how to check if parent exists in has many relation

I have 3 models:

User: has_many :comments
Video: has_many :comments
Comment: belongs_to :video, :user

videos/show.html.erb

<%= form_for(@comment, :method => :post ) do |f| %>
<%= f.text_area :body %>
<%= f.hidden_field :video_id, :value =>"4" %>
<%= f.submit "submit" %>
<% end %>

comments_controller

def create
 @comment = Comment.new(params[:comment].merge(:user_id => current_user.id))
 @comment.save
end 
开发者_运维知识库

How can i check existence of Video before i create a comment?

This should be present as a validation in your Comment model.

class Comment < ActiveRecord::Base

  validates_presence_of :video