title: how do you enforce uniqueness in ruby on rails migration?
body: <p>Based off <a href="http://guides.rubyonrails.org/v3.2.21/migrations.html" rel="nofollow">http://guides.rubyonrails.org/v3.2.21/migrations.html</a> and
  given the following migration:</p>
  
  <pre><code>class CreateVacations &lt; ActiveRecord::Migration
    def change
      create_table :vacations do |t|
        t.string :name
        t.string :slug, :uniqueness =&gt; true
        t.datetime :starts_at
        t.datetime :ends_at
        t.timestamps
      end
    end
  end
  </code></pre>
  
  <p>I ran rake db:migrate and see this schema:</p>
  
  <pre><code>ActiveRecord::Schema.define(:version =&gt; 20150825170615) do
  
    create_table "vacations", :force =&gt; true do |t|
      t.string   "name"
      t.string   "slug"
      t.datetime "starts_at"
      t.datetime "ends_at"
      t.datetime "created_at", :null =&gt; false
      t.datetime "updated_at", :null =&gt; false
    end
  
  end
  </code></pre>
  
  <p>I do not ssee any uniqueness requirement for 'slug'. If I did it wrong, how can I fix it at this point?</p>
  
  <p>If it was right, why doesn't the schema say it's unique? Thank you</p>