On 10/29/2010 11:51 AM, Hector Blanco wrote:
> Thanks Conor!
>
> The callable works like a charm! It's great news! (I've been trying to
> figure out this for 3 days... yeah... I guess I'm not that smart)
>
> Now that I have it working, a “design” question pops up. Nothing
> technical, really.
>
> As Connor mentioned in his reply:
> “Usually we consider the table with the foreign key as the "child"
> table, but that's just being picky”
>
> That's very true, and now I don't know how to design it...
It would be easier to design it if we had more concrete names instead of
"Parent" and "Child". What is the actual use case? Is this a tree
hierarchy? Does each parent have exactly two children?
> I can do it the way I asked or...
>
> class Child(rdb.Model):
> rdb.metadata(metadata)
> rdb.tablename("children_table")
> id = Column("id", Integer, primary_key=True)
> parent_id = Column("id", Integer, ForeignKey(“parent_table.id”)) # New!
> type = Column("type", ShortInteger) # New!
>
> field1 = Column(“field1”, String(64)) #Irrelevant
> def __init__(self):
> self.field1 = “hello world”
>
> class Parent(rdb.Model):
> rdb.metadata(metadata)
> rdb.tablename("parent_table”)
>
> id = Column("id", Integer, primary_key=True)
>
> child1 = relationship(
> # Well... this I still don't know how to write it down,
> # but it would be something like:
> # Give me all the children whose “parent_id” is my “id”
> # AND type == 1
> # I'll deal with the joins and that depending on your answer,
> guys
> )
>
> child2 = relationship(
> # Would be same as above
> # AND type == 2
> )
>
> This may be good for adding new children to the parent class... If I
> add a “Parent.child3”, I just need to create a new relationship very
> similar to the already existing ones.
>
> The way I asked in my former question would imply creating a new
> relationship AND adding a new foreign key to the parent.
>
> I'd like to know what people that know much more about databases think :)
I'm confused as to why you would want separate "child1", "child2", etc.
relationships instead of a single "children" relationship. Is Child.type
really something you want for distinguishing children, or is it
something you added to try and make the relationships work?
Assuming you really do want to keep separate "child1" and "child2"
relationships, and they are both one-to-one relationships, they would
look like this:
# omit uselist=False if this is a one-to-many relationship
child1 = relationship(
"Child",
primaryjoin=lambda: and_(Child.parent_id == Parent.id, Child.type == 1),
uselist=False)
child2 = relationship(
"Child",
primaryjoin=lambda: and_(Child.parent_id == Parent.id, Child.type == 2),
uselist=False)
-Conor
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