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...
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 :)
2010/10/29 Conor <conor.edward.da...@gmail.com>:
> On 10/29/2010 09:43 AM, Hector Blanco wrote:
>
> Hello list...
>
> I wrote a couple of days ago about how to model an structure of three
> classes
> (http://groups.google.com/group/sqlalchemy/browse_thread/thread/5ba5c4ad16f789d6#).
> I thing I almost have it, but I am still getting problems mapping an
> structure like this.
>
> class Child(rdb.Model):
> def __init__(self):
> self.field1 = “hello world”
>
> class Parent(rdb.Model):
> def __init__(self):
> self.child1 = Child()
> self.child2 = Child()
>
> The “Parent” class has two different instances of a Child() class. I
> am not even sure about how to treat this (two different 1:1
> relationships or a 1:2 relationship).
>
> My last try is this mapping:
>
> (don't let the rdb.Model thing fool you, is just something that
> automatically maps the class to the table specified in rdb.tablename).
>
> class Child(rdb.Model):
> rdb.metadata(metadata)
> rdb.tablename("children_table")
> id = Column("id", Integer, primary_key=True)
> 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_id = Column("child_1_id", Integer,
> ForeignKey("children_table.id"))
> child2_id = Column("child_2_id", Integer,
> ForeignKey("children_table.id"))
>
> child1 = relationship(Child,
> primaryjoin = (child1_id=="children_table.id")
> )
>
> child2 = relationship(Child,
> primaryjoin = (child2_id=="children_table.id")
> )
>
> I have tried (almost) everything. I say “almost” because obviously I
> haven't tried the right thing. I keep getting errors that sqlalchemy
> can't determine the relationship between the two tables.
>
> It looks very similar to:
> http://www.sqlalchemy.org/docs/orm/relationships.html#multiple-relationships-against-the-same-parent-child
>
> but I also understand that maybe I can't get this working that way,
> because I am actually putting two ForeignKeys from the same table
> (“children_table”) in the parent table. I am not sure how that will
> work out, or is correct or what... :-(
>
> Thank you!
>
> Usually we consider the table with the foreign key as the "child" table, but
> that's just being picky. The problem is that SQLAlchemy is treating
> "children_table.id" as a literal instead of a clause, so your join would be
> like (parent JOIN child ON parent.child1_id = 'children_table.id').
> Obviously that is not what you want. There are several ways to formulate
> primaryjoin/secondaryjoin.
>
> Pass the whole thing in as a string:
>
> child1 = relationship(Child, primaryjoin="Parent.child1_id == Child.id")
>
> Use the column objects directly (this requires that Child be defined before
> Parent):
>
> child1 = relationship(Child, primaryjoin=child1_id==Child.id)
>
> Use a callable (my favorite):
>
> child1 = relationship(Child, primaryjoin=lambda: Parent.child1_id ==
> Child.id)
>
> -Conor
>
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