Allows the RRN to perform garbage project on categories that are no longer referenced.
gem 'acts_as_sourceable'
create_table :acts_as_sourceable_registry do |t|
t.belongs_to :sourceable, :polymorphic => true
t.belongs_to :source, :polymorphic => true
t.timestamps
end
add_index :acts_as_sourceable_registry, [:sourceable_id, :sourceable_type], :name => :index_acts_as_sourceable_sourceables
add_index :acts_as_sourceable_registry, [:source_id, :source_type], :name => :index_acts_as_sourceable_sources