How to create a MongoDB Projection?

In this post, we will learn how to create a MongoDB Projection. Basically, projections allow us to retrieve selective fields of a document while querying a collection.

If you are new to MongoDB, you can check out this detailed post about MongoDB CRUD Operations. This is because the CRUD operations form the basis for other complicated operations.

1 – What is a MongoDB Projection?

In MongoDB, we have collections. Collections comprise of documents. And within those documents, we have key/value pairs. Basically, you can think of the keys as fields of a document.

Projection helps find selective data from the documents. This is incredibly useful because we will usually have a huge number of records in a typical enterprise environment. If we process all these documents along with all the fields within them, it may create a huge load on network bandwidth. More often than not, we may be only interested in a subset of those fields for fulfilling some requirement. MongoDB projections are a great way to handle such a scenario.

Projections also use the find() function. See below syntax format.

db.<collection_name>.find({},{FIELD_NAME:BOOLEAN})

As you can see, this is exactly the same find() function syntax. Only additional is the boolean parameter for field name. This parameter determines whether a particular field should be displayed or not.

2 – Writing a MongoDB Projection

Let us imagine that we have some data in a Books collection as below:

{
	"_id" : ObjectId("61cd54a29f57d2539bb251fe"),
	"bookName" : "The Eye of the World",
	"authorName" : "Robert Jordan",
	"price" : "12"
}
{
	"_id" : ObjectId("61cd54b69f57d2539bb251ff"),
	"bookName" : "The Great Hunt",
	"authorName" : "Robert Jordan",
	"price" : "13"
}
{
	"_id" : ObjectId("61cd54c89f57d2539bb25200"),
	"bookName" : "The Dragon Reborn",
	"authorName" : "Robert Jordan",
	"price" : "11"
}

Now, if we wish to only fetch the bookName for all the documents, we can use a projection.

See below example:

> db.books.find({}, {"bookName": 1}).pretty()
{
	"_id" : ObjectId("61cd54a29f57d2539bb251fe"),
	"bookName" : "The Eye of the World"
}
{
	"_id" : ObjectId("61cd54b69f57d2539bb251ff"),
	"bookName" : "The Great Hunt"
}
{
	"_id" : ObjectId("61cd54c89f57d2539bb25200"),
	"bookName" : "The Dragon Reborn"
}

As you can see, the output only contains the _id and the bookName field.

Basically, we pass a second argument which is also a JSON object. In the argument, we specify the field name and the boolean value for the same. Here, 1 means that we want the field to be in the projection. The default value of a field when we use projection is 0. Therefore, all other fields are automatically excluded from the projection.

3 – The _id Field

The _id field is a special field. Basically, this field is inserted by MongoDB for every document. It is also part of the projection by default. In other words, if we don’t specify anything, the _id field will be present in the output.

If we also want to hide the _id field, we need to pass boolean 0 for the same. See below example:

> db.books.find({}, {"_id": 0, "bookName": 1}).pretty()
{ "bookName" : "The Eye of the World" }
{ "bookName" : "The Great Hunt" }
{ "bookName" : "The Dragon Reborn" }

In above snippet, we set the boolean for _id as false. The output only contains the bookName field.

4 – Conclusion

With this, we have successfully learnt how to create a MongoDB projection and how to include fields in a projection.

If you have any comments or queries about the post, please mention in the comments section below.