how can i get the particular post id in foreach loop

I think I found the problem.
Your view builds up a series of forms (within a foreach {  }  loop).
All forms have the same name: post_comment.
In your javascript function do_something(), you submit the form with the name post_comment. Since all forms have that name, only the first one is posted.

Now my question is: when the user hits the Enter key, do you want all comment-forms to be posted, or just the one where the user has filled-in some text and hit Enter?