If you are doing this in a RJS template, then you will have the instance variables available to you, whatever you set at the controller that is rendering this. Just render the form using the @comment you created in the controller.
I just posted an answer to a different question you raised, where I show you how to get the unique IDs on the DOM elements holding members of a collection. You can use that technique with RJS or with a single JS file that does not have any ERB replacement in it.
> This is my jquery
> $('.new_comment_button').after("<%= j render('form') %>");
> I need
> $('.new_comment_button') to be contructed from the values of variables because i can't give every post the same id I don't think using class instead of id in my view would make a difference How can i do this?
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Can I have the link And once I get the DOM id's how do i plug them into my js.erb file to change $('.new_comment_button') to let's say $('.new_comment_button2')
There are a number of different ways to solve this problem. The way I imagine your app, you want to be able to comment on an item, but without having a bunch of forms rendered in the page (one for each item in a collection). Right so far? So the static (non-RJS) way to do this with jQuery is to use the DOM and JS to send the correctly unique variable to your comments controller to let it know which item you are commenting on.
@items = Item.all.page(params[:page])
<%= render @items %>
<%= content_tag_for item, :li do %>
<%= content_tag :hi, item.headline %>
<%= simple_format item.description %>
<%- end -%>
var elm = $(this);
elm.append('<a href="/comments/new?item_id=' + elm.attr('id').split('_') + '">Comment</a>');
That gets you a link to a CommentsController, which in turn will need to decode the (bare) attribute item_id out of its querystring, and figure out which item you are commenting on.
The #content_tag_for method, which you no doubt read the documentation for, will always create a unique id on the content tag it creates, using the instance variable you pass to it to figure out what that should be. For my contrived example, that will be 'item_123' or 'item_456' or similar. Model name, singular, lower-case, followed by an underscore, followed by the ID of the instance.