Add in a uniqueness constraint on mode, course, and currency.

lms/djangoapps/course_modes/migrations/0003_auto__add_unique_coursemode_course_id_currency_mode_slug.py

+# -*- coding: utf-8 -*-
+import datetime
+from south.db import db
+from south.v2 import SchemaMigration
+from django.db import models
+
+
+class Migration(SchemaMigration):
+
+    def forwards(self, orm):
+        # Adding unique constraint on 'CourseMode', fields ['course_id', 'currency', 'mode_slug']
+        db.create_unique('course_modes_coursemode', ['course_id', 'currency', 'mode_slug'])
+
+
+    def backwards(self, orm):
+        # Removing unique constraint on 'CourseMode', fields ['course_id', 'currency', 'mode_slug']
+        db.delete_unique('course_modes_coursemode', ['course_id', 'currency', 'mode_slug'])
+
+
+    models = {
+        'course_modes.coursemode': {
+            'Meta': {'unique_together': "(('course_id', 'mode_slug', 'currency'),)", 'object_name': 'CourseMode'},
+            'course_id': ('django.db.models.fields.CharField', [], {'max_length': '255', 'db_index': 'True'}),
+            'currency': ('django.db.models.fields.CharField', [], {'default': "'usd'", 'max_length': '8'}),
+            'id': ('django.db.models.fields.AutoField', [], {'primary_key': 'True'}),
+            'min_price': ('django.db.models.fields.IntegerField', [], {'default': '0'}),
+            'mode_display_name': ('django.db.models.fields.CharField', [], {'max_length': '255'}),
+            'mode_slug': ('django.db.models.fields.CharField', [], {'max_length': '100'}),
+            'suggested_prices': ('django.db.models.fields.CommaSeparatedIntegerField', [], {'default': "''", 'max_length': '255', 'blank': 'True'})
+        }
+    }
+
+    complete_apps = ['course_modes']
\ No newline at end of file

lms/djangoapps/course_modes/models.py

@@ -34,6 +34,10 @@ class CourseMode(models.Model):
 
     DEFAULT_MODE = Mode('honor', _('Honor Code Certificate'), 0, '', 'usd')
 
+    class Meta:
+        """ meta attributes of this model """
+        unique_together = ('course_id', 'mode_slug', 'currency')
+
     @classmethod
     def modes_for_course(cls, course_id):
         """