检测Django相关对象是否已通过select_lated预取

Question

假设我有以下型号：

class Product:
    name = models.CharField(max_length=128)

class ProductVersion:
    name = models.CharField(max_length=128)
    product = models.ForeignKey(Product, on_delete=models.CASCADE)

对于给定的

ProductVersion

对象，是否有办法检测

Product

对象是否通过

select_related

预取？例如：

# Yes, "product" was prefetched
ProductVersion.objects.select_related("product").first()

# No, "product" was NOT prefetched
ProductVersion.objects.first()

看起来有一种方法可以做到这一点是

prefetch_related

（请参阅此处），但我需要它

selected_related

。

Answer 1

您可以使用查询中的

JOIN

单词检查来做到这一点

queryset = ProductVersion.objects.select_related("product")
sql_query = str(queryset.query) # convert ORM query to SQL query
if 'JOIN' in sql_query:
    print("Product object was prefetched via select_related")
else:
    print("Product object was NOT prefetched via select_related")