如何在Django-Graphene中正确返回Django ORM Union合并查询结果

• 字段类型定义错误：你当前把get_farm_activities定义为单个FarmActivities对象，但resolver返回的是合并后的列表，类型不匹配导致解析失败。
• 值解析逻辑不匹配：Django ORM的values()方法返回的是字典格式结果，而Graphene默认的ObjectType字段解析逻辑是读取对象的属性，不是字典的键值，所以无法正确取到id和name的值。
• UUID类型未序列化：ORM返回的id是UUID对象，直接返回的话字符串类型字段无法自动识别转换。

第一步：修正查询字段的返回类型 #

把单个Field改成List类型，匹配返回的列表结构：

class Query(graphene.ObjectType):
    # 把graphene.Field改为graphene.List
    get_farm_activities = graphene.List(FarmActivities, farm=graphene.String(required=True))

    def resolve_get_farm_activities(self, info, farm):
        # 后续逻辑参考下方调整

第二步：调整ORM查询逻辑，直接返回模型实例 #

不需要手动用values()取字段，直接返回模型实例即可，Graphene会自动从实例属性读取对应字段，同时自动处理UUID转字符串的逻辑，用only()方法可以优化查询性能，只拉取需要的字段：

from itertools import chain

def resolve_get_farm_activities(self, info, farm):
    farm_activities = Activity.objects.filter(
        farmtypeactivityrel__farm_type__farm=farm, deleted_at=None
    ).only("id", "name")

    custom_activities = CustomActivity.objects.filter(
        farm=farm, deleted_at=None
    ).only("id", "name")

    return list(chain(farm_activities, custom_activities))

可选兼容方案：如果必须使用values()返回字典 #

如果业务要求必须用values()返回字典结构，需要给FarmActivities添加字段解析逻辑，手动读取字典键值：

class FarmActivities(graphene.ObjectType):
    id = graphene.String()
    name = graphene.String()

    def resolve_id(self, info):
        return str(self["id"])
    
    def resolve_name(self, info):
        return self["name"]

扩展方案：正确实现Union返回多类型独有字段 #

如果你需要保留Activity和CustomActivity各自的独有字段，之前提到的Union方案可以按如下方式实现：

# 定义Union类型
class FarmActivityUnion(graphene.Union):
    class Meta:
        types = (ActivityType, CustomActivityType)

# 调整查询返回类型
class Query(graphene.ObjectType):
    get_farm_activities = graphene.List(FarmActivityUnion, farm=graphene.String(required=True))

    def resolve_get_farm_activities(self, info, farm):
        farm_activities = Activity.objects.filter(
            farmtypeactivityrel__farm_type__farm=farm, deleted_at=None
        )
        custom_activities = CustomActivity.objects.filter(
            farm=farm, deleted_at=None
        )
        return list(chain(farm_activities, custom_activities))

查询时使用内联片段即可读取对应类型的独有字段：

query GetFarmActivities($farm: String!) {
  getFarmActivities(farm: $farm) {
    __typename
    ... on ActivityType {
      id
      name
      sortOrder
    }
    ... on CustomActivityType {
      id
      name
      farm
    }
  }
}

内容的提问来源于stack exchange，提问作者OpenDataAlex