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How to make more than one fields primary key and remove auto generated id in django models

Suppose in a relational database schema we have a student, a subject and a teacher which connect to each other with a relation teaches. Also, the relation has an attribute time that stores the time of the lesson. This is the most complete yet simplified example I can think to describe my case. Now, the most pythonic and django-wise way I can think of trying to reach a correct solution is, after creating a model class for student, subject and teacher, to create a new class Teaches, which has the foreign keys for the three other classes; also it has the property date field for time. This class would look something like this:
class Teaches(models.Model):
teachers = models.ForeignKey(Teacher, on_delete_models.CASCADE)
subjects = models.ForeignKey(Subject, on_delete_models.CASCADE)
students = models.ForeignKey(Student, on_delete_models.CASCADE)
time = models.DateField
class Meta:
constraints = [
fields=['teachers', 'subjects', 'students']
name='teacher_subject_student_triplet'
]
I added the Meta class because this is what this answer recommends as the correct approach.
The problem is that that in the migrations file I can still see the id field. The only way I've seen there is to remove it is to set another field as Primary Key, but in my case I cannot do that, having more than one keys. Any suggestions?

=========== model.py =============
from django.db import models
class TeacherModel(models.Model):
teacher_code = models.CharField(max_length=255)
def __str__(self):
return self.teacher_code
class SubjectModel(models.Model):
subject_code = models.CharField(max_length=255)
def __str__(self):
return self.subject_code
class StudentModel(models.Model):
student_code = models.CharField(max_length=255)
def __str__(self):
return self.student_code
class Teaches(models.Model):
custom_primary_key = models.SlugField(primary_key=True,blank=True)
teacher = models.ForeignKey(TeacherModel, on_delete=models.CASCADE)
subject = models.ForeignKey(SubjectModel, on_delete=models.CASCADE)
student = models.ForeignKey(StudentModel, on_delete=models.CASCADE)
time = models.DateField
#property
def make_key(self):
new_key = str(self.teacher.teacher_code + self.subject.subject_code + self.student.student_code)
return new_key
def save(self, *args, **kwargs):
self.custom_primary_key = self.make_key
super(Teaches, self).save(*args, **kwargs)
========= Output ==============

You can remove autogenerated id by adding primary_key=True, see below code:
class Person(models.Model):
username = CharField(primary_key=True, max_length=100)
first_name = CharField(null=True, blank=True, max_length=100)
setting a field to primary_key=True automatically makes it unique and not null.

In settings.py:
DEFAULT_AUTO_FIELD = 'django.db.models.BigAutoField'
Controls the automatic generation of primary keys of each model if defined in settings.
Read this article:
Set AutoField or BigAutoField on a per model basis

access attribute via serializer django

I got following models:
class OrderItem(models.Model):
ordered_amount = models.IntegerField(validators=[MinValueValidator(0)])
amount = models.IntegerField(default=0)
order = models.ForeignKey(
Order, on_delete=models.CASCADE, related_name="order_items"
)
class Order(models.Model):
reference = models.CharField(max_length=50)
purchase_order = models.CharField(max_length=15, blank=True, null=True)
I'm now writing a serializer for listing orders. In this OrderSerializer I need to access amount and ordered_amount in the OrderItem class. How do I do this?
This is What I have now:
class AdminOrderListSerializer(serializers.ModelSerializer):
amount = serializers.IntegerField()
ordered_amount = serializers.IntegerField()
class Meta:
model = Order
fields = [
"purchase_order",
"reference",
"amount",
"ordered_amount",
]
# noinspection PyMethodMayBeStatic
def validate_amount(self, order):
if order.order_items.amount:
return order.order_items.amount
return
# noinspection PyMethodMayBeStatic
def validate_ordered_amount(self, order):
if order.order_items.ordered_amount:
return order.order_items.ordered_amount
return
This gives me following error:
AttributeError: Got AttributeError when attempting to get a value for field amount on serializer AdminOrderItemListSerializer.
The serializer field might be named incorrectly and not match any attribute or key on the Order instance.
Original exception text was: 'Order' object has no attribute 'amount'.

There are many ways to that, one of them is SerializerMethodField:
from django.db.models import Sum
class AdminOrderListSerializer(serializers.ModelSerializer):
amount = serializers.SerializerMethodField()
ordered_amount = serializers.SerializerMethodField()
def get_amount(self,obj):
return obj.order_items.aggregate(sum=Sum('amount'))['sum']
def get_ordered_amount(self,obj):
return obj.order_items.aggregate(sum=Sum('order_amount'))['sum']
Optimized solution
Another way of achieving this is to annotate the data to queryset, and access them in serializer. For that, you need to change in view:
class SomeView(ListAPIView):
queryset = Order.objects.annotate(amount=Sum('order_items__amount'),order_amount=Sum('order_items__order_amount'))
This is a optimized solution because it reduces database hits(it only hits once).

