使用下一个和上一个对象 ID 注释查询集中的对象

Question

如何注释 Django ORM 查询集中的对象，以便每个项目都包含之前（上一个）和之后（下一个）对象的 ID？我正在使用 PostgreSQL 和 Django 1.11。

Answer 1

我认为 Django 范围内稍微更详细的示例可能会对某人有所帮助。查看官方文档here

我在视图中使用它并简单地更新响应对象：

response_data = {}
user = request.user

# put any data in this dict according to the
# logic you have in your view

response_data.update({
    "previous_object":
        get_next_or_previous_object(user, serializer.data, next_obj=False),
    "next_object":
        get_next_or_previous_object(user, serializer.data, next_obj=True),
})



def get_next_or_previous_object(user, data, next_obj=None):
    """
    Get the previous or next object from the given data list.

    If it is not possible to get the object returns None
    :param data: List of objects from which the previous or next object
                 will be taken. OrderedDict and not queryset.
    :param bool next_obj: whether the next object should be returned or
                          the previous one. True = next, False = previous
    :return: the previous/next object or None
    """

    # Get datetime field name which will be used for obtaining the
    # next/previous object
    datetime_field_name = MyView.datetime_field_name
    object_index = 0 if next_obj else -1

    # Get the original object by its id
    try:
        object_id = data[object_index]["id"]
    except (IndexError, KeyError):
        object_id = None

    try:
        parent_object = MyModel.objects.get(id=object_id)
        if next_obj:
            # Example of the method name: get_next_by_my_timestamp_field
            next_method = getattr(
                MyModel, f"get_next_by_{datetime_field_name}")
            object_to_get = next_method(
                parent_object, user=user)
        else:
            # Example of the method name: get_previous_by_my_timestamp_field
            previous_method = getattr(
                MyModel, f"get_previous_by_{datetime_field_name}")
            object_to_get = previous_method(
                parent_object, user=user)

        serializer = MyView.serializer_class(object_to_get)
        previous_next_object = serializer.data
    except MyModel.DoesNotExist:
        previous_next_object = None

    return previous_next_object

Answer 2

你可以使用window functions lag() and lead():

SELECT *
     , lag(id)  OVER (ORDER BY id) AS prev_id  -- same order as query
     , lead(id) OVER (ORDER BY id) AS next_id  -- same order as query
FROM   tbl
ORDER  BY id;