Add generator compatible with Rails 3.

lib/generators/paper_trail/USAGE

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+Description:
+  Generates (but does not run) a migration to add a versions table.

lib/generators/paper_trail/paper_trail_generator.rb

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+require 'rails/generators/named_base'
+require 'rails/generators/migration'
+
+class PaperTrailGenerator < Rails::Generators::NamedBase
+  include Rails::Generators::Migration
+  
+  desc "Generates (but does not run) a migration to add a versions table."
+
+  source_root File.expand_path('../templates', __FILE__)
+  
+  argument :name, :type => :string, :default => "create_versions"
+  
+  # Implement the required interface for Rails::Generators::Migration.
+  # taken from http://github.com/rails/rails/blob/master/activerecord/lib/generators/active_record.rb
+  def self.next_migration_number(dirname)
+    if ActiveRecord::Base.timestamped_migrations
+      Time.now.utc.strftime("%Y%m%d%H%M%S")
+    else
+      "%.3d" % (current_migration_number(dirname) + 1)
+    end
+  end
+
+  def create_migration_file
+    migration_template 'create_versions.rb', 'db/migrate/create_versions.rb'
+  end
+
+
+end

lib/generators/paper_trail/templates/create_versions.rb

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+class CreateVersions < ActiveRecord::Migration
+  def self.up
+    create_table :versions do |t|
+      t.string   :item_type, :null => false
+      t.integer  :item_id,   :null => false
+      t.string   :event,     :null => false
+      t.string   :whodunnit
+      t.text     :object
+      t.datetime :created_at
+    end
+    add_index :versions, [:item_type, :item_id]
+  end
+
+  def self.down
+    remove_index :versions, [:item_type, :item_id]
+    drop_table :versions
+  end
+end