Hi,
I'm having some trouble with what I would have thought would be a
simple query. Working with the multi-table inheritance examples
Django gives:
class Place(models.Model):
name = models.CharField(max_length=50)
address = models.CharField(max_length=80)
class Restaurant(Place):
serves_hot_dogs = models.BooleanField()
serves_pizza = models.BooleanField()
how do I get all the Places that are also Restaurants? Obviously I
can do Restaurant.objects.all(), but that returns a set of
Restaurants, whereas I want the set of Places that those Restaurants
are linked to.
I've tried (the equivalent in my app of) the following but it
generates an InternalError:
Place.objects.filter(restaurant__isnull=False)
The only surefire way of getting what I want would be to do something
like:
[r.place_ptr for r in Restaurant.objects.all()]
But this presumably incurs inefficiencies of using python rather than
SQL.
So first question: is the above possible in a more efficient manner?
-----------
I have a second question relating to this. The reason WHY I want to
do the above is because of Django's problem with serializing objects
of class Restaurant, whereby it only serializes the local fields. The
suggested solution in the documentation is:
all_objects = list(Restaurant.objects.all()) + list(Place.objects.all
())
data = serializers.serialize('xml', all_objects)
But this doesn't do at all what I would want; it in fact seems to do
exactly what it looks like it would do: serialize a list that starts
with the Restaurant objects (with their local fields), then a FULL
list of all the Place objects.
Question: am I misreading/miscopy-and-pasteing the above code; does it
in fact do something else?, or is it actually MEANT to make the
awkward list of "all Restaurants and all Places"?
If I can't find a solution using multi-table inheritance then I
imagine I'm going to have to use an abstract base class (which would
involve rethinking my architecture).
Thanks in advance and all the best
James Fisher
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