如何在DRF中获取序列化器类的计算属性？

你的问题核心在于：SerializerMethodField对应的字段值并不是序列化器实例的属性，它是在序列化过程中通过调用get_*方法动态计算的，所以直接用self.students_count访问会触发AttributeError。以下是几种可行的解决方法：

方法一：直接调用对应的get_*方法 #

既然students_count是由get_students_count方法计算的，直接在get_demand_course_percent里调用这个方法即可，既复用已有逻辑，又避免重复代码：

def get_demand_course_percent(self, obj) -> int:
    users = CustomUser.objects.all().count()
    students = self.get_students_count(obj)
    return (students / users) * 100

方法二：重写to_representation统一计算依赖字段 #

如果多个字段之间存在依赖关系，可以重写序列化器的to_representation方法，提前计算所有需要的字段值，再组装返回结果：

class CourseSerializer(serializers.ModelSerializer):
    lessons = MiniLessonSerializer(many=True, read_only=True)
    # 只声明字段，不单独写get_*方法
    lessons_count = serializers.IntegerField(read_only=True)
    students_count = serializers.IntegerField(read_only=True)
    groups_filled_percent = serializers.IntegerField(read_only=True)
    demand_course_percent = serializers.IntegerField(read_only=True)

    def get_lessons_count(self, obj) -> int:
        return obj.lessons.count()

    def get_students_count(self, obj) -> int:
        return Subscription.objects.filter().count()
    
    def get_groups_filled_percent(self, obj) -> int:
        counts = [group.users.count() for group in obj.groups.all()]
        return ((sum(counts) / len(counts)) / 30) * 100

    def to_representation(self, instance):
        # 提前计算所有需要的统计值
        students_count = self.get_students_count(instance)
        demand_course_percent = (students_count / CustomUser.objects.all().count()) * 100
        
        # 调用父类方法获取基础序列化数据，再补充自定义字段
        data = super().to_representation(instance)
        data['students_count'] = students_count
        data['demand_course_percent'] = demand_course_percent
        data['lessons_count'] = self.get_lessons_count(instance)
        data['groups_filled_percent'] = self.get_groups_filled_percent(instance)
        return data

方法三：提取重复逻辑为私有方法 #

如果某个计算逻辑被多个字段复用，把它抽成序列化器的私有方法，供各个get_*方法调用：

class CourseSerializer(serializers.ModelSerializer):
    lessons = MiniLessonSerializer(many=True, read_only=True)
    lessons_count = serializers.SerializerMethodField(read_only=True)
    students_count = serializers.SerializerMethodField(read_only=True)
    groups_filled_percent = serializers.SerializerMethodField(read_only=True)
    demand_course_percent = serializers.SerializerMethodField(read_only=True)

    def _get_total_students(self) -> int:
        # 抽取出核心计算逻辑
        return Subscription.objects.filter().count()

    def get_lessons_count(self, obj) -> int:
        return obj.lessons.count()

    def get_students_count(self, obj) -> int:
        return self._get_total_students()
    
    def get_groups_filled_percent(self, obj) -> int:
        counts = [group.users.count() for group in obj.groups.all()]
        return ((sum(counts) / len(counts)) / 30) * 100
     
    def get_demand_course_percent(self, obj) -> int:
        users = CustomUser.objects.all().count()
        return (self._get_total_students() / users) * 100

内容的提问来源于stack exchange，提问作者dasEgo