Enforce polygon minimum size at database level

arkindex/images/migrations/0006_polygon_min_size.py

+# Generated by Django 3.1.3 on 2021-01-04 11:04
+
+from django.db import migrations, models
+
+
+def remove_small_polygons(apps, schema_editor):
+    """
+    Removes polygons with less than 4 points.
+    """
+    Element = apps.get_model('documents', 'Element')
+    Zone = apps.get_model('images', 'Zone')
+
+    zone_ids = list(Zone.objects.using('default').filter(polygon__memsize__lt=96).values_list('id', flat=True))
+    if not zone_ids:
+        return
+
+    # Import the ElementQuerySet to access the trash() method
+    from arkindex.documents.managers import ElementQuerySet
+    ElementQuerySet(Element).filter(zone_id__in=zone_ids).trash()
+
+    Zone.objects.filter(id__in=zone_ids)._raw_delete(using='default')
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('images', '0005_polygon_index'),
+    ]
+    atomic = False
+
+    operations = [
+        migrations.RemoveConstraint(
+            model_name='zone',
+            name='zone_polygon_size',
+        ),
+        migrations.RunPython(
+            remove_small_polygons,
+            reverse_code=migrations.RunPython.noop,
+            elidable=True,
+        ),
+        migrations.AddConstraint(
+            model_name='zone',
+            constraint=models.CheckConstraint(
+                check=models.Q(polygon__memsize__gte=96, polygon__memsize__lte=2664),
+                name='zone_polygon_size'
+            ),
+        ),
+    ]

arkindex/images/models.py

@@ -379,9 +379,10 @@ class Zone(IndexableModel):
                 check=models.Q(polygon=SnapToGrid('polygon', 1)),
                 name='zone_polygon_integer_coordinates',
             ),
+            # Restrict to between 4 and 166 points (3+1 points for a triangle, and BTree indexes limit us to 2680 bits)
             # Checking against the memory size is faster than checking the number of points
             models.CheckConstraint(
-                check=models.Q(polygon__memsize__lte=2664),
+                check=models.Q(polygon__memsize__gte=96, polygon__memsize__lte=2664),
                 name='zone_polygon_size',
             )
         ]