On Aug 25, 2011, at 7:06 PM, Benjamin Sims wrote:
Thanks for the help with my query the other day - as ever response was swift
and bang on.
I'm now trying to set up another m:n relationship in ORM correctly.
Pseudocode:
parentA.children.append(child1)
parentA.children.append(child2)
parentB.children.append(child2)
session.delete(parentA)
At this stage, I would like child1 to be deleted and child2 to survive.
However, if I use (cascade = all), then both children will be deleted when
ParentA is. I hoped that delete-orphan would be applicable in this situation,
but that requires that single-parent be True, which I understand it cannot
for a true many-to-many.
So I guess what I am asking is - is it possible for child objects which have
still have remaining parents to survive, while deleting those with no parents
left?
That's not something supported by delete, delete-orphan cascade and its why
the single_parent=True flag is required - so that users aren't misled into
thinking it can work that way.
You'd need to roll this using attribute events most likely. The potential
expense is that you may have to emit SQL in order to load the full collection
of parents for each child in order to detect the orphan condition (which is
why SQLA doesn't support this automatically, it would be extremely inefficient
implemented generically).
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