Perform validation at the model

mayan/apps/checkouts/models.py

@@ -11,6 +11,7 @@ from documents.models import Document
 from history.api import create_history
 
 from .events import HISTORY_DOCUMENT_CHECKED_OUT
+from .exceptions import DocumentAlreadyCheckedOut
 from .managers import DocumentCheckoutManager
 
 logger = logging.getLogger(__name__)
@@ -23,6 +24,8 @@ class DocumentCheckout(models.Model):
     document = models.ForeignKey(Document, verbose_name=_(u'Document'), unique=True)
     checkout_datetime = models.DateTimeField(verbose_name=_(u'Check out date and time'), auto_now_add=True)
     expiration_datetime = models.DateTimeField(verbose_name=_(u'Check out expiration date and time'), help_text=_(u'Amount of time to hold the document checked out in minutes.'))
+
+    # TODO: simplify user_object to an instance of User
     user_content_type = models.ForeignKey(ContentType, null=True, blank=True)  # blank and null added for ease of db migration
     user_object_id = models.PositiveIntegerField(null=True, blank=True)
     user_object = generic.GenericForeignKey(ct_field='user_content_type', fk_field='user_object_id')
@@ -40,6 +43,9 @@ class DocumentCheckout(models.Model):
 
     def save(self, *args, **kwargs):
         new_checkout = not self.pk
+        if not new_checkout or self.document.is_checked_out():
+            raise DocumentAlreadyCheckedOut
+
         result = super(DocumentCheckout, self).save(*args, **kwargs)
         if new_checkout:
             create_history(HISTORY_DOCUMENT_CHECKED_OUT, source_object=self.document, data={'user': self.user_object, 'document': self.document})