On 10/29/2010 05:31 PM, Hector Blanco wrote:
> Hello, group!
>
> I am still dealing with the relationship I asked before
> (http://groups.google.com/group/sqlalchemy/browse_thread/thread/c1d46daf35116999).
>
> To tell the truth, I'm not even sure if this is a question I should
> ask in the SqlAlchemy forum because I'm also dealing with Megrok.rdb
> (http://pypi.python.org/pypi/megrok.rdb) and I don't know if that's
> what is causing the problem, but I'm pretty lost... As I explained in
> my other question, I think the rdb.Model thing that appears in the
> classes is just a tool to create the mapper "class <--> table" in a
> slightly more transparent way for the programmer. That's why I thought
> I may get some help here.
>
> 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)
The target of a ForeignKey should be a string, e.g.:
ForeignKey("parents_table.id")
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> 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)
These class definitions should be merged with those in Tables.py. You
should only have one "class Parent" statement and one "class Child"
statement. You may be confusing this with the non-declarative class
setup, where you define the table first, class 2nd, and mapper 3rd. It
looks like rdb uses the declarative approach, where the table and mapper
are defined as part of the class in one step.
Also, it is a good idea to make the first argument to relationship() a
string, as it lets you avoid worrying about which order classes are
defined. Example:
# This works even if Child hasn't been defined yet.
child1 = relationship("Child", uselist=False)
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> 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'
This is probably due to the foreign key issue above.
> 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 whatever.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)
>
> Thank you all.
In SQLAlchemy you get around circular dependencies by:
* Using strings as the target of ForeignKey()
* Using class name strings as the target of a relation (declarative
only)
* Using strings or callables as primaryjoin/secondaryjoin arguments
in a relationship()
-Conor
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