python - Django:子查询的注解

Question

我正在尝试使用 Django 2.0.3 和 PostGIS 使用最近的相邻 Station 的 id 注释 Station 的查询集(GeoDjango) 函数。

简化的 Station 模型:

class Station(models.Model):
    name = models.CharField(max_length=128)
    location = models.PointField()
    objects = StationQuerySet.as_manager()

我遇到的问题是尝试计算最近的距离，这涉及注释引用外部查询集中的 location 的子查询。

from django.db.models import OuterRef, Subquery
from django.contrib.gis.db.models.functions import Distance

class StationQuerySet(models.QuerySet):

    def add_nearest_neighbour(self):
        '''
        Annotates each station with the id and distance of the nearest neighbouring station
        '''
        # Get Station model
        Station = self.model

        # Calculate distances to each station in subquery
        subquery_with_distance = Station.objects.annotate(distance=Distance('location', OuterRef('location')) / 1000)

        # Get nearest from subquery
        nearest = subquery_with_distance.order_by('distance').values('id')[0]

        return self.annotate(
            nearest_station_id=Subquery(nearest)
        )

行 distance = Station.objects.annotate(distance=Distance('location', OuterRef('location'))/1000) 导致如下错误:

from apps.bikeshare.models import Station
stations = Station.objects.add_nearest_neighbour()

错误:

Traceback (most recent call last):
  File "/home/gbrown/Envs/bikeshare-dev/lib/python3.5/site-packages/IPython/core/interactiveshell.py", line 2847, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-3-cb35ea6d5d8b>", line 1, in <module>
    stations = Station.objects.add_nearest_neighbour()
  File "/home/gbrown/Envs/bikeshare-dev/lib/python3.5/site-packages/django/db/models/manager.py", line 82, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/home/gbrown/Development/transit_bikeshare/apps/bikeshare/querysets.py", line 162, in add_nearest_neighbour
    subquery_with_distance = Station.objects.annotate(distance=Distance('location', OuterRef('location')) / 1000)
  File "/home/gbrown/Envs/bikeshare-dev/lib/python3.5/site-packages/django/db/models/manager.py", line 82, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/home/gbrown/Envs/bikeshare-dev/lib/python3.5/site-packages/django/db/models/query.py", line 997, in annotate
    clone.query.add_annotation(annotation, alias, is_summary=False)
  File "/home/gbrown/Envs/bikeshare-dev/lib/python3.5/site-packages/django/db/models/sql/query.py", line 975, in add_annotation
    summarize=is_summary)
  File "/home/gbrown/Envs/bikeshare-dev/lib/python3.5/site-packages/django/db/models/expressions.py", line 452, in resolve_expression
    c.lhs = c.lhs.resolve_expression(query, allow_joins, reuse, summarize, for_save)
  File "/home/gbrown/Envs/bikeshare-dev/lib/python3.5/site-packages/django/contrib/gis/db/models/functions.py", line 58, in resolve_expression
    source_fields = res.get_source_fields()
  File "/home/gbrown/Envs/bikeshare-dev/lib/python3.5/site-packages/django/db/models/expressions.py", line 349, in get_source_fields
    return [e._output_field_or_none for e in self.get_source_expressions()]
  File "/home/gbrown/Envs/bikeshare-dev/lib/python3.5/site-packages/django/db/models/expressions.py", line 349, in <listcomp>
    return [e._output_field_or_none for e in self.get_source_expressions()]
AttributeError: 'ResolvedOuterRef' object has no attribute '_output_field_or_none'

Answer 1

想出了一个解决方法，使用原始查询查找最近的车站并从子查询中选择 id 和距离，奖金解释如下:

class StationQuerySet(models.QuerySet):

    def nearest_neighbour(self):
        '''
        Creates a RawQuerySet of each station with the id and distance of the nearest neighbouring station
        '''
        # Have to execute the query in order to get the list of ids to inject
        ids = tuple(self.values('id').values_list('id', flat=True))

        return self.raw('''
               SELECT
                 A0.id   as id,
                 SUB.closest_id,
                 SUB.closest_distance
               FROM "bikeshare_station" A0
                 CROSS JOIN LATERAL (
                            SELECT
                              B0.id   as closest_id,
                              st_distance_sphere(A0.location, B0.location) as closest_distance
                            FROM "bikeshare_station" B0
                            WHERE A0.id != B0.id
                            ORDER BY A0.location <-> B0.location
                            limit 1
                            ) SUB
               WHERE A0.id IN %s;
           ''', [ids])

用法

您可以将查询集调用链接在一起以在找到最近的邻居之前过滤查询集:

query = Station.objects.filter(name='Albert Gate, Hyde Park')
closest_stations = query.nearest_neighbour()
station = closest_stations[0]
station.name
[out]: 'Albert Gate, Hyde Park'
station.closest_distance
[out]: 133.52459069
station.closest_id
[out]: 6369

SQL 解释

这种类型的子查询称为相关子查询，因为它引用外部查询中的列。此外，我需要选择关于最近车站的多项信息( id 、 distance 等)。

子查询放在FROM中子句，它允许选择多个列。 LATERAL需要加入以允许子查询引用 FROM 中的同级表列表。随着子查询返回单行，CROSS join 可以应用于形成基于笛卡尔积而不是共享列的连接表。

子查询使用 PostGIS <->运算符，它在按站点之间的距离对表格进行排序时效率更高，并且 st_distance_sphere , 进行点与点之间的精确距离计算。