Fix database casting issue

watson/models.py

@@ -24,6 +24,28 @@ except AttributeError:  # Django < 2.0.
     pass
 
 
+def get_pk_output_field(model):
+    """Gets an instance of the field type for the primary key of the given model, useful for database CAST."""
+    pk = model._meta.pk
+    field_cls = type(pk)
+    field_kwargs = {}
+    if isinstance(pk, models.CharField):
+        # Some versions of Django produce invalid SQL for the CAST function (in some databases)
+        # if CharField does not have max_length passed.
+        # Therefore, it is necessary to copy over the max_length of the original field to avoid errors.
+        # See: https://code.djangoproject.com/ticket/28371
+        field_kwargs['max_length'] = pk.max_length
+    elif isinstance(pk, models.AutoField):
+        # Some versions of Django appear to also produce invalid SQL in MySQL
+        # when attempting to CAST with AutoField types.
+        # This covers for that by instead casting to the corresponding integer type.
+        if isinstance(pk, models.BigAutoField):
+            field_cls = models.BigIntegerField
+        else:
+            field_cls = models.IntegerField
+    return field_cls(**field_kwargs)
+
+
 def has_int_pk(model):
     """Tests whether the given model has an integer primary key."""
     pk = model._meta.pk

watson/search.py

@@ -446,13 +446,12 @@ class SearchEngine(object):
     def _get_deleted_entries_for_model(self, model):
         """Returns a queryset of entries associated with deleted object instances of the given model"""
         from django.contrib.contenttypes.models import ContentType
-        from watson.models import SearchEntry, has_int_pk
+        from watson.models import SearchEntry, has_int_pk, get_pk_output_field
         content_type = ContentType.objects.get_for_model(model)
-        id_output_field = type(model._meta.pk)
         object_id_field = 'object_id_int' if has_int_pk(model) else 'object_id'
         return SearchEntry.objects.annotate(
             # normalize the object id into a field of the correct type for the original table
-            normalized_pk=Cast(object_id_field, id_output_field())
+            normalized_pk=Cast(object_id_field, get_pk_output_field(model))
         ).filter(
             Q(content_type=content_type) &
             Q(engine_slug=self._engine_slug) &