Collection.map_reduce adds to an existing 'out' result collection, without cleaning it first

[baruchoxman]

MongoDB behavior is to clean the 'out' collection, before filling it with the map_reduce results.
The current mongomock behavior results in adding subsequent map-reduce calls results to the same output collection, if the collection name is reused.

I believe that the fix is to add 'out_collection.drop()' in the following block of code in collection.py:

    if isinstance(out, (str, bytes)):
        out_collection = getattr(self._Collection__database, out)
        out_collection.drop()
        out_collection.insert(reduced_rows)

[scottsexton]

I think you are correct. Map Reduce output collections are temporary and mongomock should replicate that behavior. Do you have a test for this issue?

[vmalloc]

@baruchoxman Thanks for the help on this!