How can I serialize a queryset from an unrelated model as a nested serializer?

Question

I'm trying to add a nested serializer to an existing serializer based on some criteria of the parent model, not a foreign key. The use case is to return a 'Research' object with an array of 'ResearchTemplate' objects that are identified by filtering on a Postgres ArrayField.

Models

class Research(TimeStampedModel):
 category = models.CharField(max_length=100, choices=RESEARCH_TEMPLATE_CATEGORIES, default='quote')
 body = models.CharField(max_length=1000, blank=True, default='') #The body of text comprising the nugget
 additionaldata = JSONField(null=True) # all the varying values to be merged into a wrapper
 def __str__(self):
 return self.body
 class Meta:
 ordering = ('created',)
class ResearchTemplate(TimeStampedModel):
 template = models.TextField(blank=True, default='')
 category = models.CharField(max_length=100, choices=RESEARCH_TEMPLATE_CATEGORIES, default='quote')
 mergefields = ArrayField(models.CharField(max_length=200), blank=True)
 def save(self, *args, **kwargs):
 merges = re.findall("{{(.*?)}}", self.template) #get all the template names from within the mustaches
 self.mergefields = list(set(merges)) #TODO: Make Unique
 super(TimeStampedModel, self).save(*args, **kwargs)
 def __str__(self):
 return self.wrapper
 class Meta:
 ordering = ('created',)

Serializers

class ResearchSerializer(serializers.ModelSerializer):
 templates = ResearchTemplateSerializer(many=True)
 class Meta:
 model = Research
 fields = ('id', 'created', 'speaker', 'body', 'templates')
class ResearchTemplateSerializer(serializers.RelatedField):
 def get_queryset(self, values):
 return ResearchTemplate.objects.filter(mergefields__contained_by=['django']) #This must an array of keys from the Research object's JSON field
 class Meta:
 model = ResearchTemplate
 fields = ('id', 'template')

I've been able to nest serializers when there is a foreign key mapping them, however I am unable to do so with a custom queryset. Perhaps I'm not thinking about this properly, and I require some form of 'relationship' field on the Research model.

How can I nest a serialized list of all rows that are returned from a filter with values specified from the parent model?

Answer 1

You can use DRF's SerializerMethodField.

Define your ResearchTemplateSerializer as a normal ModelSerializer, not as a RelatedField.

Then replace your ResearchSerializer with this:

class ResearchSerializer(serializers.ModelSerializer):
 templates = serializers.SerializerMethodField()
 class Meta:
 model = Research
 fields = ('id', 'created', 'speaker', 'body', 'templates')
 def get_templates(self, obj):
 values = obj.get_values() # whatever your filter values are. obj is the Research instance
 templates = ResearchTemplate.objects.filter(mergefields__contained_by=values) # Or whatever queryset filter
 return ResearchTemplateSerializer(templates, many=True).data

• Henry Goddard: I was not aware of SerializerMethodField() thanks for this! The answer to a lot of my issues.
• matshidis: I searched for a quite a while to find this, thank you.
• yasbars: Thank you! Just to add, I did not know that the many=True was important for this to work. Initially, the values were still null. Only when I added the many=True did it work! In hindsight, yes the query might return several values, but my use case only returned 1. If you are only returning 1 value, use .first(): templates = ResearchTemplate.objects.filter(mergefields__contained_by=values).first() then you can return without the many=True: return ResearchTemplateSerializer(templates).data