Refactored admin history changes to allow overriding

Leaving no easy way for users to customize this - particularly at what
point the characters are truncated - is perhaps not such a good idea.

Author: ddabble

simple_history/admin.py

@@ -7,12 +7,14 @@
 from django.contrib.auth import get_permission_codename, get_user_model
 from django.core.exceptions import PermissionDenied
 from django.shortcuts import get_object_or_404, render
+from django.template.defaultfilters import truncatechars
 from django.urls import re_path, reverse
 from django.utils.encoding import force_str
 from django.utils.html import mark_safe
 from django.utils.text import capfirst
 from django.utils.translation import gettext as _
 
+from .models import ModelChange
 from .utils import get_history_manager_for_model, get_history_model_for_model
 
 SIMPLE_HISTORY_EDIT = getattr(settings, "SIMPLE_HISTORY_EDIT", False)
@@ -22,6 +24,8 @@ class SimpleHistoryAdmin(admin.ModelAdmin):
     object_history_template = "simple_history/object_history.html"
     object_history_form_template = "simple_history/object_history_form.html"
 
+    max_displayed_history_change_chars = 100
+
     def get_urls(self):
         """Returns the additional urls used by the Reversion admin."""
         urls = super().get_urls()
@@ -83,7 +87,9 @@ def history_view(self, request, object_id, extra_context=None):
         # except the first (oldest) one
         for i in range(len(action_list) - 1):
             delta = action_list[i].diff_against(action_list[i + 1])
-            action_list[i].history_delta_changes = delta.changes
+            action_list[i].history_delta_changes = [
+                self.format_history_delta_change(change) for change in delta.changes
+            ]
 
         context = {
             "title": self.history_view_title(request, obj),
@@ -110,6 +116,17 @@ def history_view_title(self, request, obj):
         else:
             return _("Change history: %s") % force_str(obj)
 
+    def format_history_delta_change(self, change: ModelChange) -> dict:
+        """
+        Override this to customize the displayed values in the "Changes" column of
+        the object history page.
+        """
+        return {
+            "field": change.field,
+            "old": truncatechars(change.old, self.max_displayed_history_change_chars),
+            "new": truncatechars(change.new, self.max_displayed_history_change_chars),
+        }
+
     def response_change(self, request, obj):
         if "_change_history" in request.POST and SIMPLE_HISTORY_EDIT:
             verbose_name = obj._meta.verbose_name

simple_history/templates/simple_history/_object_history_list.html

@@ -47,9 +47,9 @@
               {% for change in action.history_delta_changes %}
                 <li>
                   <code>{{ change.field }}</code>:
-                  {{ change.old|truncatechars:100 }}
+                  {{ change.old }}
                   {# Add some spacing, and prevent having the arrow point to the edge of the page if `new` is wrapped #}
-                  &nbsp;&rarr;&nbsp;&nbsp;{{ change.new|truncatechars:100 }}
+                  &nbsp;&rarr;&nbsp;&nbsp;{{ change.new }}
                 </li>
               {% endfor %}
             </ul>

simple_history/tests/tests/test_admin.py

@@ -132,9 +132,9 @@ def test_history_list_doesnt_contain_too_long_diff_changes(self):
 
         response = self.client.get(get_history_url(poll))
         self.assertContains(response, "question")
-        # The limit should be 100 characters in total (including the ellipsis)
-        self.assertContains(response, f"{'A' * 99}…")
-        self.assertContains(response, f"{'B' * 99}…")
+        expected_num_chars = SimpleHistoryAdmin.max_displayed_history_change_chars - 1
+        self.assertContains(response, f"{'A' * expected_num_chars}…")
+        self.assertContains(response, f"{'B' * expected_num_chars}…")
 
     def test_history_list_custom_fields(self):
         model_name = self.user._meta.model_name