Rails Design Question

Question

I have a Post Model, and a comment Model. I would like to limit the comments to 3. How would you do this?

1. Would it be best to create a validation? If so what would this look like?

2. Would you do this in the view unless Post.comments == 3 ?

3. Would a callback make sense?

Answer 1

the post's comments count validation is the "Comment" model responsibility, so I'd like to suggest the following code:

class Comment < ActiveRecord::Base
  belongs_to :post

  before_create :limit_number

  private
  def limit_number
    if post.comments.count >= 3
      errors.add(:post, "the limit of comments is over")
    end
  end
end

class Post < ActiveRecord::Base
  has_many :comments
end

Answer 2

You should always validate at the model level as well as the views.

class Comment < ActiveRecord::Base

  belongs_to :post
  validate :check

  private

  def check
    if post.present?
      errors.add("Post", "can not have more than 3 comments") if post.comments.size >= 3
    end
  end

end

class Post < ActiveRecord::Base
  # other implementation...

  def commentable?
    comments.size <= 3
  end

end

Then just call #commentable? in your views like this. You should never hard-code values in the views.

<% if @post.commentable? %>
  <%= render "form" %>
<% end %>

Answer 3

I would make sure not to let them submit that fourth comment that you don't want to allow. Some might say you should do the check in your controller and pass a flag, but for something this simple the check in the view seems fine.

Answer 4

Validate it in the models. You could use validates_with as described here .

In the view, you'd be better off checking with an inequality like

unless Post.comments.length >= 3
  show_form
end

That way if you have four comments for some reason (race condition or an admin posts a response after 3, etc.) the form stils won't show up.