在Django中一对多地预取行为不符合预期

问题描述

例如，我在Django中有一个简单的一对多关系:

I have a simple one-to-many relationship in Django, for example:

class Team(models.Model):

    team_mascot = models.CharField(
        max_length=200, null=True)

class Player(models.Model):

    first_name = models.CharField(max_length=200, null=True)
    last_name = models.CharField(max_length=200, null=True)

    team = models.ForeignKey(
        'players.Team',
        blank=True, null=True,
        on_delete=models.SET_NULL,
        related_name='players',
        related_query_name='player',
    )

在玩家管理页面上，如果该玩家拥有一个团队，我想显示一些与该玩家相关联的团队的信息，包括有关该团队中其他玩家的信息.我想通过对当前玩家团队的所有相关玩家进行预取来优化此查询.这应该很简单，但是我似乎无法获得正确的预取.

On the Player admin page(s), I want to display some information about the Team associated with that player if the player has a team, including information about other players on that team. I want to optimize this query with a prefetch for all the related players to the current players' team. This should be fairly simple, but I can't seem to get the right prefetch.

这是我尝试过的:

def prefetch_all_team_players(self):

  return self.prefetch_related(
    Prefetch('team__players', to_attr='all_team_players')
    )

和:

def prefetch_all_team_players(self):
    return self.select_related('team').prefetch_related(
        Prefetch(
            'team__players',
            to_attr='all_team_players'
        )
    )

和:

def prefetch_all_team_players(self):
    from myproj.players.models import Team

    team_queryset = Team.objects.all()
    return self.prefetch_related(
        Prefetch('team__players', to_attr='all_team_players', queryset=team_queryset)
        )

我正在相应的管理页面上使用此功能.但是，没有按预期填充 all_team_players .我没有任何价值. player.all_team_players.all()没有给我任何东西.

I'm using this on the appropriate admin page.However, all_team_players is not being populated as expected. I'm not getting any value. player.all_team_players.all() doesn't give me anything.

当然，替代方法是仅在需要的地方使用 player.team.players.all()，这是可行的.但是我正在尝试通过预取来获得性能.

The alternative is of course just using player.team.players.all() wherever I need it, which works. But I'm trying to gain performance with the prefetch.

关于我在这里缺少什么的任何想法?

Any ideas on what I'm missing here?

推荐答案

我知道了-非常简单.第一和第二个预取语句就可以了.但是我访问它们的方式却不是.应该这样访问:

I figured it out - pretty simple. The first and second prefetch statements are just fine. However the way I was accessing them was not. Should have accessed like so:

player.team.all_team_players

忘记先通过玩家访问团队，然后可以访问该属性.h

forgot to access the team first thru player, then the attribute is accessible. Doh

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