12. Conditional Field and Fieldset Expressions

Sometimes it doesn’t make sense to render all fields of a form. Consider for instance a questionnaire of a radiologist, who wants to know if the patient is pregnant. If that patient is a male, he might even be offended by that question. The most user-friendly solution to this is to hide or disable such a field.

For this purpose django-formset offers three conditionals:

• show-if="condition": The field or fieldset is only shown if the condition evaluates to true.

• hide-if="condition": The field or fieldset is only shown if the condition evaluates to false.

• disable-if="condition": The field or fieldset is disabled if the condition evaluates to true.

The condition can be any logical JavaScript expression. It can contain comparison operators such as ==, < or != and logical operators such as &&, || and !. This conditional expression also has access to all the values in the complete context of <django-formset>. Values from fields in the same form can be accessed using a relative path, starting with dot, for instance .fieldname. Values from fields in other forms can be accessed by specifying the complete path to that field, for instance formname.fieldname. This also works for deeply nested forms.

Note

Fields using the conditionals show-if="…" or hide-if="…" shall use the attribute required=False during initialization. This is because otherwise Django’s form validation rejects that field as required, meanwhile it has been hidden by the client. In case only visible fields are required, adopt the validation code of the clean()-method in the corresponding form class.

12.1. Example Form

This form uses a conditional where the value of one field influences if another field is visible.

from django.forms import fields, forms, widgets

class QuestionnaireForm(forms.Form):
    full_name = fields.CharField(
        label="Full Name",
    )

    gender = fields.ChoiceField(
        label="Gender",
        choices=[('m', "Male"), ('f', "Female")],
        widget=widgets.RadioSelect,
    )

    pregnant = fields.BooleanField(
        label="Are you pregnant?",
        required=False,
        widget=widgets.CheckboxInput(attrs={'show-if': ".gender=='f'"})
    )

Here we add the conditional show-if=".gender=='f'" to the checkbox asking for pregnancy. Only if the field gender contains value f, then that checkbox is visible. The path for accessing that variable is relative here, if it starts with a dot, then the named field from the same form is evaluated.

12.2. Example Fieldset

Conditionals can also be used on a Fieldset element. For example by using

class CustomerForm(Fieldset):
    legend = "Customer"
    hide_if = 'register.no_customer'

we can use the value of another field, here register.no_customer to hide the whole fieldset if that value evaluates to true.