Hi folks,
I'm working on a SQLAlchemy-based app where we've decided to make some
infrastructure changes, in particular moving from reflection to declaration
for mapping the models.
However, we're now running into issues where, after switching to
declarative, relationships aren't populated the way we expect when
manipulated in Python. For example, we have code that looks like this:
class Widget(...):
def merge(self, other_widget):
"""Merge the widgets, transferring the dependent items on the other
widget to this one."""
for frobnicator in other_widget.frobnicators[:]:
frobnicator.widget = self
meta.Session.delete(other_widget)
This code worked as hoped-for when we were reflecting on the database to
create our mappers, but after switching to declarative, the dependent items
are cascade-deleted on commit when other_widget is deleted, rather than
being preserved as children of the merged widget.
It's not difficult to fix this particular issue - explicitly removing the
frobnicators from the other_widget.frobnicators collection will prevent
them from being deleted, and then the merged widget correctly has them -
but we're finding we have a class of bugs where relationships aren't being
handled the same way as before. Unfortunately, "build a comprehensive test
suite" is one of the infrastructure changes we're in the process of making
- which means it's not done yet and we can't easily track down all the
places we could get tripped up. We would really prefer to resolve this by
changing the definitions in the models, not by changing the application
code that manipulates the membership of relationships.
I've created a reduced test case here
<https://github.com/ejames/sqlalchemy_reflection_problem_reduction> which
specifically displays the behavior we're having trouble with in minimal
form. If one line in the test case is commented out, the test will pass
for reflective models and fail for declarative models; if the line is put
back in, success and failure reverse.
My question: How can we make relationships function identically in
declarative syntax as they did in reflective syntax? We thought we had
migrated mapping styles in a way that wouldn't change anything, but here we
are. What are we missing?
Thanks,
Evan James
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