how to verify if object exist in manytomany

Question

Models

class Beer(models.Model):     pass  class Salas(models.Model):     name =  models.CharField(max_length=20)     beers =     models.ManyToManyField('Beer', blank=True) 

View

beer = Beer.objects.get(user=user) id_sala = request.GET['id_sala'] sala = get_object_or_404(Salas, id=id_sala)   if beer.salas_set.filter(nombre=sala): # if beer.objects.filter(sitios__id=beer).exists():     sala.beers.remove(beer)  else:     sala.beers.add(beer) 

I want to see if there is relationship of beer with sala, how to make this??

Answer 1

I advise you to use:

if beer.salas_set.filter(pk=sala.pk).exists():     # do stuff 

If you use sala in beer.salas_set.all() instead, it selects all records from the relation table and loops over them to find, whether the given object is there or not. However, beer.salas_set.filter(pk=sala.pk).exists() only selects zero or one row from the database and immediately gives the result (without looping).

Answer 2

With your solution there might be a problem since name of salas is not unique. So, if you filter by the name of the sala expecting to find one and only one sala, that might not happen.

I advice you to do:

if sala in beer.salas_set.all():     #do stuff: else:     # do other stuff 

that way, beer.salas_set.all() will return a QuerySet in which you can check if a specific object is there with the 'in' keyword.