On 01/21/2011 01:47 AM, Andrey Popp wrote:
Hello,
On Jan 21, 2011, at 1:48 AM, Juliusz Gonera wrote:
Is it considered bad practice to make DB queries in model's methods? Or is this
the way to go?
Yes, it's a bad practice in general -- model should be persistence-agnostic.
for the record, there are lots of folks who disagree with this statement.
while there are certainly cases where the extra abstractions that you detail
below are useful, IMHO they're overkill for most applications, which are only
intended to work w/ a single persistence engine and which gain little benefit
from the added complexity.
-r
And even if you're not planning to use the same models with different storage
options or outside of SQLAlchemy context, I still recommend you to separate
queries and data manipulation routines in separate class -- ModelManager,
ModelRepository, ...
The rules that I follow are:
* If function operates on entire table (insert something into table, query
from table or etc.) -- it should become a member or model's manager.
* If function is a part of behavior of model (in a sense of its domain)-- it
should become model's method.
So, I think it's worth to delegate such relationship management to model's
manager:
class MainModelManager(object):
def add_some_related_object(self, main_obj, param1, param2):
# query something
related = SomeRelatedObject()
main_obj.some_related_objects.append(related)
If you think, that this method would require also some participation of main
model's inner state, you can replace
`main_obj.some_related_objects.append(related)` with special method of
MainModel:
class MainModel(object):
def add_some_related_object(self, related):
# some business logic, which is not require database access
main_obj.some_related_objects.append(related)
class MainModelManager(object):
def add_some_related_object(self, main_obj, param1, param2):
# query something
related = SomeRelatedObject()
main_obj.add_some_related_objects(related)
This way you can separate business logic from persistence.
It also helps to provide better testability of you code, cause you can test
domain logic inside `MainModel.add_some_related_object` without requiring
existence of test SQL database.
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