Cascade-delete progress updates

Add a warning about it, and update test to confirm it works

bookwyrm/models/readthrough.py

@@ -42,7 +42,7 @@ class ReadThrough(BookWyrmModel):
 class ProgressUpdate(BookWyrmModel):
     ''' Store progress through a book in the database. '''
     user = models.ForeignKey('User', on_delete=models.PROTECT)
-    readthrough = models.ForeignKey('ReadThrough', on_delete=models.PROTECT)
+    readthrough = models.ForeignKey('ReadThrough', on_delete=models.CASCADE)
     progress = models.IntegerField()
     mode = models.CharField(
         max_length=3,

bookwyrm/templates/snippets/components/modal.html

@@ -7,11 +7,10 @@
             </h2>
             {% include 'snippets/toggle/toggle_button.html' with label="close" class="delete" nonbutton=True %}
         </header>
-
         {% block modal-form-open %}{% endblock %}
-
-        {% block modal-body %}{% endblock %}
-
+        <section class="modal-card-body">
+            {% block modal-body %}{% endblock %}
+        </section>
         <footer class="modal-card-foot">
             {% block modal-footer %}{% endblock %}
         </footer>

bookwyrm/templates/snippets/delete_readthrough_modal.html

@@ -1,6 +1,10 @@
 {% extends 'snippets/components/modal.html' %}
 {% block modal-title %}Delete these read dates?{% endblock %}
-
+{% block modal-body %}
+{% if readthrough.progress_updates|length > 0 %}
+You are deleting this readthrough and its {{ readthrough.progress_updates|length }} associated progress updates.
+{% endif %}
+{% endblock %}
 {% block modal-footer %}
 <form name="delete-readthrough-{{ readthrough.id }}" action="/delete-readthrough" method="POST">
     {% csrf_token %}

bookwyrm/tests/actions/test_readthrough.py

@@ -77,3 +77,12 @@ class ReadThrough(TestCase):
         self.assertEqual(progress_updates[1].mode, models.ProgressMode.PAGE)
         self.assertEqual(progress_updates[1].progress, 100)
         self.assertEqual(delay_mock.call_count, 1) # Edit doesn't publish anything
+
+        self.client.post('/delete-readthrough', {
+            'id': readthroughs[0].id,
+        })
+
+        readthroughs = self.edition.readthrough_set.all()
+        updates = self.user.progressupdate_set.all()
+        self.assertEqual(len(readthroughs), 0)
+        self.assertEqual(len(updates), 0)