ModelSerializer fields don't obey default from the model

[gcbirzan]

Currently, in django, even if a field doesn't have an explicit default=None, the implicit default is going to be None (technically, it's django.db.models.fields.NOT_PROVIDED). However, even if an explicit default is set on a field on the model, the serializer will ignore it.

This comes up in the context of PUT/PATCH, where one will (I know I did) expect PUT to put the default value for fields not passed that, at the very least, have an explicit default.

To give an example, a model with a field that is:

foo = models.PositiveSmallIntegerField(null=True, blank=True, default=42)

will generate the following field in the serializer:

foo = IntegerField(allow_null=True, max_value=32767, min_value=0, required=False)

The default value will be used when creating the instance, but not on PUT updates.

So, two things need to be discussed/decided:

• Should fields that have an explicit default move that on to the serializer so that a PUT will populate the default value? - I would say a definite yes here, the current behaviour is surprising and undocumented (Or, I couldn't find it in the docs)
• Should fields with implicit default (django.db.models.fields.NOT_PROVIDED) behave similarly and null out fields not provided for a PUT? - I would say no here, this is too big a change in behaviour and also might be surprising to other people.

[tomchristie]

I'll narrow this down to one Q...

• "Should model fields that have a default move that on to the serializer"

No. If there's a model default then the serializer field is marked as not required.

---

Pasting from IRC to save myself some time here...

[10:11:49] <tomchristie>	But the short is "default on a Model" =/= "default on Serializer"
[10:12:05] <tomchristie>	If the model has a default then the serializer field should be "not required". That's all.
[10:12:09] <tomchristie>	Nothing more nothing less.
[10:12:24] <tomchristie>	There's two reasons I wouldn't be changing that
[10:12:46] <tomchristie>	1. Because I think the "reset fields using the model default on update" is less obvious behavior, not more.
[10:13:00] <tomchristie>	2. Because it'd be a huge breaking change.
[10:13:07] <tomchristie>	But let's stick with (1)
[10:13:31] <gcbirzan>	Hm. Most APIs I interacted with behaved like this
[10:14:51] <tomchristie>	gcbirzan: Eg?...
[10:15:10] <tomchristie>	You *can* set a serializer default.
[10:15:36] <gcbirzan>	True.
[10:15:44] <tomchristie>	created = models.DateTimeField(default=...)
[10:15:44] <gcbirzan>	that's what I'm doing :)
[10:16:09] <gcbirzan>	The other reason I don't care very much is that we have our own base serializer class, so it's easy to change this
[10:16:15] <tomchristie>	is v different from `updated = serializers.DateTimeField(default=...)