Projection in MongoDB

Projections in MongoDB are a way to transfer only the required data from a MongoDB server, rather than fetching the entire data and then filtering it.

This helps in reducing the load to the network and the backend applications, along with a faster data transfer.

For example, we have the below database:

hospital> db.patients.find()
output> [
  {
    _id: ObjectId("61780b10d9a6a5af8bddb297"),
    firstname: 'Aditya',
    lastname: 'Gupta',
    age: 25,
    history: [
      { disease: 'cold', treatment: 1 },
      { disease: 'xyz', treatment: 45 }
    ]
  },
  {
    _id: ObjectId("61780b10d9a6a5af8bddb299"),
    firstname: 'Robin',
    lastname: 'Sharma',
    age: 55,
    history: [ { disease: 'sneeze', treatment: 4 } ]
  },
  {
    _id: ObjectId("6181317f0802aea7b1828646"),
    firstname: 'Mark',
    lastname: 'Manson',
    age: 50,
    history: [ { disease: 'fever', treatment: 45 } ]
  }
]

Suppose, we want the names of people older than 30. Then, we can do:

hospital> db.patients.find({age: {$gt: 30}}, {firstname: 1})
output> [
  { _id: ObjectId("61780b10d9a6a5af8bddb299"), firstname: 'Robin' },
  { _id: ObjectId("6181317f0802aea7b1828646"), firstname: 'Mark' }
]

As you can notice, in the second parameter firstname: 1 means to include only the firstname in the result. But _id is also there.

This is because _id is by default present in the output of a projection and needs to be explicitly excluded.

hospital> db.patients.find({age: {$gt: 30}}, {firstname: 1, _id: 0})
output> [ { firstname: 'Robin' }, { firstname: 'Mark' } ]

Another example, if we need patients having cold as one of the diseases:

hospital> db.patients.find({"history.disease": "cold"}, {firstname: 1, _id: 0})
output> [ { firstname: 'Aditya' } ]

The find function first filters the data and then filters the columns on the server itself before sending the data to the backend application through the network.

The above examples are just a brief introduction to the capabilities of projections. To dive deeper into the topic, refer to the official documentation.

Thanks for reading.