Update a mongodb collection.

def update_collection(self, collection):
        """
        Update a mongodb collection.
        """
        node = self.node
        flow = node if node.is_flow else node.flow

        # Build the key used to store the entry in the document.
        key = node.name
        if node.is_task:
            key = "w" + str(node.pos[0]) + "_t" + str(node.pos[1])
        elif node.is_work:
            key = "w" + str(node.pos)

        db = collection.database

        # Save files with GridFs first in order to get the ID.
        if self.gridfs_files:
            import gridfs
            fs = gridfs.GridFS(db)
            for ext, gridfile in self.gridfs_files.items():
                logger.info("gridfs: about to put file:", str(gridfile))
                # Here we set gridfile.fs_id that will be stored in the mondodb document
                try:
                    with open(gridfile.path, "r" + gridfile.mode) as f:
                        gridfile.fs_id = fs.put(f, filename=gridfile.path)
                except IOError as exc:
                    logger.critical(str(exc))

        if flow.mongo_id is None:
            # Flow does not have a mongo_id, allocate doc for the flow and save its id.
            flow.mongo_id = collection.insert({})
            print("Creating flow.mongo_id", flow.mongo_id, type(flow.mongo_id))

        # Get the document from flow.mongo_id and update it.
        doc = collection.find_one({"_id": flow.mongo_id})
        if key in doc:
            raise ValueError("%s is already in doc!" % key)
        doc[key] = self.as_dict()

        collection.save(doc)