<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def primary_key(cls):
 """Return the name of the table's primary key :rtype: string """


        if cls.__from_class__:
            cls = cls.__from_class__
        return cls.__table__.primary_key.columns.values()[0].name