Hi.
I am pretty new to SQLAlchemy, so apologize me if this is an easy
one. :-)
I have three tables/classes (and their mappings), representing
pictures (P), information of pictures (D), and information categories
(C), respectively.
D has many-to-one relationships with both P and C, so it's basically
an association of P and C, having its own data too.
When my system populates the database, it starts by creating instances
of P. Adding instances of D to each P is done according to the
P.all_my_d.append(...)-pattern where "all_my_d" is a one-to-many
relationship between P and D. This is very elegant.
However, an instance of D also needs a reference to an instance of C.
If the appropriate instance of C exists in the database (or is pending
to go into it), I want to pick this one, or otherwise create a new
instance of C.
What I am looking for is the most clean way to achieve this. I have
run into trouble trying to use session.get(C, c_id) to lookup
instances of C that have not been flushed yet. (I guess this might
have something to do with primary keys not working before instances
have become persistent?)
Furthermore, I want the constructor of D to be where I lookup or
create the appropriate instance of P, which seems to require that I
pass the session object to the constructor so it can use session.get
to look for an existing instance of P. I feel this passing of the
session object around, is going to clutter the code, so I am looking
for a cleaner way.
It would be great if I could do something like this:
----8<----
#main program
for p_data in external_data:
session.save(P(p_data))
session.flush() # Now, all Ps, Ds and new Cs will be stored.
class P(object):
def __init__(self, p_data):
# ...
for (c_id, d_data) in p_data:
self.all_my_d.append(D(c_id, d_data))
class D(object):
def __init__(self, c_id, d_data):
# ...
# See if we find an instance of C having c_id as primary key.
# If not, create a new one.
self.my_c = new_or_existing_c_instance
class C(object):
def __init__(self, ...):
# whatever
----8<----
where D.my_c is the many-to-one relationship between D and C.
But what do I put into D.__init__ to achieve this in a clean way?
(I can find a dirty way myself.)
--
Sincerely,
Knut Aksel Røysland
--~--~---------~--~----~------------~-------~--~----~
You received this message because you are subscribed to the Google Groups
"sqlalchemy" group.
To post to this group, send email to sqlalchemy@googlegroups.com
To unsubscribe from this group, send email to [EMAIL PROTECTED]
For more options, visit this group at
http://groups.google.com/group/sqlalchemy?hl=en
-~----------~----~----~----~------~----~------~--~---
