Double Relationship in SQLalchemy - database

Question

I got following problem in sqlalchemy. I made three different tables in sqlite, the first has no realtion, the second has a relation to the first and the third a relation to the second. So when I want to insert things in the first and the second table everything works fine. When I want to insert datas in the third table I'm getting troubles when I'm going to do it like it's discribed in the tutorial. Here is my code for the three tables:
First table:
# Save_Data_Type.py
from sqlalchemy import ForeignKey
from sqlalchemy import Column, Float, Integer, String
from base import Base
from sqlalchemy.orm import relationship, backref
########################################################################
class Save_Data_Type(Base):
__tablename__ = "save_datas_type"
save_datas_type_id = Column(Integer, primary_key=True)
type_memory = Column(String)
comment = Column(String)
dummy1 = Column(String)
dummy2 = Column(String)
#----------------------------------------------------------------------
def __init__(self, type_memory, comment, dummy1, dummy2):
""""""
self.type_memory = type_memory
self.comment = comment
self.dummy1 = dummy1
self.dummy2 = dummy2
Second Table
# Save_Data.py
from sqlalchemy import ForeignKey
from sqlalchemy import Column, Float, Integer, String
from base import Base
from sqlalchemy.orm import relationship, backref
########################################################################
class Save_Data(Base):
""""""
__tablename__ = "save_datas"
save_datas_id = Column(Integer, primary_key=True)
save_datas_type_id = Column(Integer,ForeignKey("save_datas_type.save_datas_type_id"))
save_datas_type = relationship("Save_Data_Type", backref=backref("save_datas", order_by=save_datas_id))
value = Column(Float)
comment = Column(String)
dummy1 = Column(String)
#----------------------------------------------------------------------
def __init__(self, value, comment, dummy1):
""""""
self.value = value
self.comment = comment
self.dummy1 = dummy1
Third Table
# Station.py
from sqlalchemy import ForeignKey
from sqlalchemy import Column, Integer, Boolean, String
from base import Base
from sqlalchemy.orm import relationship, backref
########################################################################
class Station(Base):
""""""
__tablename__ = "stations"
stations_id = Column(Integer, primary_key=True)
name = Column(String)
password = Column(String)
save_datas_id = Column(Integer,ForeignKey("save_datas.save_datas_id"))
save_datas = relationship("Save_Data", backref=backref("stations", order_by=stations_id))
dummy1 = Column(String)
dummy2 = Column(String)
dummy3 = Column(String)
#----------------------------------------------------------------------
def __init__(self, name, password, dummy1, dummy2, dummy3):
""""""
self.name = name
self.password = password
self.dummy1 = dummy1
self.dummy2 = dummy2
self.dummy3 = dummy3
base.py
# base.py
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
So if I'm going to insert the datas like that:
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
import base
from Save_Data_Type import Save_Data_Type
from Save_Data import Save_Data
from Station import Station
engine = create_engine('sqlite:///Database.db', echo=True)
base.Base.metadata.create_all(engine, checkfirst=True)
# create a Session
Session = sessionmaker(bind=engine)
session = Session()
jack = Save_Data_Type("Ring Memory",'In seconds',None,None)
jack.save_datas = [Save_Data(1980,'Sometimes more, sometimes less',None)]
jack.save_datas.stations = [Station('name1','123456',None,None,None)]
session.add(jack)
session.commit()
Nothing is writing in the the third table of the database. How is the usual way to build this relationship?
Thanks in advance,
Johannes

Answer 1

Apart from some typos in the code you posted (see comment from zzzeek), it looks like the problem will be solved by replacing the code:
jack.save_datas.stations = [Station('name1','123456',None,None,None)]
with the one below:
jack.save_datas[0].stations = [Station('name1','123456',None,None,None)]
The reason is the following: in your case you assign/create a stations attribute to a class member. Not only this does not do the right job, it might actualy harm your ORM model.
In the second case you correctly assign the Station instance to the first save_datas.
The more clear code would look like this:
jack = Save_Data_Type("Ring Memory",'In seconds',None,None)
save_data1 = Save_Data(1980,'Sometimes more, sometimes less',None)
jack.save_datas.append(save_data1)
# or: jack.save_datas = [save_data1]
# or (my favourite): save_data1.Save_Data_Type = jack
save_data1.stations = [Station('name1','123456',None,None,None)]
# or... similar to the relationship above