Related object, mass mail utils

Added generic fkey on message in order to allow associating messages
with other objects
Added mass mail utilities that store outgoing messages (including link
to other object)

django_mailbox/models.py

@@ -16,6 +16,8 @@ import mimetypes
 import os.path
 import sys
 import uuid
+from django.contrib.contenttypes import generic
+from django.contrib.contenttypes.models import ContentType
 
 import six
 from six.moves.urllib.parse import parse_qs, unquote, urlparse
@@ -233,9 +235,11 @@ class Mailbox(models.Model):
 
         return msg
 
-    def record_outgoing_message(self, message):
+    def record_outgoing_message(self, message, related_object=None):
         msg = self._process_message(message)
         msg.outgoing = True
+        if related_object:
+            msg.related_object = related_object
         msg.save()
         return msg
 
@@ -413,6 +417,17 @@ class Message(models.Model):
         verbose_name=_(u'In reply to'),
     )
 
+    related_content_type = models.ForeignKey(
+        ContentType,
+        blank=True,
+        null=True
+    )
+    related_object_id = models.PositiveIntegerField(
+        blank=True,
+        null=True
+    )
+    related_object = generic.GenericForeignKey('related_content_type', 'related_object_id')
+
     from_header = models.CharField(
         _('From header'),
         max_length=255,