form_for method using slim

Was experiencing a similar issue earlier, I think you just need some child elements! Try this:

= form_for([@post, @post.comments.build]) do |f|
  div.field
    = f.label :commenter
    br
    = f.text_field :commenter
  div.field
    = f.label :body
    br
    = f.text_area :body
  div.actions
    = f.submit