AssertionError at ************** in Django-Rest Framework

Question

I am working on ModelSerializers in Django rest-framework.

models.py

from __future__ import unicode_literals
from django.db import models
from django.contrib.postgres.fields import ArrayField
from django.contrib.postgres.fields import HStoreField
from django.contrib.postgres.validators import KeysValidator

    class Component(models.Model):
        component_name = models.CharField(max_length=50)
        allowed_extensions = ArrayField(models.CharField(max_length=50))

        def __str__(self):
            return self.component_name

    class Image_Type(models.Model):
        image_name = models.CharField(max_length=30)
        def __str__(self):
            return self.image_name
    class Image_Component(models.Model):
        component_name = models.ForeignKey('Component')
        image_name = models.ForeignKey('Image_Type')
    class Image_Meta(models.Model):
        image_component = models.ForeignKey('Image_Component',on_delete=models.CASCADE,)
        component_id = models.IntegerField()
        image_count = models.IntegerField(default=0)
        version = models.CharField(max_length=10)

    class Image(models.Model):
        image_meta = models.ForeignKey('Image_Meta',on_delete=models.CASCADE,)
        image_path = models.URLField(max_length=200)
        order = models.IntegerField()
        version = models.CharField(max_length=10)


    class Feature(models.Model):
        image_component = models.ForeignKey('Image_Component',on_delete=models.CASCADE,)
        feature_value = HStoreField()
        def save(self,*args,**kwargs):
            if Feature.objects.filter(feature_value__has_keys=['size', 'quality' , 'format']):
                super(Feature, self).save(*args, **kwargs)
            else:
                print("Incorrect key entered")

serializers.py

from rest_framework import serializers
from models import Component , Image_Type , Image_Component , Image , Feature , Image_Meta
from django.contrib.postgres.fields import ArrayField
from django.contrib.postgres.fields import HStoreField


class ComponentSerializer(serializers.ModelSerializer):
    class Meta:
        model = Component
        field = {'component_name','allowed_extensions'}


class Image_TypeSerializer(serializers.ModelSerializer):
    class Meta:
        model = Image_Type
        field = {'image_name'}



class Image_ComponentSerializer(serializers.ModelSerializer):
    component_name = ComponentSerializer()
    image_name = Image_TypeSerializer()
    class Meta:
        model = Image_Component
        field = {'component_name','image_name'}

    def create(self, validated_data):
        component_data = validated_data.pop('component_name')
        image_data = validated_data.pop('image_name')
        image_component = Image_Component.objects.create(**validated_data)

        for component_data in component_data:
            Component.objects.create(image_component=image_component, **component_data)

        for image_data in image_data:
            Image_Type.objects.create(image_component=image_component, **image_data)

        return image_component

class Image_MetaSerializer(serializers.ModelSerializer):
    image_component = Image_ComponentSerializer()
    class Meta:
        model = Image_Meta
        field = {'image_component','component_id' , 'image_count', 'version'}

class ImageSerializer(serializers.ModelSerializer):
    image_meta = Image_MetaSerializer()
    class Meta:
        model = Image_Meta
        field = {'image_meta' , 'image_path' , 'order' , 'version'}

class FeatureSerializer(serializers.ModelSerializer):
    image_component = Image_ComponentSerializer()
    class Meta:
        model = Feature
        field = {'image_component' , 'feature_value'}

urls.py

from django.conf.urls import patterns , include , url
from rest_framework.urlpatterns import format_suffix_patterns
from imagedata import views

urlpatterns = patterns('',
    url(r'^api/component$',views.Component.as_view()),
    url(r'^api/image_type$',views.Image_Type.as_view()),
    url(r'^api/image_meta$',views.Image_Meta.as_view()),
    url(r'^api/image_component$',views.Image_Component.as_view()),
    url(r'^api/image$',views.Image.as_view()),
    url(r'^api/feature$',views.Feature.as_view()),
)

urlpatterns = format_suffix_patterns(urlpatterns)

It works fine for component and image_type models i.e, if I try to update these tables I can do for both. But for image_component model whenever I try to update the table I get the error:

AssertionError at /imagedata/api/image_component
The `.create()` method does not support writable nestedfields by default.
Write an explicit `.create()` method for serializer `imagedata.serializers.Image_ComponentSerializer`, or set `read_only=True` on nested serializer fields.
Request Method: POST
Request URL:    http://127.0.0.1:8000/imagedata/api/image_component
Django Version: 1.9
Exception Type: AssertionError
Exception Value:    
The `.create()` method does not support writable nestedfields by default.
Write an explicit `.create()` method for serializer `imagedata.serializers.Image_ComponentSerializer`, or set `read_only=True` on nested serializer fields.
Exception Location: /Library/Python/2.7/site-packages/rest_framework/serializers.py in raise_errors_on_nested_writes, line 724
Python Executable:  /usr/bin/python
Python Version: 2.7.10

The page at http://127.0.0.1:8000/imagedata/api/image_component looks like this :

I may be totally wrong but I think the text fields should ask to choose from the set of values of component_name and image_type that are present in component and Image_Type tables respectively but they are not.What am I doing wrong? And how to correct that??

Also there is no problem with Django-Admin. It works just fine.

Answer 1

As a first thing I want to say that I am pretty sure that the error you got was before adding the .create method on Image_ComponentSerializer.

Since Image_Component is based on 2 foreign keys if you want in the API a select field with both of them, just change your serializer to this (remove .create method since is not needed):

class Image_ComponentSerializer(serializers.ModelSerializer):
    class Meta:
        model = Image_Component

So no custom serializer, however this will affect the representation of the Image_Component object, it will show the ids of FK fields. To change this behaviour I override .to_representation method like this:

def to_representation(self, obj):  
    ret = {'component_name': ComponentSerializer(instance=obj.component_name).data,
           'image_name': ImageTypeSerializer(instance=obj.image_name).data}
    return ret

So, the input will be ids of FK and the output will be based on custom serializers for each field.