Mongoose: Search for documents with specified id in the populated document

in Mongoose i have two Schemas, which are linked using Mongoose populate:

var SystemSchema = new Schema({
    name                    : {type : String, trim : true}
    , hosts                 : [{ type: ObjectId, ref: 'Host' }]
});

var HostSchema = new Schema({
    name                    : {type : String, trim : true}
});

My question: I have a given ObjectId of a host and now want to see which which Systems do have a reference to that host.

I do this because i normally do access only the SystemsSchema and want to populate the Hosts, but i was asked if i can check the Hosts for Systems as well. Now i tried to simply search and not to populate from Hosts to Systems, but i'm a little stuck.

One of my attempts was to search with where, but this doesn't succeed:

System.where('hosts').in(['4fabca804c9d76ac0b000022']).run(function(err, hosts) {
        console.log(hosts);
    });

Is there any way to search like this? Please do not tell me to simply put the HostSchema as subdocuments to the SystemSchema, this here is just a simplified example.

Thanks in advance!

Best Regards, Uli