How to query parent based on subdocument's _id? - node.js

Question

consider the following records:
user record
{
"_id" : ObjectId("5234ccb7687ea597eabee677"),
"class" : [
{ "_id" : ObjectId("5234ccb7687ea597eabee671", "num" : 10, "color" : "blue" },
{ "_id" : ObjectId("5234ccb7687ea597eabee672", "num" : 100, "color" : "blue" }
]
}
this user has two class sub records, now I need a query that finds all users that have class property where "class._id" has a value of at least one users "class._id"
here is a more detail example:
suppose there is four user:
A:{_id:432645624232345,class:[{_id:123,name:'foo'}]}
B:{_id:432645624232555,class:[{_id:555,name:'foo'},{_id:123,name:'foo'}]}
C:{_id:432645344232345,class:[{_id:555,name:'foo'},{_id:111,name:'www'}]}
D:{_id:432644444232345,class:[{_id:222,name:'sss'},{_id:555,name:'www'},{_id:123,name:'foo'}]}
now if B login , I need to query all the user whose class subdocument contains at least one document which's _id==555 or _id==123 (555 and 123 come from B user), in this case the query result should be:
A:{_id:432645624232345,class:[{_id:123,name:'foo'}]} // match _id=123
B:{_id:432645624232555,class:[{_id:555,name:'foo'},{_id:123,name:'foo'}]} //match _id=123 and _id=555
C:{_id:432645344232345,class:[{_id:555,name:'foo'},{_id:111,name:'www'}]} //match _id=555
D:{_id:432644444232345,class:[{_id:222,name:'sss'},{_id:555,name:'www'},{_id:123,name:'foo'}]} ///match _id=123 and _id=555
which is all the user.
so far i get this:
{"class._id" : { $in : ["5234ccb7687ea597eabee671", "5234ccb7687ea597eabee672"] } }
but when different user login the class._id query condition is different. So is there any operator to do this
{"class._id" : { $in : req.user.class } }
hope I made myself clear.

Answer 1

In order to achieve what you want, first you must isolate the class _ids in an array, and then use it in the query argument.
var classIds = [];
var i = 0;
while (i < req.user.class.length) {
classIds.push(req.user.class[i]._id);
i++;
}
After that you can use classIds array in the query:
{"class._id" : { $in : classIds } }

Answer 2

The following query condition would give you all the users that have at least one class with id equal to any of the elements in the given array:
{"class._id" : { $in : ["5234ccb7687ea597eabee671", "5234ccb7687ea597eabee672"] } }
In the array for the $in clause you may provide any id's you needed , comma separated.
In addition, if you needed such, the below query condition should check for existence of nested document within "class" property that has a property "_id" :
{ "class._id" : { $exists : true } }
Both conditions should work no matter if "class._id" is a single-valued property or an array (mongo supports that).