improve taskexec admin listing performance followup (more robust migr…

…ation)

django_toosimple_q/migrations/0015_taskexec_result_preview.py

@@ -1,15 +1,14 @@
 # Generated by Django 4.1 on 2024-03-04 15:28
 
 from django.db import migrations, models
-from django.template.defaultfilters import truncatechars
 
 
 def populate_result_preview(apps, schema_editor):
     # Populate the new result preview field
     TaskExec = apps.get_model("toosimpleq", "TaskExec")
-    for task_exec in TaskExec.objects.all():
-        task_exec.result_preview = truncatechars(str(task_exec.result), 255)
-        task_exec.save()
+    TaskExec.objects.filter(result__isnull=False).update(
+        result_preview="*preview not generated*"
+    )
 
 
 class Migration(migrations.Migration):