postgresql 无法解析表达式类型，未知的output_field -返回Django的Coalesce中的模型示例？

txu3uszq  于 2023-06-29  发布在  PostgreSQL
关注(0)|答案(1)|浏览(100)

我有一个端点，它将显示来自模型的结果。每个结果都会有一张图片，如果这个类别的帖子都没有相关的媒体，我会让作者的头像显示它。
我可以在子查询中使用.values('media_source')方法获取媒体url值，但问题是序列化程序需要一个包含其他字段和属性的复杂模型。
我尝试使用下面的代码来获取示例，但收到以下错误：
Cannot resolve expression type, unknown output_field
我知道我需要指定一个字段类型（比如CharField等），但我不知道是否有一个类型实际上被解释为Django模型示例。

def get_queryset(self):
        annotation = Coalesce(
            Subquery(PostMedia.objects.filter(categories=OuterRef('pk'))), Subquery(UserMedia.objects.filter(posts__categories=OuterRef('pk')))
        )
        return Categories.objects.filter(is_public=True).annotate(media_source=annotation)

以下是序列化器：

class MediaSerializer(serializers.Serializer):
    media_detail = MediaField(source="cloudinary_resource")
    url = serializers.CharField(source="media_source")
    type = serializers.CharField()
    height = serializers.SerializerMethodField()
    width = serializers.SerializerMethodField()

    @staticmethod
    def get_height(obj):
        try:
            return obj.metadata.get("height", None)
        except AttributeError:
            if isinstance(obj, dict):
                try:
                    return obj['metadata'].get("height", None)
                except KeyError:
                    pass
            return

    @staticmethod
    def get_width(obj):
        try:
            return obj.metadata.get("width", None)
        except AttributeError:
            if isinstance(obj, dict):
                try:
                    return obj['metadata'].get("width", None)
                except KeyError:
                    pass
            return

class CategorySerializer(serializers.ModelSerializer):
    media = MediaSerializer(source='media_source', many=True)
    tags = SimpleTagSerializer(many=True)

    class Meta:
        model = Category
        fields = ("id", "name", "text", "media", "segment", "tags", "is_public", "is_validated", "metadata")

