使用select_related获取related_name对象

问题描述

我一直在寻找解决我问题的方法，但是我似乎无法弄清楚.

所以我有两个模型:

  class消息(models.Model):_db ='somedb'id = models.IntegerField(primary_key = True)文字= models.TextField()MessageTags(models.Model)类:_db ='somedb'id = models.IntegerField(primary_key = True)文字= models.TextField()标签= models.ForeignKey(标签)消息=模型.ForeignKey(消息，related_name ='标签') 

我可以使用以下调用查询给定消息的标签: test = Messages.get(id = 234124).tags.all()^返回MessageTag的列表

我想做的是在理论上执行左联接，在该联接中，我使用关联的MessageTags对象获取特定条件下的所有消息.我被指示使用select_related的方向，但是我什么也无法工作.

如何获取消息列表，并将MessageTags作为每个.tags(或其他任何内容)的属性.

谢谢！

解决方案

我不相信 select_related 将能够在您要的方法中执行.但是，实际上，左联接也不会.

这样想:一条消息可以有5个标签.如果进行联接，则结果集中该一条消息的5行，使其看起来像5条消息.

现在，正如您所指出的，您可以在单个消息上引用 .tags.all()并获取与该消息关联的所有MessageTag.如果我正确理解您的情况，这实际上就是您要寻找的.

当然，如果您要优化执行的查询数，我建议您查看 prefetch_related .这是如何使用此示例:

  messages = Messages.objects.filter(text__icontains ='foobar').prefetch_related('tags')对于消息中的消息:对于message.tags.all()中的标签:print('消息ID'，message.id，'-标签'，tag.text) 

应执行2个查询:

1. 检索过滤的邮件列表
2. 检索与这些消息相关的所有标签

I've been searching for a while now for a solution to my problem, but I can't seem to figure this out.

So I have two models:

class Messages(models.Model):
    _db = 'somedb'
    id = models.IntegerField(primary_key=True)
    text = models.TextField()

class MessageTags(models.Model):
    _db = 'somedb'
    id = models.IntegerField(primary_key=True)
    text = models.TextField()
    tag = models.ForeignKey(Tag)
    message = models.ForeignKey(Messages, related_name='tags')

I am able to query the tags for a given message using the following call:test = Messages.get(id=234124).tags.all()^ returns a list of MessageTags

What I would like to do, is to in theory perform a left join, where I get all the messages for a certain criteria with the associated MessageTags objects. I was pointed in the direction of using select_related, but I cannot get anything to work.

How do I get the list of messages, with the MessageTags as an attribute of each as .tags (or anything else..)

Thank you!!

解决方案

I don't believe select_related is going to be able to perform in the method you're looking for. But really, neither would a left join.

Think of it this way: One message could have 5 tags. If you do a join, you will get 5 rows in your result set for that one message, which makes it look like 5 messages.

Now, as you noted, you can reference .tags.all() on an individual message and get all of the MessageTags associated with that Message. If I understand your case correctly, this is actually what you're looking for.

Of course, if you're trying to optimize the number of queries executed, I would recommend looking at prefetch_related. Here is an example of how you could use this:

messages = Messages.objects.filter(text__icontains='foobar').prefetch_related('tags')
for message in messages:
    for tag in message.tags.all():
        print('Message id', message.id, '- tag', tag.text)

That should execute 2 queries:

1. Retrieve your filtered listed of messages
2. Retrieve all tags related to those messages

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