Django/Django Rest Framework - 如何允许模型序列化程序将模型的外键字段设置为空？

Question

我希望能够将我的Asset 模型中的borrower 字段（外键）设置为NULL，但我似乎无法让它工作。我正在尝试发送一个带有 JSON 数据的 PATCH 请求，其键 borrower 等于 NULL 的值，但模型实例的 borrower 字段不会更新为 NULL。可能是序列化程序存在问题，导致无法将外键字段设置为NULL？

我已经尝试将allow_null=True 传递给BorrowSerializer 类，但没有奏效。我在 StackOverflow 上搜索了有类似问题和解决方案的帖子，但我尝试过的都没有。

这是我的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)])

这是我的 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',
        )

这是我的 DRF API 视图：

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()

当我传入此 JSON 时，我希望 Asset 模型实例中的 borrower 字段更新为 NULL：

data = {
   'borrower': null
}

但是，当 borrower 字段是外键时，我的模型实例不会更新它。如果该字段是CharField 或其他东西，但在外键上，它工作正常。我打印出我的 AJAX 请求返回的数据，但 borrower 字段保持不变。

提前感谢您提供的任何帮助。

Answer 1

来自the docs：

如果您支持可写嵌套表示，则需要编写处理保存多个对象的 .create() 或 .update() 方法。

因此您需要在您的AssetSerializer 中实现.update() 方法：

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

这应该可以完成工作。因此，无论您是否传递了 allow_null=True，它都会接受 null 值，但不会更新您的嵌套关系，除非您定义自定义更新机制。