MongDB Query with DBRef



General syntax for mongodb find query is 

db.collection.find({<here comes the find conditin>})

for example : if i want to find a student with email address student@email.com , query will be like

db.students.find({"email":"student@email.com"})

where students is a collections. 

Now let us say this student belongs to a certain group and we want to find all students of the group to which the said student belongs. We will be using the DBRef for that group, following will be our query

db.students.find({group:DBRef("groups", ObjectId("52a852a844aeb67db4a3518b"))}).pretty()

this query will finds all student who belong to group with object id = ObjectId("52a852a844aeb67db4a3518b")