This is an example of Postgres Array use with Rails 4.

postgres_array_rails_migration.rb

class CreateRoles < ActiveRecord::Migration
  def change
    create_table :roles do |t|
      t.string :activities, array: true, length: 30, using: 'gin', default: '{}'
      t.timestamps
    end
  end
end

##
# You can use it since ActiveRecord 4
#
# 'array: true' is the arryfication 
# 'length : 30' applies to the string in array
# 'using: "gin"' is the index strategy for faster lookup and slower writes see: http://www.postgresql.org/docs/9.1/static/textsearch-indexes.html
# 'default: "{}"' is the default value ( same as [] )
#
#  [2] pry(main)> r = Role.last
#    Role Load (0.7ms)  SELECT "roles".* FROM "roles" ORDER BY "roles"."id" DESC LIMIT 1
#  => #<Role id: 2, activities: ["person:show"], created_at: "2013-10-15 13:21:06", updated_at: "2013-10-15 13:21:06">
#
#  [17] pry(main)> r.activities += %w(person:delete person:update)
#  => ["person:show", "person:delete", "person:update"]
#  [18] pry(main)> r.save
#     (0.3ms)  BEGIN
#    SQL (4.1ms)  UPDATE "roles" SET "activities" = $1, "updated_at" = $2 WHERE "roles"."id" = 2  [["activities", ["person:show", #  "person:delete", "person:update"]], ["updated_at", Wed, 16 Oct 2013 13:12:12 UTC +00:00]]
#     (2.3ms)  COMMIT
#  => true
#  
#  [25] pry(main)> r.activities = r.activities[0,2]
#  => ["person:show", "person:delete"]
#  [26] pry(main)> r.save
#     (0.4ms)  BEGIN
#    SQL (0.8ms)  UPDATE "roles" SET "activities" = $1, "updated_at" = $2 WHERE "roles"."id" = 2  [["activities", ["person:show", #  "person:delete"]], ["updated_at", Wed, 16 Oct 2013 13:16:01 UTC +00:00]]
#     (8.0ms)  COMMIT
#  => true
#