ruby-on-rails - Calling data from nested resource to main resource in rails app

Question

I have a blog app backed with Rails. I have Posts and Comments tables. Both are connected with has many - belongs to relationship, also resources are nested in config/routes.rb. In my posts#show, I'd like to call comments related with the post. I've written codes like below. But I got some errors like Couldn't find Comment without an ID. Here are my codes related with this issue:

# app/controllers/posts_controller.rb
def show
    @post = Post.find(params[:id])
    @comments = Comment.find(params[:post_id])

    respond_to do |format|
        format.html # show.html.erb
        format.json { render json: @post }
    end
end


# app/views/posts/show.html.erb
<h3><%= @post.title.titleize %></h3>
<p><%= @post.content %></p>
<% if @comments %>
    <% @comments.each do |com| %>
        <h4><%= com.title.titleize %></h4>
        <p><%= com.content %></p>
        <hr />
    <% end %>
<% else %>
    <p>No Comments Found , <%= link_to 'Add One?', new_post_comment_path(@post) %></p>
<% end %>
<%= link_to 'Edit', edit_post_path(@post) %> |
<%= link_to 'Back', posts_path %>


# config/routes.rb
    resources :posts do
      resources :comments
    end

EDIT:

Now I've used includes(:comments) in posts_controller.rb:

# app/controllers/posts_controller.rb
def show
    @post = Post.includes(:comments).find(params[:id])
    #@comments = Comment.find(params[:post_id])

    respond_to do |format|
        format.html # show.html.erb
        format.json { render json: @post }
    end
end

Also I've updated show.html.erb respectively:

<h3><%= @post.title.titleize %></h3>
<p><%= @post.content %></p>

<% if @post.comments %>
    <% @post.comments.each do |com| %>
    <h4><%= com.title.titleize %></h4>
    <p><%= com.content %></p>
    <hr />
<% end %>
<% else %>
    <p>No Comments Found , <%= link_to 'Add One?', new_post_comment_path(@post) %></p>
<% end %>

<%= link_to 'Edit', edit_post_path(@post) %> |
<%= link_to 'Back', posts_path %>

And here's new error: PG::Error: ERROR: column comments.commentable_type does not exist LINE 1: SELECT "comments".* FROM "comments" WHERE "comments"."comme... ^ : SELECT "comments".* FROM "comments" WHERE "comments"."commentable_type" = 'Post' A...