[Python] sqlalchemy, mixins, foreignkeys and declared_attr

I know that I need to @declared_attr when declaring columns with foreign keys on declarative mixin classes, however I would like to know the reason behind this.

thank you

#store traces
class ListContainer():
id = Column(Integer, primary_key=True)
# timestamp = Column(DateTime, default=datetime.now())
name = Column(String)
#endclass

#store flow in traces
def list_item(tablename):
class ListItem():
# @declared_attr
# def trace_id(cls):
# return Column(Integer, ForeignKey(tablename+'.id'))
trace_id = Column(Integer, ForeignKey(tablename+'.id'))
id = Column(Integer, primary_key=True)
item_pos = Column(Integer)
# start_ea = Column(BLOB)
# end_ea = Column(BLOB)
#endclass
return ListItem

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