I'm using form_for to create a chatroom and when I view the page I get the following error:

NoMethodError in Chatrooms#new 
undefined method `chatrooms_path' for #<#<Class:0xa862b94>:0xa5307f0>

Here's the code for the view, located in app/views/chatrooms/new.html.erb:

<div class="center">
  <%= form_for(@chatroom) do |f| %>
    <%=f.text_field :topic%>
    <br>
    <%=f.submit "Start a discussion", class: "btn btn-large btn-primary"%>
  <% end %>
</div>

Here's the relevant controller:

class ChatroomsController < ApplicationController
  def new
    @chatroom = Chatroom.new
  end

  def show
    @chatroom = Chatroom.find(params[:id])
  end
end

If I change the line

<%= form_for(@chatroom) do |f| %>

to

<%= form_for(:chatroom) do |f| %>

it works fine.
I've searched around for similar questions but none of the solutions have worked for me. Help?