如果已经运行了来自其他分支的更高版本的迁移，那么如何在Heroku上运行从Git分支进行的迁移？

Question

Say I create a branch ( new_branch ). In that branch, a migration ( migration_1 ) is created.

Later, I switch back to master . I then create a migration ( migration_2 ), push to Heroku, and run migrations on Heroku.

Later still, I merge new_branch into master , and then push master to Heroku.

When I try to run migrations on Heroku, won't migration_1 be skipped over and not run, because it was created before migration_2 , which has already been run?

Answer 1

Each migration has a migration timestamp attached. The list of successfully applied migrations is stored in a schema table inside your Rails application.

When you run migration_2 , this entry is added to the database. When migration_1 is merged, Rails will detect the change has not been applied yet because the entry is missing in the schema table, and will run it.