gistfile1.rb

models/product.rb 
def category_name category.try(:name) end def category_name=(name) self.category = Category.find_or_create_by_name(name) if name.present? end def category_name
  category.try(:name)
end

def category_name=(name)
  self.category = Category.find_or_create_by_name(name) if name.present?


endapp/assets/javascripts/application.js 

//= require jquery-ui 
//= require jquery-uiapp


/assets/javascripts/products.js.coffee 
jQuery -> $('#product_category_name').autocomplete source: $('#product_category_name').data('autocomplete-source') jQuery ->
  $('#product_category_name').autocomplete
    source: $('#product_category_name').data('autocomplete-source')

app/assets/stylesheets/application.css.scss
 
ul.ui-autocomplete { position: absolute; list-style: none; margin: 0; padding: 0; border: solid 1px #999; cursor: default; li { background-color: #FFF; border-top: solid 1px #DDD; margin: 0; padding: 0; a { color: #000; display: block; padding: 3px; } a.ui-state-hover, a.ui-state-active { background-color: #FFFCB2; } } } ul.ui-autocomplete {
  position: absolute;
  list-style: none;
  margin: 0;
  padding: 0;
  border: solid 1px #999;
  cursor: default;
  li {
    background-color: #FFF;
    border-top: solid 1px #DDD;
    margin: 0;
    padding: 0;
    a {
      color: #000;
      display: block;
      padding: 3px;
    }
    a.ui-state-hover, a.ui-state-active {
      background-color: #FFFCB2;
    }
  }
}

Server Side Solution

products/_form.html.erb 

<%= f.text_field :category_name, data: {autocomplete_source: categories_path} %> 


rails g controller categories rails g controller categories
config/routes.rb 
resources :categories 


categories_controller.rb 
def index @categories = Category.order(:name).where("name like ?", "%#{params[:term]}%") render json: @categories.map(&:name) end def index
  @categories = Category.order(:name).where("name like ?", "%#{params[:term]}%")
  render json: @categories.map(&:name)
end