title: How to write Nested DRF Serializer with two foreign key fields
body: <p>Suppose I have Django Models as follows:</p>
  
  <pre><code>class Car(models.Model):
      name = models.CharField()
      model = models.CharField()
      type = models.CharField()
  
  class Scooter(models.Model):
      name = models.CharField()
      model = models.CharField()
      type = models.CharField()
  
  class User(models.Model):
      name = models.CharField()
      car = models.ForeignKeyField()
      scooter = models.ForeignKeyField()
  </code></pre>
  
  <p>and the serializers as follows:</p>
  
  <pre><code>class CarSerializer(serializer.ModelSerializer):
      class Meta:
      model = Car
      fields = (
          'name',
          'model',
          'type'
  
      )
  
  class ScooterSerializer(serializer.ModelSerializer):
      class Meta:
      model = Scooter
      fields = (
          'name',
          'model',
          'type'
  
      )
  
  
  class UservehicleCreateSerializer(serializer.ModelSerializer):
      car = serializers.PrimaryKeyRelatedField(read_only=True)
      scooter = serializers.PrimaryKeyRelatedField(read_only=True)
      class Meta:
      model = User
      fields = (
      'name',
      'car',
      'scooter'
      )
  </code></pre>
  
  <p>So the scenario is something like this:
  The <code>car</code> data and <code>scooter</code> data is already created let's assume both have <code>pk=1</code>. and I want to create the <code>User</code> data using the serializer respectively.
  But struggling to find the actual resource for same in my case . The data for car and scooter is already there I need not create it again.</p>
  
  <p>have Tried what's here in <a href="http://www.django-rest-framework.org/api-guide/relations/#writable-nested-serializers" rel="nofollow noreferrer">DRF Documentation for writing serialized nested</a>.</p>
  
  <p>and Data for Api <code>post request</code>is as follows:</p>
  
  <pre><code>{
      'name': "Nikhil",
      'car': 1,
      'scooter': 1
  }
  </code></pre>
  
  <p>Getting error:</p>
  
  <pre><code>IntegrityError: (1048, "Column 'car_id' cannot be null")
  </code></pre>
  
  <p>Need help on this thing.
  Thanks!</p>