django rest framework relation field - django

Question

I have models as below:
class Bus(models.Model):
bid = models.CharField(_("Bus unique id"), max_length=45)
plate_number = models.CharField(_("Plate number"), max_length=45)
class BusData(models.Model):
bus = models.ForeignKey(Bus, on_delete=models.CASCADE)
timestamp = models.DateTimeField()
speed = models.IntegerField(_("Speed"), null=True, blank=True)
I want to retrieve the bus object with the busdata together, thus I use below serializer classes.
class BusSerializer(serializers.ModelSerializer):
class Meta:
model = Bus
fields = ('bid', 'plate_number', 'busdata_set')
class BusDataSerializer(serializers.ModelSerializer):
class Meta:
model = BusData
So far, the BusSerializer output looks like this:
In [8]: bus1 = Bus.objects.create(bid='1',plate_number='X101')
In [10]: from django.utils import timezone
In [15]: bus_data1 = BusData.objects.create(bus=bus1,timestamp=timezone.now(),speed=100)
In [18]: bus_data2 = BusData.objects.create(bus=bus1,timestamp=timezone.now(),speed=200)
In [20]: bus_s = BusSerializer(bus1)
In [21]: bus_s.data
Out[21]:
OrderedDict([('bid', u'1'),
('plate_number', u'X101'),
('busdata_set', [10, 11])])
As you can see, the busdatas associated with this bus are already taken in busdata_set fields.
However, what I want here is just get the latest busdata record to return (sort by the timestamp field desc, and return the first record), not all the busdata record.
Something looks like this,
OrderedDict([('bid', u'1'),
('plate_number', u'X101'),
('busdata', 11)])
I've no idea how to achieve this.
Any thoughts and code snippets would be appreciate.
Thanks so much!

Answer 1

There are many ways to do this. The simplest would be to have a property on the Bus class that points to the latest BusData object, and then retrieve it in the serializer.
class Bus(models.Model):
bid = models.CharField(_("Bus unique id"), max_length=45)
plate_number = models.CharField(_("Plate number"), max_length=45)
#property
def bus_data(self):
return self.busdata_set.latest('timestamp')
Then make the change in the serializer:
class BusSerializer(serializers.ModelSerializer):
bus_data = serializers.Field()
class Meta:
model = BusData

Answer 2

As Django-Rest-Framework Serialize Relations docs says You can define the queryset in your PrimaryKeyRelatedField. So you could try like this:
class BusSerializer(serializers.ModelSerializer):
busdata_set = serializers.PrimaryKeyRelatedField(queryset=BusData.objects.create(bus=bus1,timestamp=timezone.now(),speed=200), many=True)
class Meta:
model = Bus
fields = ('bid', 'plate_number', 'busdata_set')