Using select_related to get related_name objects

问题

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!!

回答1:

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

来源：https://stackoverflow.com/questions/33224827/using-select-related-to-get-related-name-objects