In MongoDB, every time we query for a document, the document returned contains all the fields by default. However, in most real-life applications, we may only want a few fields in the resulting documents.

In MongoDB queries, we can either include or exclude specific fields from the result. This technique is called projection.

Projection is expressed as a second argument to the find() or findOne() functions. In the projection expression, we can explicitly exclude a field by setting it to 0 or include one by setting it to 1.

For example, the following query finds all the products that belong to the brand Apple and returns only the price and soldQty fields:

const select = async () => {
  const result = await Product.find(
    { brand: "Apple" },
    { price: 1, soldQty: 1 }
  )

  console.log(result)
}

select()

Upon executing the query, we get the following result:

[
   {
     _id: new ObjectId("61a32cf04a960cdc89a35a89"),
     price: 305.7,
     soldQty: 244
   },
   {
     _id: new ObjectId("61a32cf04a960cdc89a35a8a"),
     price: 1499.99,
     soldQty: 110
   }
 ]

Here, we have only specific fields in the result. However, the _id field is still visible, even though it was not specified. That is because the _id field is included by default in the resulting documents. If you do not want it to be present in the result, you must exclude it explicitly:

const select = async () => {
  const result = await Product.find(
    { brand: "Apple" },
    { price: 1, soldQty: 1, _id: 0 }
  )

  console.log(result)
}

select()

The preceding query specifies that the _id field should be excluded from the result. When executed, we get the following output, which shows that the _id field is absent from all documents:

[ { price: 305.7, soldQty: 244 }, { price: 1499.99, soldQty: 110 } ]

It is important to note the three behaviors of field projections, listed as follows:

• The _id field will always be included, unless excluded explicitly.
• When one or more fields are explicitly included, the other fields (except _id) get excluded automatically.
• Explicitly excluding one or more fields will automatically include the rest of the fields, along with _id.