title: Add and drop constraints in knex migration
body: <p>I am trying to add a constrain in the <code>up</code> section of a knex migration, and then to add that constraint back on (undo the change) in the <code>down</code> section. I want to keep the table and all its columns, but remove the <code>unique</code> and <code>index</code> constrains from one of those columns. This is what I have so far:</p>
  <pre><code>exports.up = function(knex) {
      return knex.schema
          .table('menuItems', tbl =&gt; {
              tbl.dropUnique(&quot;itemName&quot;)
              tbl.dropIndex(&quot;itemName&quot;) 
     // ^these will leave the column but remove the constraints,
     // correct?
          });
  };
  
  exports.down = function(knex) {
    // What do I do here?
  };
  </code></pre>
  <p>Assuming that in my current <code>up</code>, I am indeed just dropping the <em>constraint</em> not the column, my question is: How do I add constraints back to an existing column (in the <code>down</code>) without creating a new column?</p>
  <p>(I'm pretty wobbly with knex migrations, and I'm having trouble finding examples online, so if anyone has links to complete examples of some basic migrations, that could also be helpful for me.)</p>