整个traceback：

api_1             | Internal Server Error: /api/v1/categories/
api_1             | Traceback (most recent call last):
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/core/handlers/exception.py", line 56, in inner
api_1             |     response = get_response(request)
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/core/handlers/base.py", line 197, in _get_response
api_1             |     response = wrapped_callback(request, *callback_args, **callback_kwargs)
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/views/decorators/csrf.py", line 55, in wrapped_view
api_1             |     return view_func(*args, **kwargs)
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/viewsets.py", line 125, in view
api_1             |     return self.dispatch(request, *args, **kwargs)
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/views.py", line 509, in dispatch
api_1             |     response = self.handle_exception(exc)
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/views.py", line 469, in handle_exception
api_1             |     self.raise_uncaught_exception(exc)
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/views.py", line 480, in raise_uncaught_exception
api_1             |     raise exc
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/views.py", line 506, in dispatch
api_1             |     response = handler(request, *args, **kwargs)
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/mixins.py", line 40, in list
api_1             |     page = self.paginate_queryset(queryset)
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/generics.py", line 171, in paginate_queryset
api_1             |     return self.paginator.paginate_queryset(queryset, self.request, view=self)
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/pagination.py", line 387, in paginate_queryset
api_1             |     self.count = self.get_count(queryset)
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/pagination.py", line 525, in get_count
api_1             |     return queryset.count()
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/query.py", line 470, in count
api_1             |     return self.query.get_count(using=self.db)
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/sql/query.py", line 552, in get_count
api_1             |     number = obj.get_aggregation(using, ["__count"])["__count"]
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/sql/query.py", line 537, in get_aggregation
api_1             |     result = compiler.execute_sql(SINGLE)
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/sql/compiler.py", line 1348, in execute_sql
api_1             |     sql, params = self.as_sql()
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/sql/compiler.py", line 1858, in as_sql
api_1             |     inner_query_sql, inner_query_params = self.query.inner_query.get_compiler(
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/sql/compiler.py", line 573, in as_sql
api_1             |     extra_select, order_by, group_by = self.pre_sql_setup()
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/sql/compiler.py", line 64, in pre_sql_setup
api_1             |     self.setup_query()
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/sql/compiler.py", line 55, in setup_query
api_1             |     self.select, self.klass_info, self.annotation_col_map = self.get_select()
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/sql/compiler.py", line 295, in get_select
api_1             |     sql, params = col.select_format(self, sql, params)
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/expressions.py", line 419, in select_format
api_1             |     if hasattr(self.output_field, "select_format"):
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/utils/functional.py", line 49, in __get__
api_1             |     res = instance.__dict__[self.name] = self.func(instance)
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/expressions.py", line 283, in output_field
api_1             |     raise FieldError("Cannot resolve expression type, unknown output_field")
api_1             | django.core.exceptions.FieldError: Cannot resolve expression type, unknown output_field
api_1             | Internal Server Error: /api/v1/categories/
api_1             | Traceback (most recent call last):
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/core/handlers/exception.py", line 56, in inner
api_1             |     response = get_response(request)
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/core/handlers/base.py", line 197, in _get_response
api_1             |     response = wrapped_callback(request, *callback_args, **callback_kwargs)
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/views/decorators/csrf.py", line 55, in wrapped_view
api_1             |     return view_func(*args, **kwargs)
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/viewsets.py", line 125, in view
api_1             |     return self.dispatch(request, *args, **kwargs)
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/views.py", line 509, in dispatch
api_1             |     response = self.handle_exception(exc)
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/views.py", line 469, in handle_exception
api_1             |     self.raise_uncaught_exception(exc)
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/views.py", line 480, in raise_uncaught_exception
api_1             |     raise exc
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/views.py", line 506, in dispatch
api_1             |     response = handler(request, *args, **kwargs)
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/mixins.py", line 40, in list
api_1             |     page = self.paginate_queryset(queryset)
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/generics.py", line 171, in paginate_queryset
api_1             |     return self.paginator.paginate_queryset(queryset, self.request, view=self)
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/pagination.py", line 387, in paginate_queryset
api_1             |     self.count = self.get_count(queryset)
api_1             |   File "/usr/local/lib/python3.10/site-packages/rest_framework/pagination.py", line 525, in get_count
api_1             |     return queryset.count()
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/query.py", line 470, in count
api_1             |     return self.query.get_count(using=self.db)
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/sql/query.py", line 552, in get_count
api_1             |     number = obj.get_aggregation(using, ["__count"])["__count"]
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/sql/query.py", line 537, in get_aggregation
api_1             |     result = compiler.execute_sql(SINGLE)
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/sql/compiler.py", line 1348, in execute_sql
api_1             |     sql, params = self.as_sql()
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/sql/compiler.py", line 1858, in as_sql
api_1             |     inner_query_sql, inner_query_params = self.query.inner_query.get_compiler(
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/sql/compiler.py", line 573, in as_sql
api_1             |     extra_select, order_by, group_by = self.pre_sql_setup()
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/sql/compiler.py", line 64, in pre_sql_setup
api_1             |     self.setup_query()
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/sql/compiler.py", line 55, in setup_query
api_1             |     self.select, self.klass_info, self.annotation_col_map = self.get_select()
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/sql/compiler.py", line 295, in get_select
api_1             |     sql, params = col.select_format(self, sql, params)
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/expressions.py", line 419, in select_format
api_1             |     if hasattr(self.output_field, "select_format"):
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/utils/functional.py", line 49, in __get__
api_1             |     res = instance.__dict__[self.name] = self.func(instance)
api_1             |   File "/usr/local/lib/python3.10/site-packages/django/db/models/expressions.py", line 283, in output_field
api_1             |     raise FieldError("Cannot resolve expression type, unknown output_field")
api_1             | django.core.exceptions.FieldError: Cannot resolve expression type, unknown output_field
api_1             | [21/Jun/2023 21:33:59] "GET /api/v1/categories/ HTTP/1.1" 500 165520
postgresql

1条答案

按热度按时间
cgfeq70w

cgfeq70w1#

我找到了一种方法来做我需要做的：
创建一个自定义Subquery类，将表的整个Row转换为JSONB格式。

class ToJSONB(models.Subquery):

    template = '(SELECT to_jsonb("row") FROM (%(subquery)s) "row")'

    output_field = models.JSONField()

创建序列化程序字段以仅显示所需的信息

class BJSONMediaField(serializers.Field):

    def to_representation(self, value):

        return {"type": value['resource_type'], "public_id": value['public_id'], "version": value['version']}

使用此Custom字段创建序列化程序以Map带注解的字段

class BJSONMediaSerializer(serializers.Serializer):

    media_detail = BJSONMediaField(source="metadata")

    url = serializers.CharField(source="metadata.url")

    type = serializers.CharField()

    height = serializers.IntegerField(source='metadata.height')

    width = serializers.IntegerField(source='metadata.width')

在视图集的get_queryset方法中，使用自定义Subquery创建注解

def get_queryset(self):
    category_media_source = PostMedia.objects.filter(categories=OuterRef('pk'))[:1]

    post_media_source = UserMedia.objects.filter(posts__categories=OuterRef('pk'))[:1]

    categories = Categories.objects.filter(is_public=True).annotate( 
        category_media_source=ToJSONB(category_media_source),
        post_media_source=ToJSONB(post_media_source),
    )

    return categories
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