Allow None if the field is marked as allow_null = True

[robertofd1995]

Continuing with the example of the documentation

class ProjectPolymorphicSerializer(PolymorphicSerializer):
    model_serializer_mapping = {
        Project: ProjectSerializer,
        ArtProject: ArtProjectSerializer,
        ResearchProject: ResearchProjectSerializer
    }

if we try to use this serializer in another model:

For example:

model.py

class Question(models.Model):
    question = models.CharField(max_length=240)
    project = models.ForeignKey("Project", null=True, blank=True, on_delete=models.PROTECT)

serializer.py

class QuestionSerializer(serializers.ModelSerializer):
    project = ProjectPolymorphicSerializer(allow_null=True)
    class Meta:
        model = Question
        fields = ("pk", "question", "project")

Before this fix, the code will break

[robertofd1995]

@apirobot please take a look when you can

[codecov[bot]]

Codecov Report

Merging #27 into master will decrease coverage by 0.96%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
- Coverage   93.10%   92.13%   -0.97%     
==========================================
  Files           1        1              
  Lines          87       89       +2     
==========================================
+ Hits           81       82       +1     
- Misses          6        7       +1     

[denisorehovsky]

@robertofd1995 Hi. Thanks for the PR. Could you please add some tests?

[robertofd1995]

@robertofd1995 Hi. Thanks for the PR. Could you please add some tests?

Hi, I will try to add them as soon as I have some time