ruby - How to filter with multiple parameters in Rails? -

i'm building app in users submit posts, tag them year, location, , military unit in. on index page, i'd have 3 dropdowns year location , military unit, , when user specifies different values in dropdowns, index filters through posts parameters. however, i've failed on , over.

here's posts controller:

    def index     @posts = post.all     @posts = @posts.location(params[:location]) if params[:location].present?     @posts = @posts.unit(params[:unit]) if params[:unit].present?     @posts = @posts.year(params[:year]) if params[:year].present?     end 

my index view:

    <h1>all posts</h1>     <%= form_tag 'search', method: 'get' %>     <%= select :location, options_for_select(countries) %>     <%= select :year, time.now.year.downto(1900).to_a %>     <%= select :unit, options_for_select(["army", "navy"]) %>     <%= submit_tag "filter" %>     <% end %>  </p> <ul id="posts"> <% @posts.each |post| %> <li> <%= link_to post.id, post_path(post) %> </li> <% end %> <%= link_to "new post", new_post_path %> </ul> 

and posts model:

    class post < activerecord::base     belongs_to :user     has_many :comments     has_attached_file :image     validates_attachment_content_type :image, :content_type =>     ["image/jpg","image/jpeg","image/png"] end 

with right now, error i'm receiving "nomethoderror in posts#index" : "undefined method `empty?' nil:nilclass". highlighted line view:

<%= select :location_id, options_for_select(countries) %> 

thanks this. i've searched , not found has helped.

edit: countries comes following helper:

    module countrieshelper       def countries     [      "afghanistan",      "aland islands",      "albania",      "algeria",  #truncated space        "western sahara",       "yemen",       "zambia",       "zimbabwe"     ]      end     end 

the select array needs formatted this:

def countries     [      ["afghanistan","afghanistan"],      ["aland islands","aland islands"],      ["albania","albania"],      , on . . .     ] 

then use select options in search per example snippet:

<%= select_tag :search, options_for_select(countries), prompt: "choose country", class: 'your_css_class_here' %> 

there might other issues, set helper array properly. let me know happens , can add answer.