Ability to create two or more LTI consumers through the Django admin with an…

Ability to create two or more LTI consumers through the Django admin with an empty instance_guid field.

lms/djangoapps/lti_provider/migrations/0003_auto_20161118_1040.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import openedx.core.djangolib.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('lti_provider', '0002_auto_20160325_0407'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='lticonsumer',
+            name='instance_guid',
+            field=openedx.core.djangolib.fields.CharNullField(max_length=255, unique=True, null=True, blank=True),
+        ),
+    ]

lms/djangoapps/lti_provider/models.py

@@ -13,6 +13,7 @@ from django.db import models
 import logging
 
 from openedx.core.djangoapps.xmodule_django.models import CourseKeyField, UsageKeyField
+from openedx.core.djangolib.fields import CharNullField
 
 from provider.utils import short_token
 
@@ -28,7 +29,7 @@ class LtiConsumer(models.Model):
     consumer_name = models.CharField(max_length=255, unique=True)
     consumer_key = models.CharField(max_length=32, unique=True, db_index=True, default=short_token)
     consumer_secret = models.CharField(max_length=32, unique=True, default=short_token)
-    instance_guid = models.CharField(max_length=255, blank=True, null=True, unique=True)
+    instance_guid = CharNullField(max_length=255, blank=True, null=True, unique=True)
 
     @staticmethod
     def get_or_supplement(instance_guid, consumer_key):

lms/djangoapps/lti_provider/tests/test_outcomes.py

@@ -346,6 +346,30 @@ class TestAssignmentsForProblem(ModuleStoreTestCase):
         assignment.save()
         return assignment
 
+    def test_create_two_lti_consumers_with_empty_instance_guid(self):
+        """
+        Test ability to create two or more LTI consumers through the Django admin
+        with empty instance_guid field.
+        A blank guid field is required when a customer enables a new secret/key combination for
+        LTI integration with their LMS.
+        """
+        lti_consumer_first = LtiConsumer(
+            consumer_name='lti_consumer_name_second',
+            consumer_key='lti_consumer_key_second',
+            consumer_secret='lti_consumer_secret_second',
+            instance_guid=''
+        )
+        lti_consumer_first.save()
+        lti_consumer_second = LtiConsumer(
+            consumer_name='lti_consumer_name_third',
+            consumer_key='lti_consumer_key_third',
+            consumer_secret='lti_consumer_secret_third',
+            instance_guid=''
+        )
+        lti_consumer_second.save()
+        count = LtiConsumer.objects.count()
+        self.assertEqual(count, 3)
+
     def test_with_no_graded_assignments(self):
         with check_mongo_calls(3):
             assignments = outcomes.get_assignments_for_problem(

openedx/core/djangolib/fields.py

+"""
+Custom Django fields.
+"""
+from django.db import models
+
+
+class CharNullField(models.CharField):
+    """CharField that stores NULL but returns ''"""
+
+    description = "CharField that stores NULL but returns ''"
+
+    def to_python(self, value):
+        """Converts the value into the correct Python object."""
+        if isinstance(value, models.CharField):
+            return value
+        if value is None:
+            return ""
+        else:
+            return value
+
+    def get_db_prep_value(self, value, connection, prepared=False):
+        """Converts value to a backend-specific value."""
+        if not prepared:
+            value = self.get_prep_value(value)
+        if value == "":
+            return None
+        else:
+            return value