Relationships & References

// Embedded (few comments per post)
{
  "_id": 1,
  "title": "Learn MongoDB",
  "comments": [
    { "user": "Bob", "text": "Helpful!" }
  ]
}

// Referenced (many orders per user)
// users collection
{ "_id": "user1", "name": "Alice" }

// orders collection
{ "_id": "order1", "userId": "user1", "total": 99.99 }
{ "_id": "order2", "userId": "user1", "total": 49.99 }

// Query with $lookup
db.users.aggregate([
  { $lookup: {
    from: "orders",
    localField: "_id",
    foreignField: "userId",
    as: "orders"
  }}
])

// Students and Courses — many-to-many

// students collection
{
  "_id": "s1",
  "name": "Alice",
  "courseIds": ["c1", "c2", "c3"]
}

// courses collection
{
  "_id": "c1",
  "title": "Web Development",
  "studentIds": ["s1", "s2"]
}

// Find all courses for a student
db.courses.find({ studentIds: "s1" })

// Find all students in a course
db.students.find({ courseIds: "c1" })