select_related prefetch_related後

我的模型看起來像：select_related prefetch_related後

class Book(models.Model): 
    publisher = models.ForeignKey(Publisher) # This is not important 

class Baz(models.Model): 
    a = models.CharField(max_length=100) 

class Page(models.Model): 
    book = models.ForeignKey(Book) 
    baz = models.ForeignKey(Baz)

，我試圖運行這樣的查詢：

[[x.baz.a for x in y.page_set.all()] 
for y in Book.objects.all().prefetch_related('page_set', 'page_set__baz')]

其中我認爲ORM應該能夠爲兩個查詢執行：（Page JOIN Baz）和Book。最終的連接應該在Python中進行。相反，我看到ORM執行三個查詢。我相信，我有一些如何使用select_related，但這並不工作，要麼（也不prefetch_related前移動select_related）：

Book.objects.all().prefetch_related('page_set', 'page_set__baz').select_related('page_set__baz')

的查詢（uploads是應用程序）：

QUERY = u'SELECT "uploads_book"."id", "uploads_book"."publisher_id" FROM "uploads_book"' - PARAMS =(); args=() 
QUERY = u'SELECT "uploads_page"."id", "uploads_page"."book_id", "uploads_page"."baz_id" FROM "uploads_page" WHERE "uploads_page"."book_id" IN (%s, %s)' - PARAMS = (1, 2); args=(1, 2) 
QUERY = u'SELECT "uploads_baz"."id", "uploads_baz"."a" FROM "uploads_baz" WHERE "uploads_baz"."id" IN (%s)' - PARAMS = (1,); args=(1,)

這是關係到在這裏評論：prefetch_related for multiple Levels

來源

2014-11-25 Alex Rothberg

你可以嘗試使用Prefetch object

page_qs = Page.objects.select_related('baz') 
books = Book.objects.prefetch_related(Prefetch('page_set', queryset=page_qs))

來源

2014-11-25 08:40:14 sinitsynsv

select_related prefetch_related後

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