0.13.dev2 duplicate-attribute join issue #857

>>> print(lab.Subject().definition)
   subject_id          : int   # institution 6 digit animal ID
   ---
   -> [nullable] Person        # person responsible for the animal
   cage_number         : int   # institution 6 digit animal ID
   date_of_birth       : date  # format: yyyy-mm-dd
   sex                 : enum('M','F','Unknown')
   -> [nullable] AnimalSource  # where was the animal ordered from
>>> print(lab.WaterRestriction().definition)
   -> Subject
   ---
   water_restriction_number    : varchar(16)   # WR number
   cage_number                 : int
   wr_start_date               : date
   wr_start_weight             : Decimal(6,3)
>>> lab.WaterRestriction() * lab.Subject().proj()
...
pymysql.err.OperationalError: (1052, "Column 'cage_number' in field
 list is ambiguous")