I already have this migration file that was created and the migration was run.

```
class CreateStudentContexts < ActiveRecord::Migration
  def change
    create_table :student_contexts do |t|
      t.string :student_id, index: true, null: false, limit: 40
      t.text :data
      t.timestamps null: false

      t.index [:student_id, :updated_at], unique: true, name: 'student_context_index'
    end
  end
end

```

Now i want to create a new migration file that will update the index, and the index fields should be student\_id and created\_at.