Challenge 3: Write a Projected Query — Possible Solution
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db.users.find({}, { email: 1, _id: 0 })

WHY THIS WORKS AS AN ANSWER
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The first argument, {}, is an empty filter — meaning "match every
document in the collection," since the challenge didn't ask for any
particular subset of users, only a particular SHAPE of the results.

The second argument is the projection, and it needs both pieces to
satisfy the challenge fully:
  - email: 1 includes the email field in the results.
  - _id: 0 is REQUIRED here, not optional — MongoDB includes _id in
    projection results by default even when it isn't explicitly
    listed, so without this explicit exclusion, every returned
    document would still carry its _id field alongside email, which
    the challenge specifically said not to include.

This is the one asymmetry in MongoDB's projection rules worth
remembering: every OTHER field is excluded automatically once you
start including specific fields, but _id is the one exception that
requires an explicit 0 to leave out.