Question

I have a document that has an id of another document from a different collection embedded in it.

My desired result is to return (I’m using python and pymongo) the all the fields of the first collection, and all of the friends from the document that was embedded.

I understand mongo doesn’t do joins and I understand I’ll need to make two queries. I also don’t want to duplicate my data.

My question is how to piece the two queries together in python/pymongo so I have one results with all the fields from both documents in it.

Here is what my data looks like:

db.employees

{_id: ObjectId("4d85c7039ab0fd70a117d733"), name: 'Joe Smith', title: 'junior',
manager: ObjectId("4d85c7039ab0fd70a117d730") }

db.managers

{_id: ObjectId("ObjectId("4d85c7039ab0fd70a117d730"), name: 'Jane Doe', title: 'senior manager'}

desired result

x = {_id: ObjectId("4d85c7039ab0fd70a117d733"), name: 'Joe Smith', title: 'junior',
    manager: 'Jane Doe' }

Answer 1

Your basically doing something that Mongo does not support out of the box and would actually be more painful than using the two records separately.

Basically (in pseudo/JS code since I am not a Python programmer):

var emp = db.employees.find({ name: 'Joe Smith' });
var mang = db.managers.find({ _id: emp._id });

And there you have it you have the two records separately. You cannot chain as @slownage shows and get a merged result, or any result infact since MongoDB, from one qauery, will actually return a dictionary not the value of the field even when you specify only one field to return.

So this is really the only solution, to get the the two separately and then work on them.

Edit

You could use a DBRef here but it is pretty much the same as doing it this way, only difference is that it is a helper to put it into your own model instead of doing it youself.