Django serializer, nested relation and get_or_create

I've been bugging on this issue for some time now. I have two models : Acquisitions and RawDatas.
Each RawData have one Acquisition, but many RawDatas can have the same Acquisition.
I want to create or get the instance of Acquisition automatically when I create my RawDatas. And I want to be able to have all informations using the serializer.
class Acquisitions(models.Model):
class Meta:
unique_together = (('implant', 'beg_acq', 'duration_acq'),)
id = models.AutoField(primary_key=True)
implant = models.ForeignKey("Patients", on_delete=models.CASCADE)
beg_acq = models.DateTimeField("Beggining date of the acquisition")
duration_acq = models.DurationField("Duration of the acquisition")
class RawDatas(models.Model):
class Meta:
unique_together = (('acq', 'data_type'),)
id = models.AutoField(primary_key=True)
acq = models.ForeignKey("Acquisitions", on_delete=models.CASCADE)
data_type = models.CharField(max_length=3)
sampling_freq = models.PositiveIntegerField("Sampling frequency")
bin_file = models.FileField(db_index=True, upload_to='media')
And my serializers are these :
class AcquisitionSerializer(serializers.ModelSerializer):
class Meta:
model = Acquisitions
fields = ('id', 'implant', 'beg_acq', 'duration_acq')
class RawDatasSerializer(serializers.ModelSerializer):
acq = AcquisitionSerializer()
class Meta:
model = RawDatas
fields = ('id', 'data_type', 'sampling_freq', 'bin_file', 'acq')
def create(self, validated_data):
acq_data = validated_data.pop('acq')
acq = Acquisitions.objects.get_or_create(**acq_data)
RawDatas.objects.create(acq=acq[0], **validated_data)
return rawdatas
My problem is that, using this, if my instance of Acquisitions already exists, I get a non_field_errors or another constraint validation error.
I would like to know what is the correct way to handle this please ?
So I can automatically create this using the nested serializer, and when I only want to have informations (such as a GET request), I can have all the field I need (every field of the two models).
Thanks in advance for your help !

Try this:
class AcquisitionSerializer(serializers.ModelSerializer):
class Meta:
model = Acquisitions
fields = ('id', 'implant', 'beg_acq', 'duration_acq')
class RawDatasSerializer(serializers.ModelSerializer):
class Meta:
model = RawDatas
fields = ('id', 'data_type', 'sampling_freq', 'bin_file', 'acq')
def create(self, validated_data):
acq_data = validated_data.pop('acq')
acq = Acquisitions.objects.filter(id=acq_data.get('id')).first()
if not acq:
acq = AcquisitionSerializer.create(AcquisitionSerializer(), **acq_data)
rawdata = RawDatas.objects.create(acq=acq, **validated_data)
return rawdata

Django: Can I use objects.filter() for generic foreignkey?

symbol.py
class Symbol(BaseModel):
name = models.CharField(max_length=30,)
class Meta:
abstract = True
class StockSymbol(Symbol):
market = models.CharField(max_length=10,)
my_daily_price = GenericRelation(MyDailyPrice)
daily_price.py
class DailyPrice(BaseModel):
content_type = models.ForeignKey(ContentType)
object_id = models.PositiveIntegerField()
content_object = GenericForeignKey('content_type', 'object_id')
class Meta:
abstract = True
class MyDailyPrice(DailyPrice):
open = models.DecimalField(
max_digits=15,
decimal_places=2,
)
What I want to do is,
symbol = StockSymbol.objects.first()
MyDailyPrice.objects.filter(content_object=symbol)
But it occured errors:
FieldError: Field 'content_object' does not generate an automatic reverse relation and therefore cannot be used for reverse querying. If it is a GenericForeignKey, consider adding a GenericRelation.
StockSymbol already has GenericRelation. What's wrong with it?
Or do I have to override ojbect manager?

You can filter with content_type and object_id, instead of content_object.
from django.contrib.admin.options import get_content_type_for_model
symbol = StockSymbol.objects.first()
MyDailyPrice.objects.filter(content_type=get_content_type_for_model(symbol), object_id=symbol.pk)

I wrapped up #Akash's answer in a method to be added to a custom Manager or QuerySet:
def gfks(self, **kwargs):
filters = {}
for field, obj in kwargs.items():
gfk = self.model._meta.get_field(field)
filters[gfk.ct_field] = ContentType.objects.get_for_model( obj )
filters[gfk.fk_field] = obj.pk
return self.filter(**filters)
For example, if you had a model called Comparison with two GFKs called product1 and product2, and added this method, usage would look like:
comp = Comparison.objects.gfks(product1=foo, product2=bar)
Would be nice if Django's contenttypes app provided some similar sugar automatically, but I'll settle for adding this to my BaseQuerySet class in the meantime.

Django: CharField remove default ------— choice

In my models.py I have the following CharField
class Method1(models.Model):
inputfile_param = models.FileField()
clustering_method_param = models.CharField(max_length=20,
default='ward', blank=True, choices=(
('complete', 'Complete linkage'),
('average','Average linkage'),
('ward','Ward'),))
How do I remove the default --------- choice from CharField?
I tried inserting empty_label=None but not working.
And I cannot remove blank=True because it will prevent
FileField() failed to capture the uploaded file.
My forms.py looks like this:
class Method1ClusteringForm(forms.ModelForm):
def __init__(self, *args, **kwargs):
super(Method1ClusteringForm, self).__init__(*args, **kwargs)

I have found this which may be the solution for you, too.
Try:
from django.forms import ModelForm
from django import forms as forms
class Method1ClusteringForm(ModelForm):
clustering_method_param = forms.forms.TypedChoiceField(
required=True,
initial = 'ward',
choices = (
('complete', 'Complete linkage'),
('average','Average linkage'),
('ward','Ward'),)
)
class Meta:
model = Method1
fields = ('inputfile_param', 'clustering_method_param',)

you must remove blank=True in your field definition. see here