MongoDB Relationships

Introduction to MongoDB Relationships

In MongoDB, the representation of how the number of multiple documents are connected logically to each other is known as MongoDB Relationships. There are two ways to create such relationships, which are Embedded and Referenced methods respectively. Embedded Relationships and Document Referenced Relationship, which have their ways to work with multiple types of relationships are implemented.

While Embed works great with One-to-One and One-to-Many relationships, the referenced is good for Many-to-Many relationships. These two types of relationships are also know as Denormalization, which is Embedded while Reference relationships are known as Normalization. Establishing relationships between documents can help in refining the database structure and also work in favor to develop the performance and make execution time shorter.

Types of MongoDB Relationships

Here we have two basic types that determine the relationship, Embedded or Reference. These types are distinguished based on how they are connected.

1. Embedded Relationships

Simply Stating, when we attempt to embed a BSON document, with any other document, it is known as Embedded Relationships in MongoDB. With Embedded Relationships, there are two sub relationships.

Given below are the sub relationships:

a. One-to-one relationship: This is the simplest of all relationship. Here, we have a one parent document and one child document for the parent, thats one to one.

Example:

Here, we will insert a document, which will key, with another sub document, a child document.

Code:

db.fun.insert({"Name":"Sulaksh", "Age":25,"Add":{"City":"Pune","State":"MA"}})

Above query will insert a document in fun collection, with details as mentioned and Add key will have a child document. Let’s retrieve our document and check. Upon executing find query, return will be as shown in the screenshot below:

Output:

As you can see, we have a parent document which holds some data and a sub document. The main document is the parent and sub document here is the child.

b. One-to-many relationship: To simply put it, One to Many relationship consist of one parent with multiple child documents. Similar to one to one but with many child documents. Establishing relationship using embedded method can help in reducing the number of read operations performed to retrieve data.

Code:

db.fun.insert(
{"Name": "Ajay",
"Age": 25,
"Add":
[
{"City": "Pune"},
{"State": "MH"}
] })

Above query will insert a new record with Name and Age, but with Add, it will insert two sub documents. In this example, City and State will be our many relationships to the one.

Output:

As you can see, we have sub documents in the key of add. Which is to state, One to Many relationship. Simplest example of one to many relationship can be an employee with multiple addresses or multiple phone numbers. It is usual that an employee might have multiple addresses or phone numbers and mongodb relationship helps us to save such complicated data in documents with ease.

One of the major advantage of creating Embedded Relationships in MongoDB is that it executes the queries faster, compared to referenced relationship. This relationship also boost the performance and the results are fetched quickly. This applies for large datasets too.

2. Document Referenced Relationships

Instead of inserting child document inside a parent document, we create two separate documents with parent and child relationship. So we have a parent document which has some data and then we have child documents, inserted separately in different collection but with a single connection with key from parent document. Implementing this method of creating relationships helps in reductions of errors in data and is useful in maintaining consistency with the data.

3. Many to Many Relationships

Many to Many relationship, is a mongodb relationship, where any two entities inside a document can have multiple relations with each other. Basic example of many to many relationship can be Companies and Clients. A Company can or might have multiple clients for various projects and tasks, while a individual client can have business relations with multiple companies.

Example:

Here we will implement an example with many to many relationship. For this, we will have two separate collections with names as Authors and Books respectively.

Code:

db.authors.insert( { _id : 4, authorname : "R Tagore" } )

Above query will insert a single document, inside authors collection, which is our parent collection. We have use “_id” here, which we will use as foreign key for child collection.

Code:

db.books.insert( {
_id : 9,
book_name : "The Post Office",
cat : [ "fiction", "play"],
artist_id : 4
})

Above query will insert a new document into our child collection, which artist_id, in connection with parent collection’s _id. Insert multiple such documents and then we will execute a aggregation pipeline in order to fetch these records from two separate collections.

After inserting multiple documents, our collection will look like:

Output:

db.books.find()

4. Aggregation Pipeline

It is simply a collection of commands to be executed one by one in a sequence. We will create a pipeline to return documents from books collection, where id will be local and artist_id will be foreign fields. Finally we will add a $match operator, which will only return documents which have author name as R Tagore in authors collections.

Code:

db.authors.aggregate( [
{ $lookup:
{ from: "books",
localField: "_id",
foreignField: "artist_id",
as: "All Books" } },
{ $match : { authorname : "R Tagore" }
}]).pretty()

Output for the above query will be the list of all the documents that has author name as R Tagore in books collection.

Output:

As you can see, our pipeline has returned results as expected. We have implemented many to many relationship, among books and authors.

Conclusion

MongoDB has two types of relationships. Embedded and Reference made. Every relationships has its advantages and usages. These relationships helps in improving performance. Based on situation it implements any of the relationship as One to One, One to Many or Many to Many. Implementing these relationship has resulted in betterment with data consistency.

Recommended Articles

This is a guide to MongoDB Relationships. Here we discuss the introduction and different types of MongoDB relationships respectively. You may also have a look at the following articles to learn more –

1. MongoDB Administration
2. MongoDB findAndModify
3. Indexes in MongoDB
4. MongoDB Limit()