Mongoose findOne by reference returns null - node.js

Question

I have some events in the event collection and some users in the users collection.
I have a mapping of events and users in another collection. The event_id field in that collection is a reference to the event collection(ObjectId). But when I search schema using the following command I get null as a response
db.eventusers.findOne({event_id :'57988cd30e9811750324c080'})
returns null
on the other hand when i search using user field which is not a reference, just a string containing user id, I get the result as follows.
db.eventusers.findOne({user_id :"578cdcdd56eaec041b6caf3e"})
{
"_id" : ObjectId("57988d190e9811750324c081"),
"created_at" : "1469615385595",
"updated_at" : "1469615618502",
"user_id" : "578cdcdd56eaec041b6caf3e",
"event_id" : ObjectId("57988cd30e9811750324c080"),
"deleted" : false,
"invited" : false,
"host" : true,
"status" : -1,
"__v" : 0
}

Answer 1

I have found out the solution. If the field was defined in Mongoose Schema as Schema.Types.ObjectId then in the Query id should be ObjectId rather than String. So instead of query {event_id :'57988cd30e9811750324c080'} it should be {event_id :ObjectId('57988cd30e9811750324c080')}:
db.eventusers.findOne({event_id :ObjectId('57988cd30e9811750324c080')})
{
"_id" : ObjectId("57988d190e9811750324c081"),
"created_at" : "1469615385595",
"updated_at" : "1469615618502",
"user_id" : "578cdcdd56eaec041b6caf3e",
"event_id" : ObjectId("57988cd30e9811750324c080"),
"deleted" : false,
"invited" : false,
"host" : true,
"status" : -1,
"__v" : 0
}