Added error messages for GIS DB functions when used with rasters.

django/contrib/gis/db/models/functions.py

 from decimal import Decimal
 
-from django.contrib.gis.db.models.fields import GeometryField
+from django.contrib.gis.db.models.fields import GeometryField, RasterField
 from django.contrib.gis.db.models.sql import AreaField
+from django.contrib.gis.geometry.backend import Geometry
 from django.contrib.gis.measure import (
     Area as AreaMeasure, Distance as DistanceMeasure,
 )
@@ -40,6 +41,8 @@ class GeoFunc(Func):
     def as_sql(self, compiler, connection):
         if self.function is None:
             self.function = connection.ops.spatial_function_name(self.name)
+        if any(isinstance(field, RasterField) for field in self.get_source_fields()):
+            raise TypeError("Geometry functions not supported for raster fields.")
         return super(GeoFunc, self).as_sql(compiler, connection)
 
     def resolve_expression(self, *args, **kwargs):
@@ -87,6 +90,8 @@ class GeomValue(Value):
 
 class GeoFuncWithGeoParam(GeoFunc):
     def __init__(self, expression, geom, *expressions, **extra):
+        if not isinstance(geom, Geometry):
+            raise TypeError("Please provide a geometry object.")
         if not hasattr(geom, 'srid') or not geom.srid:
             raise ValueError("Please provide a geometry attribute with a defined SRID.")
         super(GeoFuncWithGeoParam, self).__init__(expression, GeomValue(geom), *expressions, **extra)

tests/gis_tests/rasterapp/test_rasterfield.py

 import json
 
+from django.contrib.gis.db.models.functions import Distance
 from django.contrib.gis.db.models.lookups import (
     DistanceLookupBase, gis_lookups,
 )
@@ -326,3 +327,18 @@ class RasterFieldTest(TransactionTestCase):
         msg = "Couldn't create spatial object from lookup value '%s'." % obj
         with self.assertRaisesMessage(ValueError, msg):
             RasterModel.objects.filter(geom__intersects=obj)
+
+    def test_db_function_errors(self):
+        """
+        Errors are raised when using DB functions with raster content.
+        """
+        point = GEOSGeometry("SRID=3086;POINT (-697024.9213808845 683729.1705516104)")
+        rast = GDALRaster(json.loads(JSON_RASTER))
+        msg = "Please provide a geometry object."
+        with self.assertRaisesMessage(TypeError, msg):
+            RasterModel.objects.annotate(distance_from_point=Distance("geom", rast))
+        with self.assertRaisesMessage(TypeError, msg):
+            RasterModel.objects.annotate(distance_from_point=Distance("rastprojected", rast))
+        msg = "Geometry functions not supported for raster fields."
+        with self.assertRaisesMessage(TypeError, msg):
+            RasterModel.objects.annotate(distance_from_point=Distance("rastprojected", point)).count()