Django中的虚拟过滤器

Question

How do I make a dummy filter query in Django (a filter that is always matched) and an exclude query that is never matched. The reason is because I have cases where my query is None, and in those cases I want to use a dummy filter. This is the code:

MyModel.objects.filter(filterQuery).exclude(excludeQuery)

In cases where filterQuery or excludeQuery is None I get an error, so I want to add the following conditional before that query:

if filterQuery == None: filterQuery = ???
if excludeQuery == None: excludeQuery = ???
MyModel.objects.filter(filterQuery).exclude(excludeQuery)

Answer 1

You should only filter/exclude when you need to, instead of using empty statement, do this:

found = MyModel.objects.all()
if filterQuery:
    found = found.filter(filterQuery)
if excludeQuery:
    found = found.exclude(excludeQuery)

Answer 2

The best way is to use a dict, particularly if you have complex filters.

filterQuery = excludeQuery = {}

# optionally set the filters based on some logic
if 1==1:
  filterQuery['agent'] = 'John'

if 2==2:
  excludeQuery['user_id__in'] = [1, 2]

if 3==3:
  filterQuery['time__gte'] = timezone.now()

MyModel.objects.filter(**filterQuery).exclude(**excludeQuery)

You could add some ifs like Shang suggested to see if you need to apply the filter or not, but I doubt there is much of a performance loss and this is much more readable.

Answer 3

Another alternative to my answer above is to use Q() expressions, particularly if you need and/or logic:

filterQuery = Q()

filterQuery = filterQuery | Q(agent='John')
filterQuery = filterQuery & Q(time__gte=timezone.now())

...

MyModel.objects.filter(filterQuery)