[Grok-dev] [MeGrok & SqlAlchemy] Simple relationship 1:1 fails with "Foreign key assocated with column ------- could not find table"

[Hector Blanco]

Hello, list!

I am still dealing with the relationship I asked before
(https://mail.zope.org/pipermail/grok-dev/2010-October/010714.html).

I'm happy to say that I made some advances. Thank you for your help!

But I am still having troubles with it...

In this message, I have simplified the code (compared to my former
question) to make it clearer, but well... The fact is that now I'm
getting a problem with a simple 1:1 relationship (if I can fix it, I
will be able to move to the more complicated stuff as I detailed in
the former question)

I am getting this error:
Foreign key assocated with column 'children_table.id' could not find
table 'parents_table' with which to generate a foreign key to target
column 'id'

I have a file, called Tables.py where all the classes and auxiliary
(or intermediate) tables that I'm going to use in my application are
defined:

Tables.py >>

class Parent(rdb.Model):
	rdb.metadata(metadata)
	rdb.tablename("parents_table")

	id = Column("id", Integer, primary_key=True)
	_whateverField= Column("whatever_field", String(16)) #Irrelevant

	child1 = relationship("Child", uselist=False)

class Child(rdb.Model):
	rdb.metadata(metadata)
	rdb.tablename("children_table")
	id = Column("id", Integer, ForeignKey(Parent.id), primary_key = True)
	type = Column("type", String(2)) #Irrelevant (for this example)

#A few lines below, said classes are "Grokified":

def setComponents():
	"""Grok all the classes related to the database"""
	grok_component("Parent", Parent)
	grok_component("Child", Child)
	print "Grokified!"

#This method is properly run... I see a "Grokified" message properly.

     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
And then I have two different Python .py files (Parent.py and
Child.py) where the methods that manage said classes are implemented.
In those files, the static area of each class is copied from Tables.py
with some changes in the quotes (where I can use the object itself, I
use it):

Parent.py >>

from child import Child
metadata = rdb.MetaData()

class Parent(rdb.Model):
	rdb.metadata(metadata)
	rdb.tablename("parents_table")

	id = Column("id", Integer, primary_key=True)
	_whateverField= Column("whatever_field", String(16)) #Irrelevant

	child1 = relationship(Child, uselist=False) #No quotation marks on this Child
     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
And

Child.py > >
metadata = rdb.MetaData()

class Child(rdb.Model):
	rdb.metadata(metadata)
	rdb.tablename("children_table")
	id = Column("id", Integer, ForeignKey("parent_table.id"), primary_key = True)
	type = Column("type", String(2)) #Irrelevant (for this example)
     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

When I try to use these classes, I get:
Foreign key assocated with column 'children_table.id' could not find
table 'parents_table' with which to generate a foreign key to target
column 'id'

But if I take a look to the tables with a MySQL Query Browser, the
table "parents_table" is there, happily and properly created.

In some other places, I have had similar problems, but I've been able
to fix them by delaying the imports. I had been able to (kind of)
import the Parent type in the Child file so I can use the Parent
object directly. It would be a little bit as if in this case I was
able to do:

from parent import Parent
[ . . . ]

class Child(rdb.Model):
 	[ . . . ]
	id = Column("id", Integer, ForeignKey(Parent.id), primary_key = True)

and that usually fixed the problem but in this specific case, I can't
really do that: In the Parent file I need to import the Child and that
gives a very, very nasty circular dependency problem.

Is there a way to tell the Child.py file something like "Hey, dude...
Here's the parent_table that you need!" ?  (Well... In a more Pythonic
way, of course... I don't think 'dude'is a reserved keywork in Python,
or an SqlAlchemy type). I don't know, something like:

from whathever.repository.of.tables import parent_table

         so I can, without quotes, use:

id = Column("id", Integer, ForeignKey(parent_table.id), primary_key = True)

(I guess that may work)

I tried to figure out something out of these links:
http://pypi.python.org/pypi/megrok.rdb (basic documentation)
http://grok.zope.org/documentation/how-to/orm-using-megrok.rdb-and-sqlalchemy
 (more advanced)

but I still wasn't able to get anything. If anyone knows a good
megrok.rdb tutorial, that would be very helpful as well...
So I don't have to ask that much :-(
"don't give a needy person a fish, give him a fish pole and teach him
how to fish" ... type :-)

Thank you!!