How to Filter ForeignKey Choices in Django ModelForms?

Django ModelForm Filtering ForeignKey Choices

Introduction

When creating forms in Django, it can be desirable to limit the choices presented to users for a specific field based on certain criteria. This can be particularly useful in scenarios involving hierarchical data, such as when selecting a foreign key.

Case Study: Selecting ForeignKey Choices in a ModelForm

Let's consider a hypothetical Django project with the following models:

class Company(models.Model):
    name = ...

class Rate(models.Model):
    company = models.ForeignKey(Company)
    name = ...

class Client(models.Model):
    name = ...
    company = models.ForeignKey(Company)
    base_rate = models.ForeignKey(Rate)

In this case, each company has multiple rates and clients. Each client must have a base rate chosen from its parent company's rates, not another company's rates.

Limiting ForeignKey Choices Using QuerySet Filtering

To limit the choices for the Rate field in the Client form to only those rates associated with the selected company, we can modify our ClientForm class as follows:

class ClientForm(ModelForm):
    class Meta:
        model = Client
        fields = ['name', 'base_rate']

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.fields['base_rate'].queryset = Rate.objects.filter(company_id=self.instance.company_id)

In this code, we retrieve the Company ID from the instance associated with the form. This ensures that the Rate choices are filtered based on the correct company.

Additional Notes

• This approach is compatible with Django 1.0 and later.
• The limit_choices_to argument of ForeignKeyField is designed for use within the Django admin interface and may not always be suitable for use in forms.
• For more complex filtering scenarios, it may be necessary to override the get_queryset method of the ModelChoiceField class.

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