Tip #11 - Transact Your Migrations!

2008-04-17 10:23:10 +0000

If you are using Rails, you are most likely using migrations. Have you ever had a migration fail half way through? Have you ever then had to figure out how to find each change and revert it in the database? Would you like to never have to do that again? Here is how…

USE TRANSACTIONS

I sometimes wonder why this just isn’t the default. When I am doing a migration, I either want the thing to work completely all the way through, or I want nothing done to the database.

Luckily, you can do this with transactions.

A transaction (for those non SQL people out there) is an idea of a series of events being atomic. That is, either, they all are executed without error, or if any one of them fails or faults, then no change is made to the database at all.

Luckily, adding transactions to a migration is very easy.

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class ComplexMigrationThatCanNotFail < ActiveRecord::Migration

  def self.up
    transaction do
      add_column :table, :name, :type
      add_column :table, :name, :type
      # Complex task #1
      remove_column :table, :name, :type
      remove_column :table, :name, :type
      # Complex task #2
    end
  end

  def self.down
    # ... Down script
  end

end

Now, say your migration fails at the second remove column, no fear, everything is rolled back to the start point, you can read the rake error messages, fix the problem, and try again.

Of course, you are testing this on the development server, so no major harm done if it blows up a couple of times, but I don’t know about you, but I have lots more things to do than try to restore two gigabytes of staging or development database :)

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