Why django-rest-framework (using django-nonrel with mongodb) sets to null child objects PKs when updating an instance - django

Question

It happens with any object instance that contains embbeded models.
When I attempt to update the father instance, it sets to null its childs PKs.
This is an extract of a json object already updated:
{
"pk": "558d023d153bd41930b3fcf0",
"checkgroup_id": "checkgroupid 5",
"name": "checkgroup five",
"description": "this an example description",
"control_config": {
"control_config_1": {
"pk": null,
"params": {
"param2": {
"pk": null,
"value": "value2",
"mandatory": false,
"default": "default"
},
"param1": {
"pk": null,
"value": "value1",
"mandatory": false,
"default": "default"
}
},
"exceptions": {
"exception1": {
"pk": null,
"description": "description example",
"params": [
{
"pk": null,
"value": "value1",
"mandatory": false,
"default": "default"
},
{
"pk": null,
"value": "value2",
"mandatory": false,
"default": "default"
}
]
}
}
}
},
and this is the Model:
class CheckGroup(models.Model):
"""It defines a set of controls to be applied. """
checkgroup_id = models.TextField(max_length=250)
name = models.TextField(max_length=250)
description = models.TextField(max_length=250)
control_config = DictField(EmbeddedModelField(ControlConfig))
controls = DictField(EmbeddedModelField(Control))
with its serializer defined as:
class CheckGroupSerializer(serializers.ModelSerializer):
""" Transforms CheckGroup into json
"""
control_config = serializers.DictField(child=ControlConfigSerializer())
controls = serializers.DictField(child=ControlSerializer())
pk = serializers.CharField()
class Meta:
""" It lets to choose the model that will be serialize and its fields
"""
model = CheckGroup
fields = ('pk', 'checkgroup_id', 'name', 'description', 'control_config', 'controls')

Answer 1

Well, I found out the reason on django-rest-framework docs.
The problem is that the framework does not deal with that, and should be implemented by us.
The doc says:
Because the behavior of nested creates and updates can be ambiguous, and may require complex dependencies between related models, REST framework 3 requires you to always write these methods explicitly. The default ModelSerializer .create() and .update() methods do not include support for writable nested representations.