diff .svn/pristine/99/99c710840ac1685d172c3ffd9c93f67d6b1292b0.svn-base @ 926:b73a59a6acbd luisf

Merge from cannam_integration

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/.svn/pristine/99/99c710840ac1685d172c3ffd9c93f67d6b1292b0.svn-base	Fri May 11 16:10:11 2012 +0100
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+# Contains the enhancements to Rails' migrations system to support the 
+# Engines::Plugin::Migrator. See Engines::RailsExtensions::Migrations for more
+# information.
+
+require "engines/plugin/migrator"
+
+# = Plugins and Migrations: Background
+#
+# Rails uses migrations to describe changes to the databases as your application
+# evolves. Each change to your application - adding and removing models, most
+# commonly - might require tweaks to your schema in the form of new tables, or new
+# columns on existing tables, or possibly the removal of tables or columns. Migrations
+# can even include arbitrary code to *transform* data as the underlying schema
+# changes.
+# 
+# The point is that at any particular stage in your application's development, 
+# migrations serve to transform the database into a state where it is compatible
+# and appropriate at that time.
+# 
+# == What about plugins?
+# 
+# If you want to share models using plugins, chances are that you might also
+# want to include the corresponding migrations to create tables for those models.
+# With the engines plugin installed, plugins can carry migration data easily:
+# 
+#   vendor/
+#     |
+#     plugins/
+#       |
+#       my_plugin/
+#         |- init.rb
+#         |- lib/
+#         |- db/
+#             |-migrate/
+#                 |- 20081105123419_add_some_new_feature.rb
+#                 |- 20081107144959_and_something_else.rb
+#                 |- ...
+# 
+# When you install a plugin which contains migrations, you are undertaking a
+# further step in the development of your application, the same as the addition
+# of any other code. With this in mind, you may want to 'roll back' the
+# installation of this plugin at some point, and the database should be able
+# to migrate back to the point without this plugin in it too.
+#
+# == An example
+#
+# For example, our current application is at version 20081106164503 (according to the
+# +schema_migrations+ table), when we decide that we want to add a tagging plugin. The
+# tagging plugin chosen includes migrations to create the tables it requires
+# (say, _tags_ and _taggings_, for instance), along with the models and helpers
+# one might expect.
+#
+# After installing this plugin, these tables should be created in our database.
+# Rather than running the migrations directly from the plugin, they should be
+# integrated into our main migration stream in order to accurately reflect the
+# state of our application's database *at this moment in time*.
+#
+#   $ script/generate plugin_migration
+#         exists  db/migrate
+#         create  db/migrate/20081108120415_my_plugin_to_version_20081107144959.rb
+#
+# This migration will take our application to version 20081108120415, and contains the 
+# following, typical migration code:
+# 
+#   class TaggingToVersion20081107144959 < ActiveRecord::Migration
+#     def self.up
+#       Engines.plugins[:tagging].migrate(20081107144959)
+#     end
+#     def self.down
+#       Engines.plugins[:tagging].migrate(0)
+#     end
+#   end
+#
+# When we migrate our application up, using <tt>rake db:migrate</tt> as normal,
+# the plugin will be migrated up to its latest version (20081108120415 in this example). If we
+# ever decide to migrate the application back to the state it was in at version 20081106164503,
+# the plugin migrations will be taken back down to version 0 (which, typically,
+# would remove all tables the plugin migrations define).
+#
+# == Upgrading plugins
+#
+# It might happen that later in an application's life, we update to a new version of
+# the tagging plugin which requires some changes to our database. The tagging plugin
+# provides these changes in the form of its own migrations. 
+#
+# In this case, we just need to re-run the plugin_migration generator to create a 
+# new migration from the current revision to the newest one:
+#
+#   $ script/generate plugin_migration
+#        exists db/migrate
+#        create db/migrate/20081210131437_tagging_to_version_20081201172034.rb
+#
+# The contents of this migration are:
+#
+#   class TaggingToVersion20081108120415 < ActiveRecord::Migration
+#     def self.up
+#       Engines.plugins[:tagging].migrate(20081201172034)
+#     end
+#     def self.down
+#       Engines.plugins[:tagging].migrate(20081107144959)
+#     end
+#   end
+#
+# Notice that if we were to migrate down to revision 20081108120415 or lower, the tagging plugin
+# will be migrated back down to version 20081107144959 - the version we were previously at.
+#
+#
+# = Creating migrations in plugins
+#
+# In order to use the plugin migration functionality that engines provides, a plugin 
+# only needs to provide regular migrations in a <tt>db/migrate</tt> folder within it.
+#
+# = Explicitly migrating plugins
+#
+# It's possible to migrate plugins within your own migrations, or any other code.
+# Simply get the Plugin instance, and its Plugin#migrate method with the version
+# you wish to end up at:
+#
+#   Engines.plugins[:whatever].migrate(version)
+#
+#
+# = Upgrading from previous versions of the engines plugin
+#
+# Thanks to the tireless work of the plugin developer community, we can now relying on the migration 
+# mechanism in Rails 2.1+ to do much of the plugin migration work for us. This also means that we
+# don't need a seperate schema_info table for plugins.
+#
+# To update your application, run
+#
+#   rake db:migrate:upgrade_plugin_migrations
+#
+# This will ensure that migration information is carried over into the main schema_migrations table.
+# 
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