title: ransack filter criteria dropdown
body: <p>I use the current version of Ransack gem with Rails 5 and MySQL and want to implement a search form where I can have two select fields:</p>
  
  <p>1) first dropdown select list should contain the table column attributes eg. first_name, last_name, street</p>
  
  <p>2) second dropdown select list should contain the operator like "equals", "is not", "contains"</p>
  
  <p>and then an input field for the search keyword and a submit button.</p>
  
  <p>In code it should look something like this:</p>
  
  <pre><code>&lt;%= search_form_for @search do |f| %&gt;
    &lt;%= f.select :select_criteria, options_for_select([
          ['first name', :first_name], 
          ['last name', :last_name], 
          ['street', :street]
          ]), @search.select_criteria %&gt;
    &lt;%= f.select :operator, options_for_select([
        ['contains', :contains], 
        ['is not', :is_not], 
        ['equals', :equals]
        ]), @search.operator %&gt;
    &lt;%= f.search_field :term %&gt;
    &lt;%= f.submit "filter" %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>But I can't find any examples in the ransack documentation that covers this case.</p>
  
  <p>How does it look correctly?</p>