class Post < ApplicationRecord
  has_many :comments, dependent: :destroy
end

class Comment < ApplicationRecord
  belongs_to :post
  validates :comment_content, presence: true, length: { maximum: 5000 }
end

class CommentsController < ApplicationController
  def create
    @post = Post.find(params[:post_id])
    @comment = @post.comments.create(params[:comment].permit(:comment_content))
    redirect_to post_path(@post)
  end
end

<%= simple_form_for([@post, @post.comments.build]) do |f| %>
  <%= f.input :comment_content, input_html: { class: 'texture' }, wrapper: false, label_html: { class: 'label'
  } %>

  <%= f.button :submit, 'Leave a reply', class: "btn btn-primary" %>
<% end %>

<%= comment.comment_content %>

<p id="notice"><%= notice %></p>

<p>
  <!-- Post details -->
</p>

<%= render @post.comments %>

<%= render 'comments/form' %>

Rails.application.routes.draw do
  resources :posts do
    resources :comments
  end
end

<%= f.error_notification %>
<%= f.error_notification message: f.object.errors[:base].to_sentence if f.object.errors[:base].present? %>

def create
  @post = Post.find(params[:post_id])
  @comment = @post.comments.build(params[:comment].permit(:comment_content))
  if @comment.save
    redirect_to post_path(@post), notice: 'Comment was successfully created.'
  else
    render 'posts/show'
  end
end