Clarify add(), create(), remove() on ManyRelatedObjectsDescriptor for ForeignKeys

[jdunck@…]

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From: Russell Keith-Magee <freakboy3742@…>

Date: Feb 26, 2006 9:30 PM
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Hi all,

In working on an implementation for the remove() and clear() methods
for ManyRelatedObjectsDescriptor (i.e., mypolls.choices.remove() and
mypoll.choices.clear()), I have noticed that add() on a o2m
descriptor creates a new Choice object, whereas add() on a m2m
descriptor takes a list of pre-existing objects and adds them to the
membership set.

I have had a trawl through the wiki and mailing list logs, but I
didn't find any discussion covering this issue - it looks like the
inconsistency is an accident of porting the old API to the new. o2m
field behavior follows the old add_choices() behaviour; add/remove is
a new feature for m2m, replacing the set_articles() call.

To me, the difference in add() behaviours is a potentially confusing
inconsistency. In addition, the m2m interpretation of add() makes more
sense (to me, anyway) than the o2m behaviour. To that end, I would
like to propose the following:

1) Delete Manager.add(). Poll.objects.add(kwargs) doesn't strike me as
a particularly intuitive alternative to p = Poll(kwargs); p.save() -
the only reason it seems to be there is to simplify the o2m
descriptor.

2) Rename the existing mypoll.choices.add() to mypoll.choices.create()

3) Add a mypoll.choices.add() that will take a list of already
constructed objects (to match the m2m add), rather than create a
single new object. This would make mypoll.choices.add(choice) an
analog of "choice.poll_id = mypoll.id; choice.save()"

4) Delete remove() and clear() from the o2m descriptor.

I'm not entirely happy with (4) because of the lack of symmetry
between o2m and m2m descriptors, but the only alternatives I can see
are:

• have remove(*objs) delete all the named objects, and clear() delete

all objects with the matching key. This seems potentially confusing,
since the m2m remove/clear doesn't delete anything but the
relationship. Also, there is the potential for error if an object
provided to remove() isn't part of the o2m set.

• Clear/null the ForeignKey field for all objects in the removed set.

This would only work for objects where the field has blank=True set,
so blank=False fields would have to throw an error (or not have the
method available). This also strikes me as confusing.

• rename add() for o2m descriptors to something else, to remove the

implication that there should be a corresponding remove() and clear()
method. However, I can't think of a reasonable alternative.