Why should you care?

After schema changes, some users have phone and some do not. Migrations and APIs need $exists to find incomplete profiles.

See it live — copy this example

Open mongosh or MongoDB Compass, select database nosqlverse, then run the example. Change one field and run again.

db.users.insertMany([
  { username: "a", email: "a@x.com", phone: "98..." },
  { username: "b", email: "b@x.com" },
  { username: "c", email: 1001 }
])
db.users.find({ phone: { $exists: true } })
db.users.find({ email: { $type: "string" } })
db.users.find({ email: { $type: "int" } })

Run Example »

Edit the code below and click Run to see the result in Toolliyo’s live editor.

What happened?

• First query returns users with a phone field.
• Second keeps string emails (a and b).
• Third finds the bad document where email was stored as an int.
• $type protects query correctness.

Practice next

1. Insert the three users.
2. Find missing phone with phone: { $exists: false }.
3. Fix user c with updateOne $set email string.
4. Find documents where address.city $exists true.
5. Query $type: "date" on joinedAt.

Remember

$exists checks if a field is present. $type checks BSON type. Use them for data cleanup and validation.

KYC incomplete list

Compliance finds users where pan: { $exists: false } to send reminder emails.

Outcome: Only incomplete profiles are nudged.