how to join with _id as in ObjectId('dsfsdfsdfsdfdsffsdfsf') format

[inspeero]

Hey,
i have user collection and event collection so in event collection there is field userid which is normal string format such as.
User:
_id:ObjectId('123456'),

Events:
_id:ObjectId('qqqqqqqq'),
userId:'123456'

so for events join users how can i get user details using userid.