Rails%203%20In%20Action

254 CHAPTER 10 Tracking state
which define the colors of the label for this ticket. Later on, you’ll add a position field
that you’ll use to determine the sort order of the states in the select box. Let’s create this
State model and the associated migration by running this command:
rails g model state name:string color:string background:string
Before running this migration, you need to define a way that states link to comments
and to tickets, but there are a couple of things worth mentioning beforehand. For
comments, you want to track the previous state so you can display that a comment has
changed the ticket’s state. For tickets, you want to track the state for which you’ll use a
foreign key. With all of this in mind, let’s add these fields to the migration now. You
also remove the timestamps call from within create_table because it’s not important
when states were created or updated. When you’re done, the whole migration should
look like the following listing.
Listing 10.10 db/migrate/[date]_create_states.rb
class CreateStates < ActiveRecord::Migration
def up
create_table :states do |t|
t.string :name
t.string :color
t.string :background
end
add_column :tickets, :state_id, :integer
add_index :tickets, :state_id
add_column :comments, :state_id, :integer
end
def down
drop_table :states
remove_column :tickets, :state_id
remove_column :comments, :state_id
end
end
In this migration you use the add_index method to add a database index on the
tickets table’s state_id field. By adding an index on this field, you can speed up
queries made that search for this particular field. The side-effect of indexing is that it
will result in slower writes and more disk space. It’s always important to have indexes
on nonprimary-key fields 2 because of this great read speed increase.
Let’s run this migration now by running these two commands:
rake db:migrate
rake db:test:prepare
2 Primary key in this case is the id field that is automatically created for each model by create_table. Primary
key fields are, by default, indexed.