How to Use Projection in MongoDB: Syntax and Examples

In MongoDB, projection is used to specify which fields to include or exclude in the query result. You add a projection document as the second argument to find() or findOne() to control the output fields.

Syntax

The basic syntax for using projection in MongoDB is:

db.collection.find(query, projection)

Here, query filters documents, and projection specifies fields to include (1) or exclude (0).

Note: You cannot mix inclusion and exclusion except for the _id field.

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db.collection.find({ filter }, { field1: 1, field2: 1, _id: 0 })

Example

This example shows how to find all users but only return their name and email fields, excluding the _id field.

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db.users.find({}, { name: 1, email: 1, _id: 0 })

Output

[
  { "name": "Alice", "email": "[e-mail]" },
  { "name": "Bob", "email": "[e-mail]" }
]

Common Pitfalls

Common mistakes when using projection include:

• Mixing inclusion and exclusion of fields (e.g., { name: 1, age: 0 }) which is not allowed except for _id.
• Forgetting that _id is included by default unless explicitly excluded.
• Using projection on nested fields incorrectly.

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/* Wrong: mixing inclusion and exclusion */
db.users.find({}, { name: 1, age: 0 })

/* Correct: only inclusion or exclusion */
db.users.find({}, { name: 1, email: 1, _id: 0 })

/* Or exclude fields only */
db.users.find({}, { password: 0, _id: 0 })

Quick Reference

Projection Usage	Description
{ field: 1 }	Include the field in the result
{ field: 0 }	Exclude the field from the result
{ _id: 0 }	Exclude the _id field (included by default)
Mixing 1 and 0 (except _id)	Not allowed, causes error
Projection as second argument	Used in find() or findOne()

Key Takeaways

Use projection as the second argument in find() to select fields.

Specify fields with 1 to include or 0 to exclude, but don't mix both.

The _id field is included by default; exclude it explicitly if needed.

Projection helps reduce data transfer by returning only needed fields.