从django模板访问dict / object

Question

models.py

class farm_model(models.Model):
  name=models.CharField(max_length=30)

views.py

def farms(request):
farms=farm_model.objects.all()
context = { 'farms': farms }
return render(request, 'farms.html', context)

farms.html

{% for farm in farms %}
<div style="top: {{ farm.map_top }}px; left: {{ farm.map_left }}px;">{{ farm.name }}</div>

How can i add map_top and map_left to farms objects from views.py ? i've tryied

farms[0].map_top=80
setattr(farms[0],'map_top',80)

but it is not working

my second aproach was to set in views.py

farm_web_data={}
farm_web_data[1]={}
farm_web_data[1]['map_top']=80

but i don't know how to access those array from template

{% for farm in farms %}
<div style="top: {{ farm_web_data.{{ farm.pk }}.map_top }}px;

not working

any ideas ? I don't want to store map_top in DB

Answer 1

The problem with your first attempt is that slicing a queryset - as you do with farms[0] - always hits the database and gets a fresh instance. That is, the object referred to by farms[0] is not the same one you get when you later iterate through the queryset again.

The answer is to convert the queryset to a list straight away: then you can set attributes on the instances, and subsequent access will refer to the same instances.

farms = list(farm_model.objects.all())
farms[0].map_top = 80

Answer 2

It sounds like you are trying to extend your model to include more information for your template. You can create a data object to represent this. It's a common pattern to create a data class that encapsulates your actual model. You can even have methods in your data class that you can invoke in the template to do more complicated operations.

It could be something like

class FarmData(object):
    def __init__(self, farm, map_top, map_left):
        self.name = farm.name
        self.map_top = map_top
        self.map_left = map_left

    def some_fn(self):
        return some_output

and in your views.py you can do something like

farms = farm_model.objects.all()
farm_data_list = []
for farm in farms:
    farm_data = FarmData(farm, map_top, map_left) # I don't know how you are defining map data
    farm_data_list.append(farm_data)

context = { 'farms': farm_data_list }

And in your template, each farm_data in farm_data_list would have the additional properties defined in your data class. An you can use {{ farm_data.some_fn }} in your template to be able to invoke methods on your data class.

While you can go down the route of using a custom tag with dictionaries, I prefer using a data class since it's a bit easier to follow down the road when you are revisiting your code.