": [
1,
2
]
}
{
"_id": 102,
"course_name": "Physics 101",
"student_ids": [
1
]
}
- In this case, a student can enroll in many courses, and a course can have many students. The course_ids field in the Students collection and the student_ids field in the Courses collection establish the many-to-many relationship.
- Embedding is preferred when:
- The relationship is tightly coupled (e.g., profile data inside a user document).
- The related data will always be retrieved along with the parent document.
- The embedded document is relatively small and unlikely to change often.
- Referencing is preferred when:
- The relationship is loosely coupled (e.g., orders and customers).
- The related data is large or frequently updated.
- You need to maintain data consistency and avoid duplication.
- MongoDB provides the $lookup operator to join collections similar to SQL joins in referenced relationships.
- If you want to retrieve an author along with their books using the referencing approach, you can use the $lookup operator.
db.authors.aggregate([
{
$lookup: {
from: "books", // The collection to join
localField: "book_ids", // Field from the authors collection
foreignField: "_id", // Field from the books collection
as: "books" // Output array field
}
}
])
- This will return an author with an embedded array of books.
- MongoDB provides flexibility in managing relationships between documents. Depending on your data model and use case, you can choose between embedding or referencing to design relationships efficiently. Understanding the trade-offs between the two approaches is key to building scalable and performant MongoDB applications.
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