MongoDB db collection linking

Question

Suppose I have two collection in a mongoDB

students:

{_id: 1, name: "sa", teachers:[1,2,3]}
{_id: 2, name: "sb", teachers:[1,3]}

teachers:

{_id:1, name: "ta"}
{_id:2, name: "tb"}
{_id:3, name: "tc"}

now I want to query in students collection through teachers name. Like this:

db.students.find({'teachers.name':"ta"}).count()

I have read somewhere that it is possible to link collection or embed it. Is there any way to do it?

what have I tried? I have tried db.students.ensureIndex({'teachers':1}) but it does not work. I also think it should not work. I am going out of clue how to do it?

DUPLICATE: Well I know that there are a lot of post which has similar title, but yet I am confused!

Sammaye · Accepted Answer · 2013-02-11 14:22:28Z

1

Have you looked into how relational models are done with MongoDB?

I am unsure why you think ensureindex will do anything for you here. There is no way to "link" collections, MongoDB is a non-relational database. There is also no way that a query can be defined as querying two collections separately using sub selects at the moment.

The main problem with embedding here is that you have a many to many relationship:

A student has many teachers
A teacher has many students

Which could create a unperformant scenario with updating here.

The best, right now, off the top of my head is to actually do the JOIN client side like so:

var teachers = [];
db.teachers.find({'name':"ta"}).forEach(function(doc){ 
    teachers[teachers.legnth-1] = doc._id; 
});
db.students.find({teachers: {$in: teachers}});

answered Feb 11, 2013 at 14:22

Sammaye

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1 Comment

Remon van Vliet Over a year ago

+1. teachers = db.teachers.distinct("_id", {'name':"ta"}); db.students.find({teachers: {$in: teachers}}); reads a bit better perhaps and is more bandwidth friendly

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