Which approach is better in terms of performance while querying mongodb?

Question

I have two collections. One is students and the other is books . Students may select up to 100 books. So, I may design students collection as

{
'_id'=>'',
 selections['bookid1', 'bookid2', 'bookid3' ... so on]
}

And another solution would be a selections collections

{
'_id'=>''
'book_id'=>'',
'student_id'=>''
 }

Which design is better in terms of query performance. Is there any other reason to choose one over the other?

Answer 1

Mongo does not support join. When you are modeling your data there is tips in mongo docs. The references in mongo go thought databases but isn't a good approach. The preference is use manual reference. For instance

{
  'isbn': '',
  'title': ''
}

{
'name': '',
'books': [isbn numbers... ]

}

http://docs.mongodb.org/manual/core/data-modeling/

http://docs.mongodb.org/manual/reference/database-references/