autocomplete.rb

def autocomplete
  render :json => autocomplete_for_column(params[:column_name])
end

def autocomplete_for_column(column_name)
  #zipcode and mls num doesnt work
  valid_names = %w(agentlist_fullname officelist_officename officesell_officename schooldistrict)

  if valid_names.include? column_name
    Property.select(column_name).where("#{column_name} LIKE ?", "%#{params[:q]}%").uniq
  else
    "Error! Column Name Not Allowed!"
  end

end

index.html.haml

=label_tag :school_district, "School District"
= hidden_field_tag :school_district,
  '',
  :class => 'select2 ajax', 
  :data => { :source => reports_autocomplete_path('schooldistrict'), :column => 'schooldistrict' }

routes.rb

match '/reports/autocomplete/:column_name' => 'reports#autocomplete', as: :reports_autocomplete

select2-autocomplete.js

$('.select2').each(function(i, e) {
  var select = $(e);
  options = {};
  if(select.hasClass('ajax')) {
    var source     = select.data('source');
    var label      = select.prev('label').text();
    var columnName = select.data('column');

    options.id = function (result) {
      return result[columnName];
    };
    options.placeholder = "Search for " + label;
    options.minimumInputLength = 3;
    options.ajax = {
      url: source,
      dataType: 'json',
      quietMillis: 100,
      data: function (term, page) { // page is the one-based page number tracked by Select2
          return {
              q: term, //search term
              page_limit: 10, // page size
              page: page, // page number
          };
      },
      results: function (data, page) {
          return { results: data };
      }
    }
    options.formatResult = function(result) {
      return result[columnName];
    },
    options.formatSelection = function(result) {
      return result[columnName];
    },
    options.dropdownCssClass = "bigdrop";
  } else {
    options.width = "off";
    options.placeholder = "Enter some zip codes";
    options.tags = ["Type in a zip code followed by a space or comma"];
    options.tokenSeparators = [",", " "];
  }
  select.select2(options);
});