[GitHub] [trafficcontrol] zrhoffman commented on a change in pull request #6096: `db/admin`: Set migration version to 0 before migrating if DB version's migration is squashed

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zrhoffman commented on a change in pull request #6096:
URL: https://github.com/apache/trafficcontrol/pull/6096#discussion_r689615560



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File path: traffic_ops/app/db/migrations/SQUASH.md
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@@ -0,0 +1,46 @@
+# Squashing database migrations
+
+Each major release of Apache Traffic Control combines database migrations from previous ATC releases into [`create_tables.sql`](https://github.com/apache/trafficcontrol/blob/master/traffic_ops/app/db/create_tables.sql).
+
+For example, suppose the latest version of Apache Traffic Control is 147.5.8 and contains these migrations:
+* `1_my-migration.up.sql`
+* `1_my-migration.down.sql`
+* `3_another-migration.up.sql`
+* `3_another-migration.down.sql`
+
+And suppose the ATC [`master`](https://github.com/apache/trafficcontrol/commits/master) branch contains these migrations:
+* `1_my-migration.up.sql`
+* `1_my-migration.down.sql`
+* `3_another-migration.up.sql`
+* `3_another-migration.down.sql`
+* `4_migration-name.up.sql`
+* `4_migration-name.down.sql`
+* `9_add-column-to-table.up.sql`
+* `9_add-column-to-table.down.sql`
+
+1. In order to prepare database migrations for the next major release, in this case, ATC 148.0.0, migrations `1` and `3` should be collapsed into `create_tables.sql` and migrations `4` and `9` should remain in [`traffic_ops/app/db/migrations/`](https://github.com/apache/trafficcontrol/tree/master/traffic_ops/app/db/migrations/).
+
+2. Note that `3` is the migration version version of the last up/down migration set. Find the definition for `LastSquashedMigrationVersion` in [`traffic_ops/app/db/admin.go`](https://github.com/apache/trafficcontrol/blob/master/traffic_ops/app/db/admin.go) and change it to `3`.

Review comment:
       > version repeated twice.
   
   Removed one *version* in 84f2871c99




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