如何从查询中获取另一个对象内的对象？

Question

I don't know how to ask this but is very simple

I have an entity called "State" and another entity called "City".

I'm doing a query to get specific cities with a given parameter:

cities = City.objects.filter(code__exact=12345).values("id","name","state")

And then I serialize the list ( or dict? = in order to get them via JSON:

for c in cities:
    result.append(c)
return HttpResponse(json.dumps(result))

The problem is I'm getting only the state ID but I need another attributes from this object, how I can initialize the state object inside the city or at least get specific attributes from state object.

Answer 1

The result from a values() call is a ValueQuerySet , which is special in a few ways. One of them is:

A ValuesQuerySet is useful when you know you're only going to need values from a small number of the available fields and you won't need the functionality of a model instance object. It's more efficient to select only the fields you need to use.

The part in bold is important. It means that the result of the queryset will not have the instances of the models, you have to tell it exactly what you need to be fetched.

So, if you know the fields of the state model you want in your result, you can add them in your values clause. If you just put state , it will give you the state id, which is the default identity field.

cities = City.objects.filter(code__exact=12345).values("id",
                                                       "name",
                                                       "state__name",
                                                       "state__id")

If you are doing this just to convert the results to json, use the built-in serializers :

from django.core import serializers
result = serializers.serialize('json',
                               City.objects.filter(code__exact=12345),
                               fields=('id', 'name', 'state__name', 'state__id'))