Question

I have a model similar to this one:

class MyModel(models.Model):
    name = models.CharField(max_length = 30)
    a    = models.ForeignKey(External)
    b    = models.ForeignKey(External, related_name='MyModels_a')

    def __unicode__(self):
        return self.a + self.b.name + self.b.name

So when I query it I get something like this:

>>> MyModel.objects.all()
[<MyModel: Name1AB>,<MyModel: Name2AC>,<MyModel: Name3CB>,<MyModel: Name4BA>,<MyModel: Name5BA>]

And I’d like to represent this data similar to the following.

[[ []                 , [Name1AB] , [Name2AC] ]
 [ [Name4BA, Name5BA] , []        , []        ]
 [ []                 , [Name3CB] , []        ]]

As you can see the rows would be ‘a’ in the model; and the columns would be ‘b’
I can do this, but it takes a long of time because in the real database I have a lot of data. I’d like to know if there’s a Django built in way to do this.

I’m doing it like this:

mymodel_list  = MyModel.objects.all()
external_list = External.objects.all()

for i in external_list:
    for j in external_list:

        print(mymodel_list.filter(a=i).filter(arrl=j).all(),end='')
        print()

Thanks

Answer 1

Three ways of doing it but you will have to research a bit more. The third option one may be the most suitable for what you are looking for.

1) Django queries

The reason it is taking a long time is because you are constantly accessing the database in this line:

print(mymodel_list.filter(a=i).filter(arrl=j).all(),end='')

You may have to start reading what the Django documentation say about the way of working with queries. For what you are doing you have to create the algorithm to avoid the filters. Using MyModel.objects.order_by(‘a’) may help you to build a efficient algorithm.

2) {% ifchanged …%} tag

I suppose you are using print to post your answer but you probably need it in html. In that case, you may want to read about the ifchanged tag. It will allow you to build your matrix in html with just one db access.

3) Many to many relations

It seems you are sort of simulating a many to many relation in a very peculiar way. Django has support for many to many relations. You will need an extra field, so you will also have to read this.

Finally, for doing what you are trying with just one access to the database, you will need to read prefetch_related