title: Laravel Eloquent increment a column based on a relation
body: <p>I currently have a table of image links associate with another model as follows</p>
  
  <pre><code>+----+-------------+----------------+
  | id | property_id |      url       |
  +----+-------------+----------------+
  |  1 |           2 | /example.jpg   |
  |  2 |           7 | /example-2.jpg |
  |  3 |           5 | /example-3.jpg |
  +----+-------------+----------------+
  </code></pre>
  
  <p>I would like to add a column to the table to order the images as follows</p>
  
  <pre><code>+----+-------------+----------------+------------+
  | id | property_id |      url       | sort_order |
  +----+-------------+----------------+------------+
  |  1 |           2 | /example.jpg   |          1 |
  |  2 |           7 | /example-2.jpg |          1 |
  |  3 |           2 | /example-3.jpg |          2 |
  +----+-------------+----------------+------------+
  </code></pre>
  
  <p>Is there way to make a database level constraint (i.e. put it in the migration) that the sort_order value auto_increments but is dependent on the propery_id value, such that the sort_order value is essentially an index with each property_id having it's own index?</p>