Querying and grouping in mongoDb? - mongodb

Question

Part 1:
I have (student) collection:
{
sname : "",
studentId: "123"
age: "",
gpa: "",
}
im trying to get only two keys from it :
{
sname : "",
studentId: "123"
}
so i need to eliminate age and gpa to have only name and studentId , how could i do that ?
Part2:
Then I have 'subject' collection :
{
subjectName : "Math"
studentId : "123"
teacherName: ""
}
I need to match/combine the previous keys (in part1) with the correct studentId so I will end up with something like this :
{
sname : "",
studentId: "123",
subjectName : "Math"
}
How can i do this and is that the right way to think to get the result? i tried to read about group and mapReduce but i didnt find a clear example.

Answer 1

To answer your first question, you can do this:
db.student.find({}, {"sname":1, "studentId":1});
The first {} in that is the limiting query, which in this case includes the entire collection. The second half specifies keys with a 1 or 0 depending on whether or not you want them back. Don't mix include and excludes in a single query though. Except for a couple special cases, mongo won't accept it.
Your second question is more difficult. What you're asking for is a join and mongo doesn't support that. There is no way to connect the two collections on studentId. You'll need to find all the students that you want, then use those studentIds to find all the matching subjects. Then you'll need to merge the two results in your own code. You can do this through whatever driver you're using, or you can do this in javascript in the shell itself, but either way, you'll have to merge them with your own code.
Edit:
Here's an example of how you could do this in the shell with the output going to a collection called "out".
db.student.find({}, {"sname":1, "studentId":1}).forEach(
function (st) {
db.subject.find({"studentId":st.studentId}, {"subjectName":1}).forEach(
function (sub) {
db.out.insert({"sname":st.sname, "studentId":st.studentId, "subjectName":sub.subjectName});
}
);
}
);
If this isn't data that changes all that often, you could just drop the "out" collection and repopulate it periodically with this shell script. Then your code could query directly from "out". If the data does change frequently, you'll want to do this merging in your code on the fly.
Another, and possibly better, option is to include the "subject" data in the "student" collection or vice versa. This will result in a more mongodb friendly structure. If you run into this joining problem frequently, mongo may not be the way to go and a relational database may be better suited to your needs.

Answer 2

Mongo's find() operator lets you include or exclude certain fields from the results
Check out Field Selection in the docs for more info. You could do either:
db.users.find({}, { 'sname': 1, 'studentId': 1 });
db.users.find({}, { 'age': 0, 'gpa': 0 });
For relating your student and subject together, you could either lookup which subjects a student has separately, like this:
db.subjects.find({ studentId: 123 });
Or embed subject data with each student, and retrieve it together with the student document:
{
sname : "Roland Browning",
studentId: "123"
age: 14,
gpa: "B",
subjects: [ { name : "French", teacher: "Mr Bronson" }, ... ]
}