title: Incorrect type. Expected pk value, received dict. - Django REST
body: <p>I'm new with Django Rest, I'm having trouble saving my data, I get "Incorrect type. Expected pk value, received dict", do I have to convert the dictionary into a list? </p>
  
  <p>Company Profile Model:</p>
  
  <pre><code>class CompanyProfile(models.Model):
      name = models.CharField(_('nombre'), max_length=255, db_index=True)
      email = models.EmailField(_('email de contacto'), max_length=255, blank=True)
      description = HTMLField(_('descripción'), blank=True)
      countries = models.ManyToManyField('countries.Country', blank=True, related_name='countries')
  </code></pre>
  
  <p>Country Model: </p>
  
  <pre><code>class Country(models.Model):
      name = models.CharField(_('nombre'), max_length=255)
      description = HTMLField(_('descripción'), blank=True)
  </code></pre>
  
  <p>Serializers:</p>
  
  <pre><code>class CompanyWriteSerializer(serializers.ModelSerializer):
      countries_id = serializers.PrimaryKeyRelatedField(queryset=Country.objects.all(), write_only=True, many=True)
      def create(self, validated_data):
          countries = validated_data.pop('countries_id')
          company = CompanyProfile.objects.register_company(**validated_data,
                                                        country=self.context['request'].user.country.id)
          for c in countries:
              print(c)
              company.countries.add(c)
          return company
  
      def update(self, instance, validated_data):
          instance.countries = validated_data.get('countries_id', instance.countries)
          instance.save()
          return instance
  
      class Meta:
          model = CompanyProfile
          fields = ('countries_id', 'countries')
  </code></pre>