Explicitly handle too-long field values

If you use a traditional database engine (such as MySQL/MariaDB) then
maximum character field lengths are enforced, however depending on the
configuration this may result in an exception rather than a warning and
truncation and will also break the per-layer transaction. To avoid that
ugliness, add a signal handler to do it internally, which as a bonus
lets us know if field lenghts are too short for data when using database
engines that don't enforce lengths (e.g. SQLite).

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>

layerindex/models.py

@@ -9,10 +9,31 @@ from datetime import datetime
 from django.contrib.auth.models import User
 from django.core.urlresolvers import reverse
 from django.core.validators import URLValidator
+from django.db.models.signals import pre_save
+from django.dispatch import receiver
 import os.path
 import re
 import posixpath
 
+from . import utils
+
+
+logger = utils.logger_create('LayerIndexModels')
+
+
+@receiver(pre_save)
+def truncate_charfield_values(sender, instance, *args, **kwargs):
+    # Instead of leaving this up to the database, check and handle it
+    # ourselves to avoid nasty exceptions; as a bonus we won't miss when
+    # the max length is too short with databases that don't enforce
+    # the limits (e.g. sqlite)
+    for field in instance._meta.get_fields():
+        if isinstance(field, models.CharField):
+            value = getattr(instance, field.name)
+            if value and len(value) > field.max_length:
+                logger.warning('%s.%s: length %s exceeds maximum (%s), truncating' % (instance.__class__.__name__, field.name, len(value), field.max_length))
+                setattr(instance, field.name, value[:field.max_length])
+
 
 class PythonEnvironment(models.Model):
     name = models.CharField(max_length=50)