title: Is necessary put "add_index" in a migration one to many?
body: <p>For example in this migration I have a relation one to many "Category has many Subcategories". If I not put "add_index :subcategories, :category_id", it work anyway.</p>
  
  <pre><code>class CreateSubcategories &lt; ActiveRecord::Migration
    def change
      create_table :subcategories do |t|
        t.string :nombre
        t.string :descripcion
        t.integer :category_id
  
        t.timestamps
      end
      add_index :subcategories, :category_id
    end
  end
  </code></pre>
  
  <p>For validate foreign key I use this</p>
  
  <pre><code>validates :category, presence: true
  </code></pre>