Use OneToOneField instead of a ForeignKey with unique=True.

mayan/apps/checkouts/migrations/0005_auto_20160122_0756.py

@@ -0,0 +1,19 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('checkouts', '0004_auto_20150617_0330'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='documentcheckout',
+            name='document',
+            field=models.OneToOneField(verbose_name='Document', to='documents.Document'),
+        ),
+    ]

mayan/apps/checkouts/models.py

@@ -24,9 +24,7 @@ class DocumentCheckout(models.Model):
     """
     Model to store the state and information of a document checkout
     """
-    document = models.ForeignKey(
-        Document, unique=True, verbose_name=_('Document')
-    )
+    document = models.OneToOneField(Document, verbose_name=_('Document'))
     checkout_datetime = models.DateTimeField(
         auto_now_add=True, verbose_name=_('Check out date and time')
     )