MongoDB (Lesson 12 field judgment)

3.4.2.7 determine if a field exists

Use "$exists" to determine if a field exists, or if it is set to true, false to indicate that it does not exist.

Example : Querying for data with parents members

Db.students.find ({"Parents": {"$exists": True}}). Pretty ()

Example: querying for data that does not have course members

Db.students.find ({"course": {"$exists": false}}). Pretty ()

You can use this type of query to do some filtering of unwanted data.

MongoDB (Lesson 12 field judgment)