[PATCH] migrations: Optimize 0007 patch comment for psql

[Andy Doan]

By handling comment copying/deletion via SQL we can make migration take
<3 minutes rather than >30minutes for big instances. The SQL is vendor
specific, so a similar query would need to be added to support MySQL.

Signed-off-by: Andy Doan <andy.doan at linaro.org>
---
 .../0007_move_comment_content_to_patch_content.py  | 89 +++++++++++++---------
 1 file changed, 51 insertions(+), 38 deletions(-)

diff --git a/patchwork/migrations/0007_move_comment_content_to_patch_content.py b/patchwork/migrations/0007_move_comment_content_to_patch_content.py
index 63d57ba..aea65fe 100644
--- a/patchwork/migrations/0007_move_comment_content_to_patch_content.py
+++ b/patchwork/migrations/0007_move_comment_content_to_patch_content.py
@@ -1,47 +1,60 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
-from django.db import migrations
-
-
-def copy_comment_field(apps, schema_editor):
-    Comment = apps.get_model('patchwork', 'Comment')
-    Patch = apps.get_model('patchwork', 'Patch')
-
-    for patch in Patch.objects.all():
-        try:
-            # when available, this can only return one entry due to the
-            # unique_together constraint
-            comment = Comment.objects.get(patch=patch, msgid=patch.msgid)
-        except Comment.DoesNotExist:
-            # though there's no requirement to actually have a comment
-            continue
-
-        patch.content = comment.content
-        patch.save()
+from django.db import connection, migrations
+
+if connection.vendor == 'postgresql':
+    def copy_comment_field(apps, schema_editor):
+        schema_editor.execute('''
+            UPDATE patchwork_patch
+              SET content = patchwork_comment.content
+            FROM patchwork_comment
+              WHERE patchwork_patch.id=patchwork_comment.patch_id
+                    AND patchwork_patch.msgid=patchwork_comment.msgid
+        ''')
+
+    def remove_duplicate_comments(apps, schema_editor):
+        schema_editor.execute('''
+            DELETE FROM patchwork_comment
+              USING patchwork_patch
+              WHERE patchwork_patch.id=patchwork_comment.patch_id
+                    AND patchwork_patch.msgid=patchwork_comment.msgid
+        ''')
+else:
+    def copy_comment_field(apps, schema_editor):
+        Comment = apps.get_model('patchwork', 'Comment')
+        Patch = apps.get_model('patchwork', 'Patch')
+
+        for patch in Patch.objects.all():
+            try:
+                # when available, this can only return one entry due to the
+                # unique_together constraint
+                comment = Comment.objects.get(patch=patch, msgid=patch.msgid)
+            except Comment.DoesNotExist:
+                # though there's no requirement to actually have a comment
+                continue
+
+            patch.content = comment.content
+            patch.save()
+
+    def remove_duplicate_comments(apps, schema_editor):
+        Comment = apps.get_model('patchwork', 'Comment')
+        Patch = apps.get_model('patchwork', 'Patch')
+
+        for patch in Patch.objects.all():
+            try:
+                # when available, this can only return one entry due to the
+                # unique_together constraint
+                comment = Comment.objects.get(patch=patch, msgid=patch.msgid)
+                comment.delete()
+            except Comment.DoesNotExist:
+                # though there's no requirement to actually have a comment
+                continue
 
 
 def uncopy_comment_field(apps, schema_editor):
-    Patch = apps.get_model('patchwork', 'Patch')
-
-    for patch in Patch.objects.all():
-        patch.content = None
-        patch.save()
-
-
-def remove_duplicate_comments(apps, schema_editor):
-    Comment = apps.get_model('patchwork', 'Comment')
-    Patch = apps.get_model('patchwork', 'Patch')
-
-    for patch in Patch.objects.all():
-        try:
-            # when available, this can only return one entry due to the
-            # unique_together constraint
-            comment = Comment.objects.get(patch=patch, msgid=patch.msgid)
-            comment.delete()
-        except Comment.DoesNotExist:
-            # though there's no requirement to actually have a comment
-            continue
+    # This is no-op because the column is being deleted
+    pass
 
 
 def recreate_comments(apps, schema_editor):
-- 
2.7.3