From 693408eb4ea0f0fef8b1e93fd6af14b8fd4f4d6d Mon Sep 17 00:00:00 2001
From: Dmitry Viskov <dmitry.viskov@webenterprise.ru>
Date: Fri, 3 Jun 2016 15:44:39 +0300
Subject: [PATCH] Ability to create two or more LTI consumers through the
 Django admin with an empty instance_guid field.

---
 .../migrations/0003_auto_20161118_1040.py     | 20 +++++++++++++
 lms/djangoapps/lti_provider/models.py         |  3 +-
 .../lti_provider/tests/test_outcomes.py       | 24 ++++++++++++++++
 openedx/core/djangolib/fields.py              | 28 +++++++++++++++++++
 4 files changed, 74 insertions(+), 1 deletion(-)
 create mode 100644 lms/djangoapps/lti_provider/migrations/0003_auto_20161118_1040.py
 create mode 100644 openedx/core/djangolib/fields.py

diff --git a/lms/djangoapps/lti_provider/migrations/0003_auto_20161118_1040.py b/lms/djangoapps/lti_provider/migrations/0003_auto_20161118_1040.py
new file mode 100644
index 0000000000..394439e891
--- /dev/null
+++ b/lms/djangoapps/lti_provider/migrations/0003_auto_20161118_1040.py
@@ -0,0 +1,20 @@
+# -*- 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),
+        ),
+    ]
diff --git a/lms/djangoapps/lti_provider/models.py b/lms/djangoapps/lti_provider/models.py
index 34cf9d3492..2dcc0cb61d 100644
--- a/lms/djangoapps/lti_provider/models.py
+++ b/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):
diff --git a/lms/djangoapps/lti_provider/tests/test_outcomes.py b/lms/djangoapps/lti_provider/tests/test_outcomes.py
index 85394a6336..e7a195554d 100644
--- a/lms/djangoapps/lti_provider/tests/test_outcomes.py
+++ b/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(
diff --git a/openedx/core/djangolib/fields.py b/openedx/core/djangolib/fields.py
new file mode 100644
index 0000000000..f1087c55d5
--- /dev/null
+++ b/openedx/core/djangolib/fields.py
@@ -0,0 +1,28 @@
+"""
+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