do you wish to export data from two collections ?

I wish to fetch data from two collections using a single query.

Hi Sachin,

unfortunately there is no merging/joining operation directly in MongoDB. Anyway, you can use the map-and-reduce functionality in order to combine the data appropriately. 

Depending on your actual problem, the following links might be of help:

http://stackoverflow.com/questions/5681851/mongodb-combine-data-from-multiple-collections-in-to-one-how

http://stackoverflow.com/questions/9696940/merging-two-collections-in-mongodb

http://www.pal-blog.de/entwicklung/mongodb/mapreduce-multiple-mongodb-collections-into-one.html

https://groups.google.com/forum/#!topic/mongodb-user/OQUH_wxYyRg

https://groups.google.com/forum/#!topic/mongodb-user/qxa7ZBTk7sQ

Best,

  Marc

I think doing it in programming language level would be much easier than the map-reduce approach. 

Hi Juggapong,

i would definitely agree on this, it will be easier on application level. Anyway, the question is where you want to put the additional load. Since usually the distribution between the database and the application layer is somewhat like 70% (80%) on the application layer and 30% (20%) on the database layer, it is often necessary to put additional load to the database layer instead of the application layer. 

Anyway, if you have the possibility, it would certainly be much easier on the application level.

Best,

  Marc

As of MongoDB 3.2, joins become possible. 

https://www.mongodb.com/blog/post/joins-and-other-aggregation-enhancements-coming-in-mongodb-3-2-part-1-of-3-introduction

You can get data from two or more collection use aggregate method suing $lookup.

db.test.aggregate([{$lookup:{from:"test1",localField:"user_id",foreignField:"emp_id",as:"data"}}]).pretty();

Here test is 1st collection, test1 is 2nd collection and user_id and emp_id are join key from respective collections. And we will get result data in data key as the result.

If you are using Mongoose.js, a good approach would be to use the deep-populate plugin which should be added to the model schema.

https://github.com/buunguyen/mongoose-deep-populate

In one of our Java projects we are doing it like this:

https://github.com/hzg-wpi/xenv-hq/blob/6d9b96cf36efa21857356a3ad5d859608b09ba72/src/test/java/de/hzg/wpi/xenv/hq/configuration/ConfigurationTest.java#L112