Remove foreign key constraint on CourseOverviewTab -> CourseOverview

This will allow v1 code of CourseOverview to delete entries. While
this is not a good thing in general (and future versions will ignore
entries with higher versions than they support), this is necessary
to prevent errors with the existing code in a rollback situation.
Otherwise, old code trying to delete CourseOverview entries will
fail with a foreign key constraint violation.

openedx/core/djangoapps/content/course_overviews/migrations/0008_drop_course_tabs_fk_constraint.py

+# -*- coding: utf-8 -*-
+from south.utils import datetime_utils as datetime
+from south.db import db
+from south.v2 import SchemaMigration
+from django.db import models
+
+
+class Migration(SchemaMigration):
+
+    def forwards(self, orm):
+        # Foreign keys aren't enforced by default on our version of SQLite3, and
+        # trying to delete them will throw an error. See:
+        #   http://south.aeracode.org/ticket/775
+        if db.backend_name != 'sqlite3':
+            db.delete_foreign_key('course_overviews_courseoverviewtab', 'course_overview_id')
+
+    def backwards(self, orm):
+        pass
+
+    models = {
+        'course_overviews.courseoverview': {
+            'Meta': {'object_name': 'CourseOverview'},
+            '_location': ('xmodule_django.models.UsageKeyField', [], {'max_length': '255'}),
+            '_pre_requisite_courses_json': ('django.db.models.fields.TextField', [], {}),
+            'advertised_start': ('django.db.models.fields.TextField', [], {'null': 'True'}),
+            'cert_html_view_enabled': ('django.db.models.fields.BooleanField', [], {'default': 'False'}),
+            'cert_name_long': ('django.db.models.fields.TextField', [], {}),
+            'cert_name_short': ('django.db.models.fields.TextField', [], {}),
+            'certificates_display_behavior': ('django.db.models.fields.TextField', [], {'null': 'True'}),
+            'certificates_show_before_end': ('django.db.models.fields.BooleanField', [], {'default': 'False'}),
+            'course_image_url': ('django.db.models.fields.TextField', [], {}),
+            'created': ('model_utils.fields.AutoCreatedField', [], {'default': 'datetime.datetime.now'}),
+            'days_early_for_beta': ('django.db.models.fields.FloatField', [], {'null': 'True'}),
+            'display_name': ('django.db.models.fields.TextField', [], {'null': 'True'}),
+            'display_number_with_default': ('django.db.models.fields.TextField', [], {}),
+            'display_org_with_default': ('django.db.models.fields.TextField', [], {}),
+            'end': ('django.db.models.fields.DateTimeField', [], {'null': 'True'}),
+            'end_of_course_survey_url': ('django.db.models.fields.TextField', [], {'null': 'True'}),
+            'enrollment_domain': ('django.db.models.fields.TextField', [], {'null': 'True'}),
+            'enrollment_end': ('django.db.models.fields.DateTimeField', [], {'null': 'True'}),
+            'enrollment_start': ('django.db.models.fields.DateTimeField', [], {'null': 'True'}),
+            'facebook_url': ('django.db.models.fields.TextField', [], {'null': 'True'}),
+            'has_any_active_web_certificate': ('django.db.models.fields.BooleanField', [], {'default': 'False'}),
+            'id': ('xmodule_django.models.CourseKeyField', [], {'max_length': '255', 'primary_key': 'True', 'db_index': 'True'}),
+            'invitation_only': ('django.db.models.fields.BooleanField', [], {'default': 'False'}),
+            'lowest_passing_grade': ('django.db.models.fields.DecimalField', [], {'null': 'True', 'max_digits': '5', 'decimal_places': '2'}),
+            'max_student_enrollments_allowed': ('django.db.models.fields.IntegerField', [], {'null': 'True'}),
+            'mobile_available': ('django.db.models.fields.BooleanField', [], {'default': 'False'}),
+            'modified': ('model_utils.fields.AutoLastModifiedField', [], {'default': 'datetime.datetime.now'}),
+            'social_sharing_url': ('django.db.models.fields.TextField', [], {'null': 'True'}),
+            'start': ('django.db.models.fields.DateTimeField', [], {'null': 'True'}),
+            'version': ('django.db.models.fields.IntegerField', [], {}),
+            'visible_to_staff_only': ('django.db.models.fields.BooleanField', [], {'default': 'False'})
+        },
+        'course_overviews.courseoverviewtab': {
+            'Meta': {'object_name': 'CourseOverviewTab'},
+            'course_overview': ('django.db.models.fields.related.ForeignKey', [], {'related_name': "'tabs'", 'to': "orm['course_overviews.CourseOverview']"}),
+            'id': ('django.db.models.fields.AutoField', [], {'primary_key': 'True'}),
+            'tab_id': ('django.db.models.fields.CharField', [], {'max_length': '50'})
+        }
+    }
+
+    complete_apps = ['course_overviews']
\ No newline at end of file