Damien MATHIEU Damien MATHIEU -4 years ago 60
Python Question

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

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 Source

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.

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