Going by my application, I have an index.html.erb file that lists all articles and i was in need of a search box that would search all articles depending on the selected category.So firstly ,I created a search box within index.html.erb like this :

 <h3> Search according to category</h3>

 <%= form_tag( show_category_wise_path, method: "get") do  %>

       <%= select_tag(:category, options_for_select(@categories.collect{ |category|   [category.category_name, category.id]})) %>


   <%= submit_tag("Search", :class => "btn btn-primary" )%>
     
 <% end %>

Here , It is important to understand what parameters  select_tag receives. The First parameter :category provides us  a category key in our params hash for the show_category_wise action of the ArticlesController (in my case) .

For the second parameter options_for_select method receives a collection . Here @categories object has an array of all category objects .It is something like this:

def index

    @articles = Article.paginate page: params[:page] ,per_page: '2'

    @categories = Category.all

end

Now understand the working of collect method through this simple example:

a = [1,2,3,4]

a.collect {|x| x*2}

returns [2,4,6,8].

Remember category_name and id are columns in the categories table.So we get something like this :

[["ruby",1],["java",2],["html",3],["c lang",4]]

Now if user selects html , the category key in params hash  will have  value 3.Now Let's look at the show_category_wise method :

def show_category_wise

    @category = Category.find(params[:category])

    @articles = @category.articles.paginate page: params[:page] ,per_page: '2'

end

Please take cognizance of association and pagination here .Now show_category_wise.html.erb 

will simply list all those articles belonging to a particular category.

Thank You.