permit the establishment of a field with the name of size or other

Example:

size = ReferenceField(Size, verbose_name='Size')

# currently error

examples = Example.objects(size=instance_size)

# caused an error

"""
File "/home/czr/environments/vasko/lib/python2.7/site-packages/mongoengine/queryset/transform.py", line 50, in query
if parts[-1] == 'not':
IndexError: list index out of range
"""