Django query single underscore behaving like double underscore?

问题

I made a typo in my code recently and noticed that I got the same behavior, so I was wondering what the difference between single and double underscores are in django queries.

>>> underscore = MyModel.objects.filter(foreign_key_id=var)
>>> double_underscore =  MyModel.objects.filter(foreign_key__id=var)
>>> underscore == double_underscore
False
>>> list(underscore) == list(double_underscore)
True

I'm not sure what equality method is being used to compare the querysets, but when I convert to python lists I find exactly the same elements contained within. Does anyone have some insight into what's going on here?

回答1:

Those two fields just happen to both exist.

foreign_key_id is an automatically created column on the MyModel object, whereas foreign_key__id is the ID on the foreign key table itself.

These values would both be the same..

MyModel1.foreign_key_id == 5  # this is stored on the model
                              # and does not require a lookup.
MyModel1.foreign_key.id == 5  # this is stored on the target table
                              # and requires a DB hit. 

回答2:

foreign_key_id is a (hidden) field name of MyModel, foreign_key__id is a reference to a field on whatever model the foreign_key field references. In other words, it is a specific detail of foreign key fields.

回答3:

From my observations, Django is smart enough to not do a join with the ForeignKey if only filtering or fetching foreign_key__id. You can test this with:

>>> print(MyModel.objects.filter(foreign_key_id=var).query)
>>> print(MyModel.objects.filter(foreign_key__id=var).query)

Since underscore and double_underscore are separate objects in memory, I believe this is why underscore == double_underscore returns False.

来源：https://stackoverflow.com/questions/14945242/django-query-single-underscore-behaving-like-double-underscore