When (if) to consolidate ActiveRecord migrations?

Yes, this makes sense. There is a practice of consolidating migrations. To do this, simply copy the current schema into a migration, and delete all the earlier migrations. Then you have fewer files to manage, and the tests can run faster. You need to be careful doing this, especially if you have migrations running automatically on production. I generally replace a migration that I know everyone has run with the new schema one.

Other people have slightly different ways to do this.

I generally haven't done this until we had over 100 migrations, but we can hit this after a few months of development. As the project matures, though, migrations come less and less often, so you may not have to do it again.

This does go against a best practice: Once you check in a migration to source control, don't alter it. I make a rare exception if there is a bug in one, but this is quite rare (1 in 100 maybe). The reason is that once they are out in the wild, some people may have run them. They are recorded as being completed in the db. If you change them and check in a new version, other people will not get the benefit of the change. You can ask people to roll back certain changes, and re-run them, but that defeats the purpose of the automation. Done often, it becomes a mess. It's better left alone.

I think that there are two kinds of migrations:

• those you made during design/development, because you changed your mind on how your db should be like;

• those you made between releases, reflecting some behaviour changes.

I get rid of the first kind of migrations as soon as I can, as they do not really represent working releases, and keep the second kind, so that it is possible, in theory, to update the app.

About symbols vs strings: many argue that only strings should be used in migrations: symbols are meant to be "handles" to objects, and should not be used to represent names (column and table names, in this case). This is a mere stylistic consideration, but convinced me, and I'm no more using symbols in migrations.

I've read of another point for using strings: "ruby symbols are memory leaks", meaning that, when you create a symbol, it never gets disposed for all the application life time. This seems quite pointless to me, as all your db columns will be used as symbols in a Rails (and ActiveRecord) app; the migrating task, also, will not last forever, so I don't think that this point actually makes sense.

Having lots of migrations are a good thing. Combined with your version control system, they allow you to see what developer made a change to the database and why. This helps with accountability. Removing them just makes this a big hassle.

If you really want to get a new database up and running quickly you can just load the schema with rake db:schema:load RAILS_ENV=your_environment and if you want to get your test database setup quick you can just use rake db:test:prepare

That being said, if you really want to consolidate your migrations then I'd create a new migration that checks to see if the very last migration in your set has been performed (ex: does the column you added exist?) and if not, then it will fire. Otherwise the migration will just add itself to the schema table as completed so it doesn't attempt to fire again.

Just communicate what you're doing to the rest of your team so that they understand what is going on lest they blindly fire off a rake db:migrate and screw up something they already had.