Database
[{"_id":{"$oid":"61a9085af9733d0274c41990"},"myArray":[{"$oid":"61a9085af9733d0274c41991"},{"$oid":"61a9085af9733d0274c41992"},{"$oid":"61a9085af9733d0274c41993"}],"myField":{"$oid":"61a9085af9733d0274c41991"}/** < In 'myArray' collection*/},{"_id":{"$oid":"61a9085af9733d0274c41995"},"myArray":[{"$oid":"61a9085af9733d0274c41991"},{"$oid":"61a9085af9733d0274c41992"},{"$oid":"61a9085af9733d0274c41993"}],"myField":{"$oid":"61a9085af9733d0274c41994"}/** < Not in 'myArray' collection*/}]
Query
db.collection.aggregate([{"$match":{$expr:{"$in":["$myField","$myArray"]}}}])