Presuming your model is called Employee:
Employee.find({ "departments.0": { "$exists": true } },function(err,docs) {
})
As $exists asks for the 0 index of an array which means it has something in it.
The same applies to a maximum number:
Employee.find({ "departments.9": { "$exists": true } },function(err,docs) {
})
So that needs to have at least 10 entries in the array to match.
Really though you should record the length of the array and update with $inc every time something is added. Then you can do:
Employee.find({ "departmentsLength": { "$gt": 0 } },function(err,docs) {
})
On the “departmentsLength” property you store. That property can be indexed, which makes it much more efficient.