undefined method `model_name' for nested routes - ruby-on-rails-4

Question

I want to set up my models, such that my Posts has_many Comments and Comments has_many Replies. Comments are working fine. But my application starts breaking saying
undefined method `model_name' for "/posts/adas/comments/11/replies":String
when I try to setup the replies for comments. I cannot figure out what's triggering this error. I can add replies from my Rails console, and see them in the view, but adding the form_for tag breaks the code. Can anybody please point out what is the mistake and how I should be routing it?
Posts#show.html.erb
<h2>Comments</h2>
<% #post.comments.each do |comment| %>
<p>
<b><%= comment.username %></b>
<%= comment.name %>
<% if current_user.email == comment.username || current_user.admin? %>
<%= link_to 'Delete', [comment.post, comment],
:confirm => 'Are you sure?', :method => :delete %>
<% end %>
<p style = "text-indent: 3em">
<% comment.replies.each do |reply| %>
<i><%= reply.author %></i>
<%= reply.content %>
<% end %>
<%= form_for [#reply, post_comment_replies_path(#post, comment)] do |f| %>
<%= f.label :reply %>
<%= f.text_field :content %>
<%= f.submit("Reply") %>
<% end %>
<% end %>
</p>
</p>
<h3>Add a comment:</h3>
<%= form_for([#post, #post.comments.build]) do |f| %>
<%= f.label :comment %><br />
<%= f.text_area :name %>
<%= f.submit %>
<% end %>
replies_controller.rb
class RepliesController < ApplicationController
def create
#reply = #comment.replies.create(reply_params)
redirect_to post_path(#post)
end
private
def reply_params
params.require(:reply).permit(:content)
end
end
routes.rb
Rails.application.routes.draw do
devise_for :users, :controllers => { :omniauth_callbacks => "callbacks" }
root 'welcome#index'
resources :posts do
resources :comments do
resources :replies
end
member do
put "like", to: "posts#upvote"
end
end
end

Answer 1

You're almost there, but you're not passing the parameters in the proper way to get the result that you want. At the moment you're passing in a url string where rails is expecting a resource object, hence the undefined method exception.
form_for(record, options = {}, &block)
The method takes a record and a hash of options that are optional.
The suspect line is
<%= form_for [#reply, post_comment_replies_path(#post, comment)] do |f| %>
The square brackets should only be used to define a resource and its associated resources which are then used to generate a url path. You should also explicitly state your url in the options hash unless it can be inferred from the resources you pass into form_for.
So it should look something like this, keeping with the way you are doing things.
<%= form_for #reply, url: post_comment_replies_path(#post, comment) do |f| %>
That's the case if you want to specify a url in form_for. However, why don't you just allow rails to generate the url for you?
<%= form_for [#post, comment, #reply] do |f| %>
This way rails should handle the routes.