title: Change string to text type in Laravel migration
body: <p>I have create a table <strong>stack_links</strong> as below</p>
  
  <pre><code>Schema::create('stack_links', function (Blueprint $table) {
              $table-&gt;increments('id');
              $table-&gt;string('site_key');
              $table-&gt;string('url');
              $table-&gt;string('path_run')-&gt;nullable();
              $table-&gt;integer('state');
              $table-&gt;integer('parent')-&gt;nullable();
              $table-&gt;text('data');
              $table-&gt;timestamps();
  
              $table-&gt;unique(['site_key', 'url']);
  
              $table-&gt;index(['site_key']);
              $table-&gt;index(['state']);
              $table-&gt;index(['parent']);
              $table-&gt;index(['url']);
          });
  </code></pre>
  
  <p>Now i want to change url column to text type. So i have tried this</p>
  
  <pre><code> $table-&gt;text('url')-&gt;change();
  </code></pre>
  
  <p>Hower it return this err </p>
  
  <pre><code>1170 BLOB/TEXT column 'url' used in key specification without a key length
  </code></pre>
  
  <p>After searching, i found that the err because url is indexed(<a href="https://stackoverflow.com/questions/1827063/mysql-error-key-specification-without-a-key-length">MySQL error: key specification without a key length</a>). So i remove index but it still the same err. i follow : <a href="https://laravel.com/docs/5.5/migrations#dropping-indexes" rel="nofollow noreferrer">https://laravel.com/docs/5.5/migrations#dropping-indexes</a></p>
  
  <pre><code>Schema::table('stack_links', function (Blueprint $table) {
              $table-&gt;dropUnique(['url']);
              $table-&gt;dropIndex(['url']);
  
              $table-&gt;text('url')-&gt;change();
              $table-&gt;string('md5_url');
  
              $table-&gt;unique('md5_url');
              $table-&gt;index('md5_url');
          });
  </code></pre>
  
  <p>Can anyone tell me where am i wrong.</p>
  
  <p><strong>Update</strong> : The problem is the parameter of dropIndex. When i replace it with the name of index. It work</p>