Add an "empty" option to a ChoiceField based on model datas

Question

Hello,

I'm defining a ChoiceField based on a model's datas.

field = forms.ChoiceField(choices=[[r.id, r.name] for r in Model.objects.all()])

However I'd like to prepend my options with an empty one to select "no" objects. But I can't find a nice way to prepend that.

All my tests like :

field = forms.ChoiceField(choices=[[0, '----------']].extend([[r.id, r.name] for r in Model.objects.all()]))

Returns me a "NoneType object not iterable" error. The only way I've found until now is the following :

def append_empty(choices):
    ret = [[0, '----------']]
    for c in choices:
        ret.append(c)
   return ret

And when I define my field :

forms.ChoiceField(choices=append_empty([[r.id, r.name] for r in
    Restaurant.objects.all()]), required=False)

However I'd like to keep my code clean and not have that kind of horrors. Would you have an idea for me ? :p Thanks by advance.

Answer 1

An easy answer is to do:

field = forms.ChoiceField(choices=[[0, '----------']] + [[r.id, r.name] for r in Model.objects.all()])

Unfortunately, your approach is flawed. Even with your 'working' approach, the field choices are defined when the form is defined, not when it is instantiated - so if you add elements to the Model table, they will not appear in the choices list.

You can avoid this by doing the allocation in the __init__ method of your Form.

However, there is a much easier approach. Rather than messing about with field choices dynamically, you should use the field that is specifically designed to provide choices from a model - ModelChoiceField. Not only does this get the list of model elements dynamically at instantiation, it already includes a blank choice by default. See the documentation.