For loop in views - django

Question

In views I want to get car-object, then change first_year var.
def cars(request, mark_id, year=year):
#BMW, etc.
mark = get_object_or_404(Mark, pk=mark_id, active=1)
#M, X-series, etc.
for group in mark.groups.iterator():
group.first_year.year = int(year)-int(group.first_year.year)
return render(request, 'cars.html', { 'mark':mark, })
So, in template I use:
{% for i in mark.groups.all %}
{{i.first_year}}
{% endfor %}
And it returns the values from db, not my group.first_year.year. As you can understand, there is 3 models - mark, groups and first_year. If you need, I can publish they here, but I think, that something wrong in my views.
Thanks.
Edit. Models.
class First_Year(models.Model):
year = models.IntegerField()
def __unicode__(self):
return str(self.year)
class Groups(models.Model):
many_other_fields
mark = models.ForeignKey(Mark, related_name='groups')
last_update = models.DateTimeField()
first_year = models.ForeignKey(First_Year, related_name='first_year')
def __unicode__(self):
return self.name
def altered_date(self, year):
altered_year = int(year)-int(self.first_year.year)
return altered_year
Without model Mark, bacause it has many-many fields without year and etc.

Answer 1

The for loop making changes in your view is not saving the changes anywhere so you will not see the changes when the values are passed to your template.
One solution here is to add a new model method to your model and do the date comparison there.
In your First_Year model add the altered_date function like so:
class First_Year(models.Model):
year = models.IntegerField()
def __unicode__(self):
return str(self.year)
def altered_date(self, this_year):
altered_year = int(this_year)-int(self.year)
return altered_year
This gets the altered year for every First_Year model by calling the function. Unfortunately this function requires an additional parameter - year - so it cannot be called directly from the template. You can create a custom template filter to get around this:
#register.filter
def get_altered_year(obj, gar):
return obj.altered_date(gar)
Now you just need to make sure that you pass year to your view by modifying your return like so:
def cars(request, mark_id, year=year):
...
return render(request, 'cars.html', {'mark':mark, 'year':year, })
And then in your template you can do:
{% for i in mark.groups_set.all %}
{{i.first_year|get_altered_year:year }}
{% endfor %}
You can look at the model method documentation here. And the documentation for custom template filters here.

Answer 2

You mean:
{% for i in mark.groups_set.all %}
{{i.first_year}}
{% endfor %}
This will only allow if groups has a foreign key from mark