Migration

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Migration
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Whats Migration?

• Migration
• Isolates database differences
• Allows you to write schema updates without
  worries about differences
• Helps you keep up with changes not only to code,
  but to database schema in an iterative/agile
  development
• Not only allows you to move forward, but also to
  roll back changes as well

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How does it work?

• You run the rake migrate to migrate database
• Ruby code tells what to do
• You write a class (or generate and edit it) that
  inherits from ActiveRecordMigration
• The up method moves it forward
• The down method moves it backward

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But, how does it how where to go?

• Database has a special table with one column and
  one row of the version number
• Based on the current version, it knows to move
  forward by how many versions, one after another

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Operations in Migration

• You can perform any of the following
• create_table
• drop_table
• rename_table
• add_column
• rename_column
• change_column
• remove_column
• add_index
• remove_index

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Creating a Migration

• We will play with Migrations for an Alumni
  application
• Creates a file 001_create_degree.rb

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Lets take it for a drive
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Creating Table and Initializing
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Making a Change
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Change related

• For irreversible options (like data deleted in
  move up) you can throw IrreversibleMigration
  exception

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Some Extras

• What is you already have a schema that youve
  developed along the way without migration?
• rake db_schema_dump can to get a Migration file
  created for your existing schema
• rake db_schema_import can import schema file into
  the current database