Projections — MongoDB Technical Interview Questions & Answers

Choosing which fields find() returns.

Projections

  • Simple meaning: Projection picks which fields come back, so you do not download the whole document.
  • How to use: db.students.find({ city: "Pune" }, { name: 1, marks: 1, _id: 0 }). 1 = include, 0 = exclude.
  • You usually cannot mix include and exclude except for _id.
  • Try this: find with only name. Check _id still appears unless _id: 0.
  • Common mistake: { name: 1, city: 0 } mixed projection error.
  • Interview tip: Second argument of find. _id is included unless you exclude it.

Technical Interview Tip: Show { name: 1, _id: 0 }.

Code Examples for Projections

Projections — working example

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

Projections — projection (2nd argument) includes (1) or excludes (0) fields — do not mix include/exclude except for _id.

How to Explain Projections to an Interviewer

How do you hide _id?

Add _id: 0 in the projection