Adding Referenced Documents in MongoDB

MongoDB is a popular NoSQL database that is known for its flexible and scalable document-oriented data model. One of the key features of MongoDB is the ability to store complex and hierarchical data structures, such as nested documents and arrays, within a single document. This can be useful in many situations, including when you need to store a reference to a document from another collection in a separate document.

In this article, we will discuss how to add a document from one collection to another document in another collection in MongoDB.

 

Storing a Reference to a Document

In MongoDB, you can store a reference to a document from one collection in another document by saving the _id of the referenced document in a field. For example, if you have a collection of posts and a collection of users, you can store a reference to a post document in a user document by adding the _id of the post document to a field, such as posts, in the user document.

Here is an example of how you can store a reference to a post document in a user document using the Mongoose library:

// Create a new post document
const post = new Post({ title: 'Hello World' });

// Save the post document to the posts collection
post.save((error, post) => {
  if (error) return console.error(error);

  // Update the user document with the post _id
  User.updateOne({ _id: userId }, { $push: { posts: post._id } }, (error) => {
    if (error) return console.error(error);
  });
}); 

In this example, the new post document is created and saved to the posts collection. Then, the _id of the post document is added to the posts field of the user document in the users collection using the updateOne method.

 

Populating the Referenced Document

Once you have stored a reference to a document in another document, you can retrieve the referenced document and all of its data using the populate method. The populate method retrieves the referenced document and adds it to the original document as a new field.

Here is an example of how you can retrieve the referenced post document in a user document using the Mongoose library:

User.findOne({ _id: userId }).populate('posts').exec((error, user) => {
  if (error) return console.error(error);
  console.log(user);
});

In this example, the findOne method is used to retrieve the user document with the specified _id. Then, the populate method is used to retrieve the referenced post document and add it to the user document as a new field named posts.

 

Conclusion

In this article, we discussed how to add a document from one collection to another document in another collection in MongoDB. By storing a reference to a document in another document, you can store complex data structures and retrieve the referenced document and all of its data using the populate method. This can be a useful feature in many situations, including when you need to store relationships between documents in different collections.