title: How to get authenticated user on serializer class for validation
body: <p>I'm working on a project using django-rest-framework. In my API view, an authenticated user can create other users. But, only five. Then if there are five users registered by one user, I want to send him in the response that hit the limit. Then, I need to get on my serializer the authenticated user but, I can't find a way to pass it from my ModelViewSet to my serializer. </p>
  
  <p>This is my code:</p>
  
  <p>View:</p>
  
  <pre><code>class ChildUserViewSet(viewsets.ModelViewSet):
      serializer_class = ChildUserSerializer
      queryset = User.objects.all()
  
      authentication_classes = (
          TokenAuthentication,
      )
      permission_classes = (
          IsAuthenticated,
      )
  
      def perform_create(self, serializer):
          account_group = self.request.user.userprofile.get_account_group
  
          mobile_number = serializer.data.get('mobile_number')
          password = serializer.data.get('password')
          user = serializer.save()
          user.set_password(password)
          user.save()
  
          # Generate user profile
          UserProfile.objects.create(
              user=user,
              mobile_number=mobile_number,
              user_type=CHILD,
              related_account_group=account_group,
          )
  </code></pre>
  
  <p>Serializer:</p>
  
  <pre><code>class ChildUserSerializer(serializers.ModelSerializer):
      mobile_number = serializers.CharField()
  
      class Meta:
          model = User
  
          fields = (
              'first_name',
              'last_name',
              'email',
              'password',
              'mobile_number',
          )
  
      def validate(self, data):
          """
          Check that the start is before the stop.
          """
          # Get authenticated user for raise hit limit validation
  
  
  
      def validate_email(self, value):
          if User.objects.filter(email=value):
              raise serializers.ValidationError("This field must be unique.")
          return value
  
      def create(self, validated_data):
          username = generate_unique_username(
              u'{0}{1}'.format(
                  validated_data['first_name'],
                  validated_data['last_name'],
              )
          )
  
          user = User(
              username=username,
              first_name=validated_data['first_name'],
              last_name=validated_data['last_name'],
              email=validated_data['email'],
          )
  
          user.set_password(validated_data['password'])
          user.save()
  
          return user
  </code></pre>
  
  <p>Then, in the <code>def validate(self, data)</code> function of my serializer, I want to get the currently authenticated user. </p>
  
  <p>How can I pass the request.user from my APIView to my serializer?</p>