What is the best way to resolve Rails orphaned migrations?

By | December 28, 2017
Questions:

I have been switching between branches in a project and each of them have different migrations… This is the scenario:

$ rake db:migrate:status

 Status   Migration ID    Migration Name
--------------------------------------------------
   ...
   up     20130307154128  Change columns in traffic capture
   up     20130311155109  Remove log settings
   up     20130311160901  Remove log alarm table
   up     20130320144219  ********** NO FILE **********
   up     20130320161939  ********** NO FILE **********
   up     20130320184628  ********** NO FILE **********
   up     20130322004817  Add replicate to root settings
   up     20130403190042  ********** NO FILE **********
   up     20130403195300  ********** NO FILE **********
   up     20130403214000  ********** NO FILE **********
   up     20130405164752  Fix ap hostnames
   up     20130410194222  ********** NO FILE **********

The problem is rake db:rollback don’t work at all because of the missing files…

What should I do to be able to rollback again and get rid of the NO FILE messages?

Btw, rake db:reset or rake db:drop are not an option, I cannot lose data from other tables…

Answers:

I ended up solving the problem like this:

(1) Go to the branches that has the migration files and roll them back. This is not trivial when you have many branches which are will result in many conflicts if you try to merge them. So I use this commands to find out the branches of each orphan migration belongs to.

So, I need to find commit of the last time the migration was modified.

git log --all --reverse --stat | grep <LASTEST_ORPHAN_MIGRATION_ID> -C 10

I take the commit hash and determine which branch it belongs like this:

git branch --contains <COMMIT_HASH>

Then I can go back to that branch, do a rollback and repeat this process for all the missing files.

(2) Run migrations: checkout the branch you finally want to work on and run the migrations and you should be good to go.

Troubleshooting

I also ran in some cases where orphaned migrations where on deleted branches.

To solve this I created dummy migration files with the same migration_id of the missing files and roll them back. After that, I was able to delete they dummy migrations and have a clean migration status 🙂

Another alternative is deleting the missing files from the database directly:

delete from schema_migrations where version='<MIGRATION_ID>';

Questions:
Answers:

Edit: As said in the comments, the following WILL DROP YOUR DATABASE

A simpler approach that has worked for me (note that this command will drop the database and all your data will be lost):

rake db:migrate:reset

..and then:

rake db:migrate:status

The orphan(s) should disappear.

Questions:
Answers:

Migrations are stored in your database. If you want to remove the abandoned migrations, remove them from the db.

Example for Postgres:

  1. Open psql:

    psql
    
  2. Connect to your db:

    \c your_database
    
  3. If you’re curious, display schema_migrations:

    SELECT * FROM schema_migrations;
    
  4. If you’re curious, check if the abandoned migrations are present:

    SELECT version FROM schema_migrations WHERE version IN 
    ('20130320144219', '20130320161939', '20130320184628', '20130403190042',
     '20130403195300', '20130403214000', '20130410194222');
    
  5. Delete them:

    DELETE FROM schema_migrations WHERE version IN (<version list as above>);
    

Now if you run bundle exec rake db:migrate:status, you’ll see the orphaned migrations have been successfully removed.

Questions:
Answers:

Assuming that you are using Git, it should be relatively simple to grab these migrations and bring them into your current branch. If you have a specific commit you want a file from, you can use:

git checkout <commit hash> <file_name>

(Thanks to this answer)

Alternatively, you can check out from a specific branch HEAD:

git checkout <branch name> -- <file_name>

According to this blog post

Assuming these are, in fact, the versions of the migrations run on the database, you should be good to rollback.

Questions:
Answers:

You could merge the two branches back into the master so that you have all migrations available. If you really don’t want those migrations there, but want to be able to roll back, you could edit the schema_migrations table in your database to remove the rows corresponding to the migrations for which you don’t have files. However, this will cause problems if you then switch to another branch with different migrations.

Questions:
Answers:

If the migration files are truly missing (e.g. ran migration, forgot to roll back migration, then deleted migration file before commit), I was able to reproduce the missing migration as follows:

  1. go back in git history to get a copy of the schema.rb file and save outside of git repo (git log; git checkout xxxxxx; cp schema.rb ~/schema_old.rb, git checkout master).
  2. run a diff on the two files, and copy the migration commands into a migration file that matches the missing migration ID (diff schema.rb ~/schema_old.rb > migration_file.rb; vi migration_file.rb)
  3. Check your migration status and rollback (rake db:migrate:status; rake db:rollback; rake db:migrate:status;)
Category: Git

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