show better error message when recipe deletion breaks due to constraints

cookbook/templates/generic/delete_template.html

@@ -1,4 +1,6 @@
 {% extends "base.html" %}
+{% load custom_tags %}
+{% load custom_tags %}
 {% load crispy_forms_tags %}
 {% load i18n %}
 
@@ -12,16 +14,40 @@
 
     <h3>{% trans 'Delete' %} {{ title }}</h3>
 
-    
+
     <form action="." method="post">
         {% csrf_token %}
-        <div class="alert alert-danger" role="alert">
+        <div class="alert alert-warning" role="alert">
             {% blocktrans %}Are you sure you want to delete the {{ title }}: <b>{{ object }}</b> {% endblocktrans %}
         </div>
         {{ form|crispy }}
-        <button class="btn btn-success" type="submit" href="{{ success_url }}"><i class="fas fa-trash-alt"></i> {% trans 'Confirm' %}</button>
-        <a href="javascript:history.back()"  class="btn btn-danger"><i class="fas fa-undo-alt"></i> {% trans 'Cancel' %}</a>
+
+        {% if related_objects %}
+            {% blocktrans %} <i>{{ object }}</i> could not be deleted because it is still referenced by the following instances: {% endblocktrans %}
+            <br/>
+            <br/>
+            {% for o in related_objects %}
+                {% class_name o.model as name %}
+                <h5>{{ name }}</h5>
+                <ul>
+                    {% for e in o %}
+                        <li>
+                            <span class="badge badge-info">#{{ e.id }}</span> {{ e }}
+                        </li>
+                    {% endfor %}
+                </ul>
+
+            {% endfor %}
+
+
+            <br/>
+        {% endif %}
+
+        <button class="btn btn-success" type="submit" href="{{ success_url }}"><i
+                class="fas fa-trash-alt"></i> {% trans 'Confirm' %}</button>
+        <a href="javascript:history.back()" class="btn btn-danger"><i class="fas fa-undo-alt"></i> {% trans 'Cancel' %}
+        </a>
     </form>
-    
+
 
 {% endblock %}

cookbook/templatetags/custom_tags.py

@@ -26,6 +26,11 @@ def get_class(value):
     return value.__class__
 
 
+@register.simple_tag
+def class_name(value):
+    return value.__class__.__name__
+
+
 @register.simple_tag
 def delete_url(model, pk):
     try:

cookbook/views/delete.py

@@ -1,7 +1,7 @@
 from django.contrib import messages
 from django.db.models import ProtectedError
 from django.http import HttpResponseRedirect
-from django.shortcuts import get_object_or_404
+from django.shortcuts import get_object_or_404, render
 from django.urls import reverse, reverse_lazy
 from django.utils.translation import gettext as _
 from django.views.generic import DeleteView
@@ -23,9 +23,31 @@ class RecipeDelete(GroupRequiredMixin, DeleteView):
     model = Recipe
     success_url = reverse_lazy('index')
 
+    def delete(self, request, *args, **kwargs):
+        obj = self.get_object()
+
+        related_objects = []
+        for x in obj._meta.get_fields():
+            try:
+                related = x.related_model.objects.filter(**{x.field.name: obj})
+                if related.exists():
+                    related_objects.append(related)
+            except AttributeError:
+                pass
+
+        if related_objects:
+            self.object = obj
+            return render(request, template_name=self.template_name, context=self.get_context_data(related_objects=related_objects))
+
+        success_url = self.get_success_url()
+        obj.delete()
+        return HttpResponseRedirect(success_url)
+
     def get_context_data(self, **kwargs):
         context = super(RecipeDelete, self).get_context_data(**kwargs)
         context['title'] = _("Recipe")
+        if 'related_objects' in kwargs:
+            context['related_objects'] = kwargs.pop('related_objects')
         return context