如何为Django序列化器不同字段复用同一验证器？

嘿，这个问题我熟！你想让ChildSerializer里的sister字段复用FatherSerializer中brother的验证逻辑，还得能返回Person对象，之前用自定义验证器没搞定？别担心，有几个实用的方案可以解决：

把brother字段的验证逻辑抽成一个通用的私有方法，这样父类的brother和子类的sister都能调用，而且后续修改逻辑只需要改一处，代码结构也更清晰：

class FatherSerializer(...):
    class Meta(object):
        model = Person
        fields = ('name', 'email', 'brother')
    name = serializers.CharField()
    email = serializers.EmailField()
    brother = serializers.CharField()
    
    def validate_email(self, value):
        if Person.objects.filter(email=value).exists():
            raise serializers.ValidationError('Such email already exists')
        return value
    
    # 提取通用的验证逻辑为私有方法
    def _validate_person_exists(self, value):
        if not Person.objects.filter(name=value).exists():
            raise serializers.ValidationError('Such person doesnt exist')
        return Person.objects.get(name=value)
    
    def validate_brother(self, value):
        return self._validate_person_exists(value)

class ChildSerializer(FatherSerializer):
    class Meta(object):
        model = Person
        fields = ('name', 'email', 'sister')
    sister = serializers.CharField()
    
    def validate_sister(self, value):
        return self._validate_person_exists(value)

你之前用validators参数没法返回Person对象，是因为普通验证器函数如果直接传，没处理好返回值逻辑？其实可以写一个工厂函数，返回的验证器函数既能做校验，又能返回查询到的对象：

def person_exists_validator():
    def validator(value):
        if not Person.objects.filter(name=value).exists():
            raise serializers.ValidationError('Such person doesnt exist')
        return Person.objects.get(name=value)
    return validator

class FatherSerializer(...):
    class Meta(object):
        model = Person
        fields = ('name', 'email', 'brother')
    name = serializers.CharField()
    email = serializers.EmailField()
    # 调用工厂函数获取验证器
    brother = serializers.CharField(validators=[person_exists_validator()])
    
    def validate_email(self, value):
        if Person.objects.filter(email=value).exists():
            raise serializers.ValidationError('Such email already exists')
        return value

class ChildSerializer(FatherSerializer):
    class Meta(object):
        model = Person
        fields = ('name', 'email', 'sister')
    # 同样使用这个验证器
    sister = serializers.CharField(validators=[person_exists_validator()])

注意这里要调用person_exists_validator()而不是直接传函数名，这样每个字段都会拿到独立的验证器实例，而且验证器会返回查询到的Person对象，完美解决你之前的问题。

如果sister的验证逻辑和brother完全一致，没有任何差异，那可以直接在子类的validate_sister方法里调用父类的validate_brother：

class ChildSerializer(FatherSerializer):
    class Meta(object):
        model = Person
        fields = ('name', 'email', 'sister')
    sister = serializers.CharField()
    
    def validate_sister(self, value):
        # 直接复用父类的validate_brother逻辑
        return self.validate_brother(value)

内容的提问来源于stack exchange，提问作者akhavro