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)
});

```
Yes you can: Take this utility function that I have written today:

```
function shangMergeCol() {
  tcol= db.getCollection(arguments[0]);
  for (var i=1; i<arguments.length; i++){
    scol= db.getCollection(arguments[i]);
    scol.find().forEach(
        function (d) {
            tcol.insert(d);
        }
    )
  }
}

```

You can pass to this function any number of collections, the first one is going to be the target one. All the rest collections are sources to be transferred to the target one.