Is it a good idea to purge old Rails migration files?

问题

I have been running a big Rails application for over 2 years and, day by day, my ActiveRecord migration folder has been growing up to over 150 files.

There are very old models, no longer available in the application, still referenced in the migrations. I was thinking to remove them.

What do you think? Do you usually purge old migrations from your codebase?

回答1:

They are relatively small, so I would choose to keep them, just for the record.

You should write your migrations without referencing models, or other parts of application, because they'll come back to you haunting ;)

Check out these guidelines:

http://guides.rubyonrails.org/migrations.html#using-models-in-your-migrations

回答2:

Once I hit a major site release, I'll roll the migrations into one and start fresh. I feel dirty once the migration version numbers get up around 75.

回答3:

The Rails 4 Way page 177: Sebastian says...

A little-known fact is that you can remove old migration files (while still keeping newer ones) to keep the db/migrate folder to a manageable size. You can move the older migrations to a db/archived_migrations folder or something like that. Once you do trim the size of your migrations folder, use the rake db:reset task to (re-)create your database from db/schema.rb and load the seeds into your current environment.

回答4:

I occasionally purge all migrations, which have already been applied in production and I see at least 2 reasons for this:

1. More manageable folder. It is easier to spot if some new migration is added.
2. Cleaner text search results (Global text search within a project does not lead to tons of useless matches because of old migration, say, when someone created some column 3 years ago).

回答5:

Why? Unless there is some kind of problem with disk space, I don't see a good reason for deleting them. I guess if you are absolutely certain that you are never going to roll back anything ever again, than you can. However, it seems like saving a few KB of disk space to do this wouldn't be worth it. Also, if you just want to delete the migrations that refer to old models, you have to look through them all by hand to make sure you don't delete anything that is still used in your app. Lots of effort for little gain, to me.

回答6:

Personally I like to keep things tidy in the migrations files. I think once you have pushed all your changes into prod you should really look at archiving the migrations. The only difficulty I have faced with this is that when Travis runs it runs a db:migrate, so these are the steps I have used:

1. Move historic migrations from /db/migrate/ to /db/archive/release-x.y/

2. Create a new migration file manually using the version number from the last run migration in the /db/archive/release-x.y directory and change the description to something like from_previous_version. Using the old version number means that it won't run on your prod machine and mess up.

3. Copy the schema.rb contents from inside the ActiveRecord::Schema.define(version: 20141010044951) do section and paste into the change method of your from_previous_version changelog

4. Check all that in and Robert should be your parent's brother.

The only other consideration would be if your migrations create any data (my test scenarios contain all their own data so I don't have this issue)

回答7:

See http://edgeguides.rubyonrails.org/active_record_migrations.html#schema-dumping-and-you

Migrations are not a representation of the database: either structure.sql or schema.rb is. Migrations are also not a good place for setting/initializing data. db/seeds or a rake task are better for that kind of task.

So what are migrations? In my opinion they are instructions for how to change the database schema - either forwards or backwards (via a rollback). Unless there is a problem, they should be run only in the following cases:

1. On my local development machine as a way to test the migration itself and write the schema/structure file.
2. On colleague developer machines as a way to change the schema without dropping the database.
3. On production machines as a way to change the schema without dropping the database.

Once run they should be irrelevant. Of course mistakes happen, so you definitely want to keep migrations around for a few months in case you need to rollback.

CI environments do not ever need to run migrations. It slows down your CI environment and is error prone (just like the Rails guide says). Since your test environments only have ephemeral data, you should instead be using rake db:setup, which will load from the schema.rb/structure.sql and completely ignore your migration files.

If you're using source control, there is no benefit in keeping old migrations around; they are part of the source history. It might make sense to put them in an archive folder if that's your cup of coffee.

With that all being said, I strongly think it makes sense to purge old migrations, for the following reasons:

• They could contain code that is so old it will no longer run (like if you removed a model). This creates a trap for other developers who want to run rake db:migrate.
• They will slow down grep-like tasks and are irrelevant past a certain age.

Why are they irrelevant? Once more for two reasons: the history is stored in your source control and the actual database structure is stored in structure.sql/schema.rb. My rule of thumb is that migrations older than about 12 months are completely irrelevant. I delete them. If there were some reason why I wanted to rollback a migration older than that I'm confident that the database has changed enough in that time to warrant writing a new migration to perform that task.

So how do you get rid of the migrations? These are the steps I follow:

1. Delete the migration files
2. Write a rake task to delete their corresponding rows in the schema_migrations table of your database.
3. Run rake db:migrate to regenerate structure.sql/schema.rb.
4. Validate that the only thing changed in structure.sql/schema.rb is removed lines corresponding to each of the migrations you deleted.
5. Deploy, then run the rake task from step 2 on production.
6. Make sure other developers run the rake task from step 2 on their machines.

The second item is necessary to keep schema/structure accurate, which, again, is the only thing that actually matters here.

回答8:

It's fine to remove old migrations once you're comfortable they won't be needed. The purpose of migrations is to have a tool for making and rolling back database changes. Once the changes have been made and in production for a couple of months, odds are you're unlikely to need them again. I find that after a while they're just cruft that clutters up your repo, searches, and file navigation.

Some people will run the migrations from scratch to reload their dev database, but that's not really what they're intended for. You can use rake db:schema:load to load the latest schema, and rake db:seed to populate it with seed data. rake db:reset does both for you. If you've got database extensions that can't be dumped to schema.rb then you can use the sql schema format for ActiveRecord and run rake db:structure:load instead.

回答9:

Yes. I guess if you have completely removed any model and related table also from database, then it is worth to put it in migration. If model reference in migration does not depend on any other thing, then you can delete it. Although that migration is never going to run again as it has already run and even if you don't delete it from existing migration, then whenever you will migrate database fresh, it cause a problem.

So better it to remove that reference from migration. And refactore/minimize migrations to one or two file before big release to live database.

回答10:

I agree, no value in 100+ migrations, the history is a mess, there is no easy way of tracking history on a single table and it adds clutter to your file finding. Simply Muda IMO :)

Here's a 3-step guide to squash all migrations into identical schema as production:

Step1: schema from production

# launch rails console in production
stream = StringIO.new
ActiveRecord::SchemaDumper.dump(ActiveRecord::Base.connection, stream); nil
stream.rewind
puts stream.read

This is copy-pasteable to migrations, minus the obvious header

Step 2: making the migrations without it being run in production

This is important. Use the last migration and change it's name and content. ActiveRecord stors the datetime number in it's schema_migrations table so it knows what it has run and not. Reuse the last and it'll think it has already run.

Example: rename 20161202212203_this_is_the_last_migration -> 20161202212203_schema_of_20161203.rb

And put the schema there.

Step 3: verify and troubleshoot

Locally, rake db:drop, rake db:create, rake db:migrate

Verify that schema is identical. One issue we encountered was datetime "now()" in schema, here's the best solution I could find for that: https://stackoverflow.com/a/40840867/252799

来源：https://stackoverflow.com/questions/4248682/is-it-a-good-idea-to-purge-old-rails-migration-files