Django / Django Rest Framework-如何允許模型序列化程序將模型的外鍵字段設置為null？

Question

我希望能夠在我的Asset模型中將borrower字段（外鍵）設置為NULL但似乎無法使其正常工作。 我正在嘗試使用具有密鑰borrower等於NULL值的JSON數據發送PATCH請求，但是對於模型實例， borrower字段不會更新為NULL 。 也許序列化程序存在一個問題，即無法將外鍵字段設置為NULL ？

我已經嘗試過將allow_null=True傳遞給BorrowSerializer類，但這沒有用。 我已經在StackOverflow的高處和低處搜索了具有類似問題和解決方案的帖子，但我嘗試過的任何方法都沒有奏效。

這是我的models.py：

from django.conf import settings
from django.db import models
from django.utils import timezone
from datetime import date
from django.contrib.auth.models import User
from django.urls import reverse

import uuid

class Category(models.Model):
    """Model representing an Asset category"""

    name = models.CharField(max_length=128)

    def __str__(self):
        return self.name

class Borrower(models.Model):
    first_name = models.CharField(max_length=64)
    last_name = models.CharField(max_length=128)
    associated_user = models.ForeignKey(User, on_delete=models.CASCADE, null=True)

    def __str__(self):
        return f'{self.first_name} {self.last_name}'

class Asset(models.Model):
    """Model representing an Asset"""
    # Unique identifier for an instance of an asset (a barcode of sorts)
    uid = models.UUIDField(primary_key=True, default=uuid.uuid4)
    name = models.CharField(max_length=200)
    manufacturer = models.CharField(max_length=64)
    model = models.CharField(max_length=128)
    description = models.TextField()
    category = models.ManyToManyField(Category)
    owner = models.ForeignKey(User, on_delete=models.CASCADE)
    borrower = models.ForeignKey(Borrower, on_delete=models.CASCADE, null=True, blank=True)
    checked_out = models.BooleanField(default=False)
    return_date = models.DateField(null=True, blank=True)    

    CONDITION_TYPE = (
        ('e', 'Excellent'),
        ('g', 'Good'),
        ('f', 'Fair'),
        ('p', 'Poor'),
    )

    condition = models.CharField(
        max_length=1,
        choices=CONDITION_TYPE,
        blank=True,
        help_text='Asset condition')

    class Meta:
            ordering = ['return_date']

    @property
    def is_dueback(self):
        if self.return_date and date.today() > self.return_date:
            return True
        return False

    def display_category(self):
        """Create a string for the Category. This is required to display category in Admin."""
        return ', '.join(category.name for category in self.category.all())

    display_category.short_description = 'Category'

    def __str__(self):
        return f'{self.uid} - {self.name}'

    def get_absolute_url(self):
        return reverse('asset-detail', args=[str(self.uid)])

這是我的serializers.py文件：

from rest_framework import serializers
from inventory.models import Asset, Borrower, Category

class BorrowerSerializer(serializers.ModelSerializer):
    class Meta:
        model = Borrower
        fields = ('first_name',
                  'last_name',
                  'associated_user'
        )

    def update(self, instance, validated_data):
        print('Update method triggered.')
        instance.first_name = validated_data.get('first_name', instance.first_name)
        instance.last_name = validated_data.get('last_name', instance.last_name)
        instance.associated_user = validated_data.get('associated_user'. instance.associated_user)
        instance.save()
        return instance

class CategorySerializer(serializers.ModelSerializer):
    class Meta:
        model = Category
        fields = '__all__'

class AssetSerializer(serializers.ModelSerializer):
    borrower = BorrowerSerializer(allow_null=True)
    category = serializers.StringRelatedField(many=True)
    condition = serializers.CharField(source='get_condition_display')

    class Meta:
        model = Asset
        fields = ('name', 
                  'manufacturer', 
                  'model',
                  'description',
                  'condition',
                  'category',
                  'borrower',
                  'checked_out',
                  'return_date',
                  'is_dueback',
        )

這是我的DRF API視圖：

class AssetRetrieveUpdateDestroy(RetrieveUpdateDestroyAPIView):
    lookup_field = 'uid'
    serializer_class = AssetSerializer

    def get_queryset(self):
        user = self.request.user
        return Asset.objects.filter(owner=user)

class BorrowerRetrieveUpdateDestroy(RetrieveUpdateDestroyAPIView):
    lookup_field = 'id'
    serializer_class = BorrowerSerializer

    def get_queryset(self):
        return Borrower.objects.all()

當我傳遞此JSON時，我希望我的Asset模型實例中的borrower字段更新為NULL：

data = {
   'borrower': null
}

但是，我的模型實例是外鍵時，不會更新borrower字段。 如果該字段是CharField或其他名稱，但在外鍵上，則可以正常工作。 我打印了從我的AJAX請求返回的數據，但borrower字段保持不變。

預先感謝您提供的任何幫助。

Answer 1

從文檔 ：

如果您支持可寫的嵌套表示形式，則需要編寫用於保存多個對象的.create()或.update()方法。

因此，你需要實現.update()在你的方法AssetSerializer ：

class AssetSerializer(serializers.ModelSerializer):
    borrower = BorrowerSerializer(allow_null=True)
    ...

    class Meta:
        model = Asset
        fields = (...)

    def update(self, instance, validated_data):
        instance.borrower = validated_data.get('borrower')
        instance.save()

        return instance

這應該做的工作。 因此，無論您傳遞了allow_null=True ，它都將接受null值 ，但是除非您定義自定義更新機制，否則它不會更新嵌套關系。