JSON serializer for subset of fields.

[Sonu kumar]

class Country(models.Model):
    country = CountryField()

class City(models.Model):
    name = models.CharField(max_length=1000)
    country = models.ForeignKey(Country)

country = request.POST['country']
country = get_object_or_404(Country, country=country)
cities = serializers.serialize("json", country.city_set.all(), fields=('name', 'id'))

​https://docs.djangoproject.com/en/1.8/topics/serialization/#s-subset-of-fields

According to above code snippet and document it should return JSON object with only name and id field but it returns

[{"fields": {"name": "Chennai"}, "model": "job.city", "pk": 1}]

instead of

[{"name": "Chennai", "id": 1}]

which includes all fields of a model City.

[Claude Paroz]

The behavior is consistent. However the documentation could be a little clearer about the structure which will be serialized for a subset of fields. That is the basic structure with fields/model/pk is still output, but the fields content is filtered.

[Sonu kumar]

Replying to claudep:

The behavior is consistent. However the documentation could be a little clearer about the structure which will be serialized for a subset of fields. That is the basic structure with fields/model/pk is still output, but the fields content is filtered.

Yeah that is correct. But serializers output fields with only one value "name" and "id" is also missing.

[Claude Paroz]

So the question is if the id field when explicitly provided should be present even when it is already given by the pk field at the first level. Once again, this might be documented like "the primary key field is always produced in the pk field, even if you specify it in the fields parameter".

[Claude Paroz]

A suggestion:

docs/topics/serialization.txt

@@ -59 +59 @@
     data = serializers.serialize('xml', SomeModel.objects.all(), fields=('name','size'))
 
 In this example, only the ``name`` and ``size`` attributes of each model will
-be serialized.
+be serialized. The primary key is always serialized as the ``pk`` element in the
+resulting output, it never appears in the ``fields`` part.
 
 .. note::
 

[Tim Graham]

Makes sense to me. Since we have two independent clauses, the comma could be a semicolon.

[Claude Paroz <claude@…>]

In c5fda55e:

Fixed #26256 -- Added note about primary key serialization

Thanks Sonu kumar for the report and Tim Graham for the review.

[Claude Paroz <claude@…>]

In 0e80ac4:

[1.9.x] Fixed #26256 -- Added note about primary key serialization

Thanks Sonu kumar for the report and Tim Graham for the review.
Backport of c5fda55edc from master.