¿Cuál es un ejemplo de una migración de Ruby on Rails?

Which is an example of a Ruby on rails migration?

Before we dive into the details of a migration, here are a few examples of the sorts of things you can do: This migration adds a table called products with a string column called name and a text column called description. A primary key column called id will also be added, however since this is the default we do not need to ask for this.

Active Record will work out which migrations should be run. It will also update your db/schema.rb file to match the structure of your database. Migrations also allow you to describe these transformations using Ruby.

This migration adds a table called products with a string column called name and a text column called description. A primary key column called id will also be added, however since this is the default we do not need to ask for this. The timestamp columns created_at and updated_at which Active Record populates automatically will also be added.

While it’s not required you might want to add foreign key constraints to guarantee referential integrity. This adds a new foreign key to the author_id column of the articles table. The key references the id column of the authors table. If the column names cannot be derived from the table names, you can use the :column and :primary_key options.

Rails 3.1 makes migrations smarter by providing a new change method. This method is preferred for writing constructive migrations (adding columns or tables). The migration knows how to migrate your database and reverse it when the migration is rolled back without the need to write a separate down method.

Prior to Rails 2.1 the migration number started at 1 and was incremented each time a migration was generated. With multiple developers it was easy for these to clash requiring you to rollback migrations and renumber them. With Rails 2.1+ this is largely avoided by using the creation time of the migration to identify them.

If you have already run the migration then you cannot just edit the migration and run the migration again: Rails thinks it has already run the migration and so will do nothing when you run rake db:migrate.