Fetching foreign key's value in serializer

Question

I have a relation whereby many hosts can belong to one network (imagine a database cluster in Miami - mysql is the host, mia is the network site name). A hostname could be mysql-1c0a-mia.

serializers.py:

class HostSerializer(serializers.ModelSerializer):
    host = serializers.SerializerMethodField()

    def get_host(self, obj):

        site = models.ForeignKey(Network, related_name='site', on_delete=models.DO_NOTHING)

        return '{}-{}-{}'.format(obj.service, obj.instance, site)

    class Meta:
        model = Host
        depth = 1  # to show JOINed stuff from Network

        fields = [
            'id',
            'network',
            'service',
            'instance',
            'ipv4_data',
            'host',
        ]

Network class (from models.py):

class Network(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    name = models.CharField(max_length=63)
    site = models.CharField(max_length=3)
    domain = models.CharField(max_length=63)
    vlan = models.PositiveIntegerField(null=True, validators=[MaxValueValidator(4094)])
    ipv4_net = models.GenericIPAddressField(protocol='IPv4', null=True)
    ipv4_gateway = models.GenericIPAddressField(protocol='IPv4', null=True)
    ipv4_netmask = models.PositiveIntegerField(null=True, validators=[MaxValueValidator(32)])

    class Meta:
        verbose_name = "Network"
        verbose_name_plural = "Networks"

    def __str__(self):
        return self.name

Host class (from models.py):

class Host(models.Model):
    network = models.ForeignKey(Network, on_delete=models.SET_NULL, null=True)
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    service = models.CharField(max_length=6)
    instance = RandomSlugField(length=4, exclude_upper=True)
    ipv4_data = models.GenericIPAddressField(protocol='IPv4', null=True)

    class Meta:
        verbose_name = "Host"
        verbose_name_plural = "Hosts"

    def __unicode__(self):
        return '%s-%s-net.%s' % (self.name, "placeholder-site-name", self.domain)

What's going on with host at the very end? I essentially just want to pull in the network's site name - mia.

sample json output:

{
    "id": "b2994407-6c02-41fa-816c-745c55269ac8",
    "network": {
        "id": "fa73846a-cfa5-4d7a-97c2-73c1adf0d9a0",
        "name": "stackoverflow",
        "site": "mia",
        "domain": "core.example.com",
        "vlan": 1234,
        "ipv4_net": "10.23.45.0",
        "ipv4_gateway": "10.23.45.1",
        "ipv4_netmask": 24
    },
    "service": "mysql",
    "instance": "1c0a",
    "ipv4_data": "10.23.45.34",
    "host": "mysql-1c0a-<django.db.models.fields.related.ForeignKey>"
},

Answer 1

You are declaring a new ForeignKey field inside the serializer class, but you just need to access the related ForeignKey field value:

def get_host(self, obj):
    return '{}-{}-{}'.format(obj.service, obj.instance, obj.network.site)