_form.html.haml

.input
  = f.text_area :body
  = f.hidden_field :commented_by, :value => current_user.email

comments_controller.rb

  def create
    @comment = @issue.comments.build(params[:comment])
    if @comment.save
      redirect_to project_issue_path(@issue.project, @issue)
    else
      render :new
    end
  end

new.html.haml

- content_for(:heading) do
  = @project.name
  
#inner
  %h2= "Comment on #{@issue.name}"
  
  = form_for [:project, :issue, @comment] do |f|
    = render :partial => "form", :locals => { :f => f, :comment => @comment }
    .input
      = f.submit "Add Comment", :class => "blue-button medium"