Question: How can I (in MongoDB) combine data from multiple collections into one collection?

Can I use map-reduce and if so then how? 

I would greatly appreciate some example as I am a novice.

Answer: 
If there is no bulk insert into mongodb, we loop all objects in the `small_collection` and insert them one by one into the `big_collection`:

```
db.small_collection.find().forEach(function(obj){ 
   db.big_collection.insert(obj)
});

```
Mongorestore has this feature of appending on top of whatever is already in the database, so this behavior could be used for combining two collections:

1. mongodump collection1
2. collection2.rename(collection1)
3. mongorestore

Didn't try it yet, but it might perform faster than the map/reduce approach.