[ADD] runbot_merge: missing staging migration

44084e303c changed the interpretation
and schema of the `statuses_cache` field on stagings, but I forgot to
add a migration, so it would just blow up on opening the home
dashboard or the staging lists.

runbot_merge/__manifest__.py

@@ -1,6 +1,6 @@
 {
     'name': 'merge bot',
-    'version': '1.14',
+    'version': '1.15',
     'depends': ['contacts', 'mail', 'website'],
     'data': [
         'security/security.xml',

runbot_merge/migrations/15.0.1.15/pre-migration.py

@@ -0,0 +1,22 @@
+"""Completely missed that in 44084e303ccece3cb54128ab29eab399bd4d24e9 I
+completely changed the semantics and structure of the statuses_cache, so the
+old caches don't actually work anymore at all.
+
+This rewrites all existing caches.
+"""
+def migrate(cr, version):
+    cr.execute("""
+WITH statuses AS (
+    SELECT
+        s.id as staging_id,
+        json_object_agg(c.sha, c.statuses::json) as statuses
+    FROM runbot_merge_stagings s
+    LEFT JOIN runbot_merge_stagings_heads h ON (h.staging_id = s.id)
+    LEFT JOIN runbot_merge_commit c ON (h.commit_id = c.id)
+    GROUP BY s.id
+)
+UPDATE runbot_merge_stagings
+SET statuses_cache = statuses
+FROM statuses
+WHERE id = staging_id
+    """)