validate GeometryField and cleanup intersection errors if possible

src/c3nav/mapdata/fields.py

@@ -1,19 +1,32 @@
 import json
 
+from django.core.exceptions import ValidationError
 from django.db import models
+from shapely import validation
 from shapely.geometry import mapping, shape
+from shapely.geometry.base import BaseGeometry
 
-from c3nav.mapdata.utils import format_geojson
+from c3nav.mapdata.utils import clean_geometry, format_geojson
+
+
+def validate_geometry(geometry):
+    if not isinstance(geometry, BaseGeometry):
+        raise ValidationError('GeometryField expexted a Shapely BaseGeometry child-class.')
+
+    if not geometry.is_valid:
+        raise ValidationError('Invalid geometry: %s' % validation.explain_validity(geometry))
 
 
 class GeometryField(models.TextField):
+    default_validators = [validate_geometry]
+
     def from_db_value(self, value, expression, connection, context):
         if value is None:
             return value
         return shape(json.loads(value))
 
     def to_python(self, value):
-        return shape(json.loads(value))
+        return clean_geometry(shape(json.loads(value)))
 
     def get_prep_value(self, value):
         return json.dumps(format_geojson(mapping(value)))

src/c3nav/mapdata/utils.py

@@ -1,6 +1,8 @@
 import json
 from collections import OrderedDict
 
+from shapely.geometry import Polygon
+
 
 def _preencode(data, magic_marker, in_coords=False):
     if isinstance(data, dict):
@@ -45,3 +47,18 @@ def round_coordinates(data):
         return tuple(round_coordinates(item) for item in data)
     else:
         return round(data, 2)
+
+
+def clean_geometry(geometry):
+    if geometry.is_valid:
+        return geometry
+
+    if isinstance(geometry, Polygon):
+        p = Polygon(list(geometry.exterior.coords))
+        for interior in geometry.interiors:
+            p = p.difference(Polygon(list(interior.coords)))
+
+        if p.is_valid:
+            return p
+
+    return geometry