def addFile(data=None, filename=None):
"""
Put the file data into the GenericFile collection,
return the _id for use as an id or ReferenceField.
If a filename kwarg is given, read data from the file.
If a data kwarg is given or data is the 1st arg, store the data.
If both or neither is given, raise an error.
"""
if filename is not None:
if data is not None:
raise ValueError('both filename and data kwargs given. can only use one.')
else:
with open(filename, 'r') as file: # do we need 'b' for binary?
data = file.read() # is this blocking? might not always get everything at once?!
elif data is None:
raise ValueError('neither filename or data kwargs given. need one.')
f = GenericFile(data=data)
f.save()
f.reload() # to fetch generated id
return f.to_dbref()
