Why use database migrations instead of a version controlled schema

Question

Migrations are undoubtedly better than just firing up phpMyAdmin and changing the schema willy-nilly (as I did during my php days), but after using them for awhile, I think they're fatally flawed.

Version control is a solved problem. The main function of migrations is to keep a history of changes to your database. But storing a different file for each change is a clumsy way to track them. You don't create a new version of post.rb (or a file representing the delta) when you want to add a new virtual attribute -- why should you create a new migration when you want to add a new non-virtual attribute?

Put another way, just as you check post.rb into version control, why not check schema.rb into version control and make the changes to the file directly?

This is functionally the same as keeping a file for each delta, but it's much easier to work with. My mental model is "I want table X to have such and such columns (or really, I want model X to have such and such properties)" -- why should you have to infer from this how to get there from the existing schema; just open up schema.rb and give table X the right columns!

But even the idea that classes wrap tables is an implementation detail! Why can't I just open up post.rb and say:

 Class Post
    t.string :title
    t.text :body
 end

If you went with a model like this, you'd have to make a decision about what to do with existing data. But even then, migrations are overkill -- when you migrate data, you're going to lose fidelity when you use a migration's down method.

Anyway, my question is, even if you can't think of a better way, aren't migrations kind of gross?

Answer 1

why not check schema.rb into version control and make the changes to the file directly?

Because the database itself is not in sync with version control.

For instance, you could be using the head of the source tree. But you're connecting to a database that was defined as some past version, not the version you have checked out. The migrations allow you to upgrade or downgrade the database schema from any version and to any version, incrementally.

But to answer your last question, yes, migrations are kind of gross. They implement a redundant revision control system on top of another revision control system. However, neither of these revision control systems is really in sync with the database.

Answer 2

Just to paraphrase what others have said: migrations allow you to protect the data as your schema evolves. The notion of maintaining a single schema.rb file is attractive only until your app goes into production. Thereafter, you'll need a way to migrate your existing users' data as your schema changes.