Irreversible Migrations in Java

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Some transformations are destructive in a manner that cannot be reversed Migrations of that kind should raise an ActiveRecord::IrreversibleMigration exception in their down method For example, what if someone on your team made a silly mistake and defined the telephone column of your clients table as an integer You can change the column to a string and the data will migrate cleanly, but going from a string to an integer Not so much
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def selfup # Phone number fields are not integers, duh! change_column :clients, :phone, :string end def selfdown raise ActiveRecord::IrreversibleMigration end
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Getting back to the Migration API itself, here is the 20090124223305_create_ clientsrb file again, from earlier in the chapter, after adding a couple of column definitions for the clients table:
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class CreateClients < ActiveRecord::Migration def selfup create_table :clients do |t| tstring :name tstring :code ttimestamps end end def selfdown drop_table :clients end end
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As you can see in the example, migration directives happen within two class method definitions, selfup and selfdown If we go to the command line in our project folder and type rake db:migrate, the clients table will be created Rails gives us informative output during the migration process so that we see what is going on:
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$ rake db:migrate == CreateClients: migrating ========================================== -- create_table(:clients) -> 00448s == CreateClients: migrated (00450s) =================================
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Normally, only the code in the up method is run, but if you ever need to rollback to an earlier version of the schema, the down method specifies how to undo what happened in up To execute a rollback, use the migrate task, but pass it a version number to rollback to, as in rake db:migrate VERSION=20090124223305
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6: Active Record Migrations
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613 create table(name, options, & block)
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The create_table method needs at minimum a name for the table and a block containing column definitions Why do we specify identifiers with symbols instead of strings Both will work, but symbols require one less keystroke1 The create_table method makes a huge, but usually true assumption that we want an autoincrementing, integer-typed, primary key That is why you don t see it declared in the list of columns If that assumption happens to be wrong, it s time to pass create_table some options in a hash For example, how would you define a simple join table consisting of two foreign key columns and not needing its own primary key Just pass the create_table method an :id option set to false, as a boolean, not a symbol! It will stop the migration from autogenerating a primary key altogether:
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create_table :ingredients_recipes, :id => false do |t| tcolumn :ingredient_id, :integer tcolumn :recipe_id, :integer end
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If all you want to do is change the name of the primary key column from its default of id , pass the :id option a symbol instead For example, let s say your corporation mandates that primary keys follow the pattern tablename id Then the earlier example would look as follows:
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create_table :clients, :id => :clients_id do |t| tcolumn :name, :string tcolumn :code, :string tcolumn :created_at, :datetime tcolumn :updated_at, :datetime end
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The :force => true option tells the migration to go ahead and drop the table being defined if it exists Be careful with this one, since it will produce (possibly unwanted) data loss when run in production As far as I know, the :force option is mostly useful for making sure that the migration puts the database in a known state, but isn t all that useful on a daily basis The :options option allows you to append custom instructions to the SQL CREATE statement and is useful for adding database-specific commands to your migration Depending on the database you re using, you might be able to specify things such as character set, collation, comments, min/max sizes, and many other properties using this option The :temporary => true option specifies creation of a temporary table that will only exist during the current connection to the database In other words, it only exists
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