mongodbHow to create a many to many relation in MongoDB?

MongoDB supports many-to-many relationships through the use of embedded documents and document references. To create a many-to-many relationship in MongoDB, we need to create two collections, one for each entity in the relationship. For example, if we have a users collection and a groups collection, we can create a many-to-many relationship between them by embedding a groups array in each user document and a users array in each group document.

Example code

// Create users collection
db.createCollection("users")

// Create groups collection
db.createCollection("groups")

// Insert a user
db.users.insert({
  name: "John Doe",
  groups: []
})

// Insert a group
db.groups.insert({
  name: "Group A",
  users: []
})

// Add user to group
db.groups.update(
  { name: "Group A" },
  { $push: { users: { $ref: "users", $id: ObjectId("5f3f9f9f9f9f9f9f9f9f9f9f") } } }
)

// Add group to user
db.users.update(
  { name: "John Doe" },
  { $push: { groups: { $ref: "groups", $id: ObjectId("5f3f9f9f9f9f9f9f9f9f9f9f") } } }
)

Output example

WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

Code explanation

• db.createCollection("users"): This creates a collection called users in the current database.

• db.users.insert({ name: "John Doe", groups: [] }): This inserts a document into the users collection with the name John Doe and an empty groups array.

• db.groups.insert({ name: "Group A", users: [] }): This inserts a document into the groups collection with the name Group A and an empty users array.

• db.groups.update({ name: "Group A" }, { $push: { users: { $ref: "users", $id: ObjectId("5f3f9f9f9f9f9f9f9f9f9f9f") } } }): This updates the groups collection by pushing a reference to the users collection with the _id of the user document to the users array in the Group A document.

• db.users.update({ name: "John Doe" }, { $push: { groups: { $ref: "groups", $id: ObjectId("5f3f9f9f9f9f9f9f9f9f9f9f") } } }): This updates the users collection by pushing a reference to the groups collection with the _id of the group document to the groups array in the John Doe document.

Helpful links

• MongoDB Documentation - Relationships

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