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diff --git a/guides/source/migrations.md b/guides/source/migrations.md index 64c4e1e07e..bfee55a95d 100644 --- a/guides/source/migrations.md +++ b/guides/source/migrations.md @@ -18,9 +18,10 @@ After reading this guide, you will know: Migration Overview ------------------ -Migrations are a convenient way to alter your database schema over time in a -consistent and easy way. They use a Ruby DSL so that you don't have to write -SQL by hand, allowing your schema and changes to be database independent. +Migrations are a convenient way to +[alter your database schema over time](http://en.wikipedia.org/wiki/Schema_migration) +in a consistent and easy way. They use a Ruby DSL so that you don't have to +write SQL by hand, allowing your schema and changes to be database independent. You can think of each migration as being a new 'version' of the database. A schema starts off with nothing in it, and each migration modifies it to add or @@ -818,159 +819,6 @@ The `revert` method can be helpful when writing a new migration to undo previous migrations in whole or in part (see [Reverting Previous Migrations](#reverting-previous-migrations) above). -Using Models in Your Migrations -------------------------------- - -When creating or updating data in a migration it is often tempting to use one -of your models. After all, they exist to provide easy access to the underlying -data. This can be done, but some caution should be observed. - -For example, problems occur when the model uses database columns which are (1) -not currently in the database and (2) will be created by this or a subsequent -migration. - -Consider this example, where Alice and Bob are working on the same code base -which contains a `Product` model: - -Bob goes on vacation. - -Alice creates a migration for the `products` table which adds a new column and -initializes it: - -```ruby -# db/migrate/20100513121110_add_flag_to_product.rb - -class AddFlagToProduct < ActiveRecord::Migration - def change - add_column :products, :flag, :boolean - reversible do |dir| - dir.up { Product.update_all flag: false } - end - end -end -``` - -She also adds a validation to the `Product` model for the new column: - -```ruby -# app/models/product.rb - -class Product < ActiveRecord::Base - validates :flag, inclusion: { in: [true, false] } -end -``` - -Alice adds a second migration which adds another column to the `products` -table and initializes it: - -```ruby -# db/migrate/20100515121110_add_fuzz_to_product.rb - -class AddFuzzToProduct < ActiveRecord::Migration - def change - add_column :products, :fuzz, :string - reversible do |dir| - dir.up { Product.update_all fuzz: 'fuzzy' } - end - end -end -``` - -She also adds a validation to the `Product` model for the new column: - -```ruby -# app/models/product.rb - -class Product < ActiveRecord::Base - validates :flag, inclusion: { in: [true, false] } - validates :fuzz, presence: true -end -``` - -Both migrations work for Alice. - -Bob comes back from vacation and: - -* Updates the source - which contains both migrations and the latest version - of the Product model. -* Runs outstanding migrations with `rake db:migrate`, which - includes the one that updates the `Product` model. - -The migration crashes because when the model attempts to save, it tries to -validate the second added column, which is not in the database when the _first_ -migration runs: - -``` -rake aborted! -An error has occurred, this and all later migrations canceled: - -undefined method `fuzz' for #<Product:0x000001049b14a0> -``` - -A fix for this is to create a local model within the migration. This keeps -Rails from running the validations, so that the migrations run to completion. - -When using a local model, it's a good idea to call -`Product.reset_column_information` to refresh the Active Record cache for the -`Product` model prior to updating data in the database. - -If Alice had done this instead, there would have been no problem: - -```ruby -# db/migrate/20100513121110_add_flag_to_product.rb - -class AddFlagToProduct < ActiveRecord::Migration - class Product < ActiveRecord::Base - end - - def change - add_column :products, :flag, :boolean - Product.reset_column_information - reversible do |dir| - dir.up { Product.update_all flag: false } - end - end -end -``` - -```ruby -# db/migrate/20100515121110_add_fuzz_to_product.rb - -class AddFuzzToProduct < ActiveRecord::Migration - class Product < ActiveRecord::Base - end - - def change - add_column :products, :fuzz, :string - Product.reset_column_information - reversible do |dir| - dir.up { Product.update_all fuzz: 'fuzzy' } - end - end -end -``` - -There are other ways in which the above example could have gone badly. - -For example, imagine that Alice creates a migration that selectively -updates the `description` field on certain products. She runs the -migration, commits the code, and then begins working on the next feature, -which is to add a new column `fuzz` to the products table. - -She creates two migrations for this new feature, one which adds the new -column, and a second which selectively updates the `fuzz` column based on -other product attributes. - -These migrations run just fine, but when Bob comes back from his vacation -and calls `rake db:migrate` to run all the outstanding migrations, he gets a -subtle bug: The descriptions have defaults, and the `fuzz` column is present, -but `fuzz` is `nil` on all products. - -The solution is again to use `Product.reset_column_information` before -referencing the Product model in a migration, ensuring the Active Record's -knowledge of the table structure is current before manipulating data in those -records. - Schema Dumping and You ---------------------- |