content: import time
  from decimal import Decimal
  
  from django import forms
  from django.core.validators import MaxValueValidator, MinValueValidator
  from django.utils.translation import gettext_lazy as _
  
  __all__ = (
      'BackgroundJobMixin',
      'CheckLastUpdatedMixin',
      'DistanceValidationMixin',
  )
  
  
  class BackgroundJobMixin(forms.Form):
      background_job = forms.BooleanField(
          label=_('Background job'),
          help_text=_("Execute this task via a background job"),
          required=False,
      )
  
      def __init__(self, *args, **kwargs):
          super().__init__(*args, **kwargs)
  
          # Declare background_job a meta field
          if hasattr(self, 'meta_fields'):
              self.meta_fields.append('background_job')
          else:
              self.meta_fields = ['background_job']
  
  
  class CheckLastUpdatedMixin(forms.Form):
      """
      Checks whether the object being saved has been updated since the form was initialized. If so, validation fails.
      This prevents a user from inadvertently overwriting any changes made to the object between when the form was
      initialized and when it was submitted.
  
      This validation does not apply to newly created objects, or if the `_init_time` field is not present in the form
      data.
      """
      _init_time = forms.DecimalField(
          initial=time.time,
          required=False,
          widget=forms.HiddenInput()
      )
  
      def clean(self):
          super().clean()
  
          # Skip for absent or newly created instances
          if not self.instance or not self.instance.pk:
              return
  
          # Skip if a form init time has not been specified
          if not (form_init_time := self.cleaned_data.get('_init_time')):
              return
  
          # Skip if the object does not have a last_updated value
          if not (last_updated := getattr(self.instance, 'last_updated', None)):
              return
  
          # Check that the submitted initialization time is not earlier than the object's modification time
          if form_init_time < last_updated.timestamp():
              raise forms.ValidationError(_(
                  "This object has been modified since the form was rendered. Please consult the object's change "
                  "log for details."
              ))
  
  
  class DistanceValidationMixin(forms.Form):
      distance = forms.DecimalField(
          required=False,
          validators=[
              MinValueValidator(Decimal(0)),
              MaxValueValidator(Decimal(100000)),
          ]
      )
  