.

cards/migrations/0004_convert_stats_to_units.py

@@ -0,0 +1,61 @@
+from django.db import migrations, models
+
+
+def forwards(apps, schema_editor):
+    Dinosaur = apps.get_model('cards', 'Dinosaur')
+    # Constants must match those used in models at conversion time
+    SPEED_MAX_KMH = 60.0
+    WEIGHT_MAX_KG = 15000.0
+    HEIGHT_MAX_M = 12.0
+
+    for obj in Dinosaur.objects.all():
+        try:
+            # previous values were 0-100 percentages
+            old_speed = getattr(obj, 'speed', None)
+            old_weight = getattr(obj, 'weight', None)
+            old_height = getattr(obj, 'height', None)
+            if old_speed is not None:
+                new_speed = int(round(float(old_speed) / 100.0 * SPEED_MAX_KMH))
+                obj.speed = new_speed
+            if old_weight is not None:
+                new_weight = int(round(float(old_weight) / 100.0 * WEIGHT_MAX_KG))
+                obj.weight = new_weight
+            if old_height is not None:
+                new_height = round(float(old_height) / 100.0 * HEIGHT_MAX_M, 1)
+                # assign as string to avoid Decimal conversion issues
+                obj.height = new_height
+            obj.save(update_fields=['speed', 'weight', 'height'])
+        except Exception:
+            # skip errors during migration for safety; admins can fix manually
+            continue
+
+
+def reverse(apps, schema_editor):
+    # Not reversible automatically
+    pass
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('cards', '0003_dinosaur_facts'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='dinosaur',
+            name='speed',
+            field=models.PositiveIntegerField(default=30, help_text='Speed in km/h'),
+        ),
+        migrations.AlterField(
+            model_name='dinosaur',
+            name='weight',
+            field=models.PositiveIntegerField(default=1000, help_text='Weight in kg'),
+        ),
+        migrations.AlterField(
+            model_name='dinosaur',
+            name='height',
+            field=models.DecimalField(default=1.5, max_digits=5, decimal_places=1, help_text='Height in metres'),
+        ),
+        migrations.RunPython(forwards, reverse_code=migrations.RunPython.noop),
+    ]