Rails - Generating migration script from model

Question

I am learning rails, and I came across Migrations. It seems that everytime I want to edit a model, I would need to add a migration script, even though I am not yet in production.

Can you edit your model, add all your attributes you need to it, and before releasing it, have the migration script autogenerated?

Thanks!

It really isn't too hard to write migration script manually. — Sergio Tulentsev
– Sergio Tulentsev, Commented Mar 14, 2012 at 15:45
well, I guess I could edit the original migration script, and drop and reload the database — Karan
– Karan, Commented Mar 14, 2012 at 15:49
At the end of the day you probably don't want to run your migrations in production anyway, but rather just load the completed schema. — numbers1311407
– numbers1311407, Commented Mar 14, 2012 at 15:49
Or you could "compress" migration scripts before going to production. — Sergio Tulentsev
– Sergio Tulentsev, Commented Mar 14, 2012 at 15:50

ggorlen · Accepted Answer · 2023-05-26 01:37:53Z

5

If you're using Rails 3+, you might want to consider DataMapper instead of ActiveRecord. It lets you define the data model in the model instead of multiple migration files. As I understand DataMapper lets you generate migrations from changes.

This is a tried and trusted pattern often used in the wider ORM community.

edited May 26, 2023 at 1:37

ggorlen

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answered Oct 19, 2012 at 21:59

Greg

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Comments

Tom Harrison · Accepted Answer · 2012-03-14 18:38:37Z

I agree with the comments so far. The idea of migrations is to make it simple to fluidly adapt your data schema to fit your application as you want to add new fields. It's a simple and beautiful system.

So yes, you can (and should) use rails generate migration... as not only does this generate the proper code in many common cases, it also keeps track of which migrations have been run in different versions of the database. See http://guides.rubyonrails.org/migrations.html#creating-a-migration

A common workflow might be something like this:

create a new model, for example User with fields like first_name, last_name, user_name
this will create an associated migration, which you can run using bundle exec rake db:migrate -- your database schema will be updated
you decide you want additional information, such as birthdate, so run rails generate migration AddBirthdateToUser birthdate:date. For some simple operations like adding a column, index, etc., the full migration code will be generated; in other cases you'll need to write the migration. When done, run the migration.
If you find a problem in development, for example a field type should be float, not integer, or you forgot to add an index, you can roll back the migration (bundle exec rake db:rollback), fix the migration and re-run it.
run your tests (which will run the migrations), and when it all works for you locally, check in the files (including the migrations) and deploy to a QA or staging server, which has its own copy of the database.
run rake db:migrate on the staging server. If you're on a team and other developers have checked in migrations, their will run, too. Now your code and data schema are in sync.
repeat :-)

There's no harm whatsoever running migrations during a production deployment (I respectfully disagree with a comment above) -- you should embrace the idea that change, even changes like this (which can be incredibly difficult in other environments) are a normal part of everyday Rails life!

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