Merge pull request #11748 from edx/jibsheet/course_overview_0009_on_migrate_from_scratch

Migrate correctly from scratch also

openedx/core/djangoapps/content/course_overviews/migrations/0009_readd_facebook_url.py

@@ -1,8 +1,27 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
-from django.db import migrations, models, OperationalError, connection
-from openedx.core.djangoapps.content.course_overviews.models import CourseOverview
+from django.db import migrations, models, connection
+
+def table_description():
+    """Handle Mysql/Pg vs Sqlite"""
+    # django's mysql/pg introspection.get_table_description tries to select *
+    # from table and fails during initial migrations from scratch.
+    # sqlite does not have this failure, so we can use the API.
+    # For not-sqlite, query information-schema directly with code lifted
+    # from the internals of django.db.backends.mysql.introspection.py
+
+    if connection.vendor == 'sqlite':
+        fields = connection.introspection.get_table_description(connection.cursor(), 'course_overviews_courseoverview')
+        return [f.name for f in fields]
+    else:
+        cursor = connection.cursor()
+        cursor.execute("""
+            SELECT column_name
+            FROM information_schema.columns
+            WHERE table_name = 'course_overviews_courseoverview' AND table_schema = DATABASE()""")
+        rows = cursor.fetchall()
+        return [r[0] for r in rows]
 
 
 class Migration(migrations.Migration):
@@ -11,15 +30,17 @@ class Migration(migrations.Migration):
         ('course_overviews', '0008_remove_courseoverview_facebook_url'),
     ]
 
-    # An original version of 0008 removed the facebook_url field
-    # We need to handle the case where our noop 0008 ran AND the case
-    # where the original 0008 ran.  We do that by using Django's introspection
-    # API to query INFORMATION_SCHEMA.  _meta is unavailable as the
-    # column has already been removed from the model.
-    fields = connection.introspection.get_table_description(connection.cursor(),'course_overviews_courseoverview')
+    # An original version of 0008 removed the facebook_url field We need to
+    # handle the case where our noop 0008 ran AND the case where the original
+    # 0008 ran.  We do that by using the standard information_schema to find out
+    # what columns exist. _meta is unavailable as the column has already been
+    # removed from the model
     operations = []
+    fields = table_description()
 
-    if not any(f.name == 'facebook_url' for f in fields):
+    # during a migration from scratch, fields will be empty, but we do not want to add
+    # an additional facebook_url
+    if fields and not any(f == 'facebook_url' for f in fields):
         operations += migrations.AddField(
             model_name='courseoverview',
             name='facebook_url',