Add new migration to prefill the end dates for nutritional plans

This just sets each user's nutrition plan to the start date of their next
one and should just make the nutritional plan stats ( wger-project/flutter/pull/855 )
get a bit less crowded since not all plans since the beginning of time should
show up in the comparison view.

Author: rolandgeider

wger/nutrition/migrations/0027_prefill_end_date.py

@@ -0,0 +1,34 @@
+# Generated by Django 5.2.6 on 2025-09-16 20:54
+
+from django.db import migrations
+
+
+def prefill_end_date(apps, schema_editor):
+    """
+    Sets the end date of each user's nutrition plan to the start date of their
+    next one. The last one is left unchanged.
+    """
+    NutritionPlan = apps.get_model('nutrition', 'NutritionPlan')
+    user_ids = NutritionPlan.objects.order_by().values_list('user_id', flat=True).distinct()
+
+    for user_id in user_ids:
+        plans = list(
+            NutritionPlan.objects.filter(user_id=user_id).order_by('start', 'id')
+        )
+        for i in range(len(plans) - 1):
+            current = plans[i]
+            next_plan = plans[i + 1]
+            NutritionPlan.objects.filter(pk=current.pk).update(end=next_plan.start)
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('nutrition', '0026_add_start_and_end_fields'),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            prefill_end_date,
+            reverse_code=migrations.RunPython.noop,
+        ),
+    ]