django queryset include more columns in select statement

问题

I been trying to create a backward relation using queryset and the joining is working fine, accept that its not including the other joined table in the selected columns. Below is my models, queryset and query.str() print

class Main(models.Model):
    slug       = models.SlugField()
    is_active  = models.BooleanField(default=True)
    site       = models.ForeignKey(Site)
    parent     = models.ForeignKey('self', blank=True, null=True, limit_choices_to={'parent' : None})
    class Meta:
        unique_together = (("slug", "parent"))
    def __unicode__(self):
        return self.slug

class MainI18n(models.Model):
    main                = models.ForeignKey(Main)
    language            = models.CharField(max_length=2, choices=settings.LANGUAGES)
    title               = models.CharField(max_length=100)
    label               = models.CharField(max_length=200, blank=True, null=True)
    description         = models.TextField(blank=True, null=True)
    disclaimer          = models.TextField(blank=True, null=True)
    class Meta:
        unique_together = (("language", "main"))
    def __unicode__(self):
        return self.title
class List(models.Model):
    main        = models.ForeignKey(Main)
    slug        = models.SlugField(unique=True)
    is_active   = models.BooleanField(default=True)
    parent      = models.ForeignKey('self', blank=True, null=True)

    def __unicode__(self):
        return self.slug
class ListI18n(models.Model):
    list        = models.ForeignKey(List)
    language    = models.CharField(max_length=2, choices=settings.LANGUAGES)
    title       = models.CharField(max_length=50)
    description = models.TextField()
    class Meta:
        unique_together = (("language", "list"))
    def __unicode__(self):
        return self.title

and my queryset is

Main.objects.select_related('main', 'parent').filter(list__is_active=True, maini18n__language='en', list__listi18n__language='en')

and this is what my query is printing

'SELECT `category_main`.`id`, `category_main`.`slug`, `category_main`.`is_active`, `category_main`.`site_id`, `category_main`.`parent_id`, T5.`id`, T5.`slug`, T5.`is_active`, T5.`site_id`, T5.`parent_id` FROM `category_main` INNER JOIN `category_maini18n` ON (`category_main`.`id` = `category_maini18n`.`main_id`) INNER JOIN `category_list` ON (`category_main`.`id` = `category_list`.`main_id`) INNER JOIN `category_listi18n` ON (`category_list`.`id` = `category_listi18n`.`list_id`) LEFT OUTER JOIN `category_main` T5 ON (`category_main`.`parent_id` = T5.`id`) WHERE (`category_maini18n`.`language` = en  AND `category_list`.`is_active` = True  AND `category_listi18n`.`language` = en )'

anyone can help show columns from list and listi18n? I tried extra but It doesn't allow me to pass things like category_list.*

thanks

UPDATE

Thanks for Daniel approach, I managed to get it to work but instead I had to start from ListI18n

ListI18n.objects.select_related('list', 'list__main', 'list__main__parent', 'list__main__i18nmain').filter(list__is_active=True, list__main__maini18n__language='en', language='en').query.__str__()

Its working perfectly now, but I couldn't include list_main_maini18n, below is the output query

'SELECT `category_listi18n`.`id`, `category_listi18n`.`list_id`, `category_listi18n`.`language`, `category_listi18n`.`title`, `category_listi18n`.`description`, `category_list`.`id`, `category_list`.`main_id`, `category_list`.`slug`, `category_list`.`is_active`, `category_list`.`parent_id`, `category_main`.`id`, `category_main`.`slug`, `category_main`.`is_active`, `category_main`.`site_id`, `category_main`.`parent_id`, T5.`id`, T5.`slug`, T5.`is_active`, T5.`site_id`, T5.`parent_id` FROM `category_listi18n` INNER JOIN `category_list` ON (`category_listi18n`.`list_id` = `category_list`.`id`) INNER JOIN `category_main` ON (`category_list`.`main_id` = `category_main`.`id`) INNER JOIN `category_maini18n` ON (`category_main`.`id` = `category_maini18n`.`main_id`) LEFT OUTER JOIN `category_main` T5 ON (`category_main`.`parent_id` = T5.`id`) WHERE (`category_list`.`is_active` = True  AND `category_listi18n`.`language` = en  AND `category_maini18n`.`language` = en )'

Any idea how can I include MainI18n in the query result? should I use extra and include the tables and do the relation in the where clause? or is there a better approach?

回答1:

The relationship from Main to List is a backwards ForeignKey (ie the FK is on List pointing at Main), and select_related doesn't work that way. When you think about it, this is correct: there are many Lists for each Main, so it doesn't make sense to say "give me the one List for this Main", which is what select_related is all about.

If you started from List, it would work:

List.objects.select_related('main__parent').filter(is_active=True, main__maini18n__language='en', listi18n__language='en')

because that way you're only following forwards relationships. You may find you're able to reorder your views/templates to use the query this way round.

回答2:

Looks to me it is actually working (T5 in your select statement). You can always access fields from related instances in django via something like my_obj.parent.is_active. If you used select_related before they are included in the first query. If you didn't specify it in select_related a call to my_obj.parent.is_active for example would perform an extra db query.

来源：https://stackoverflow.com/questions/5889751/django-queryset-include-more-columns-in-select-statement

标签

python

django

orm

django-models

django-queryset