app.rb

get '/posts/:id/edit' do
  @post = Post.find(params[:id])
  haml :"posts/edit"
end

put '/posts/:id' do
  @post = Post.find(params[:id])
  @post.update_attributes(params)
  cleanup @post
  redirect "/posts/#{@post.id}"
end

edit.haml

%form{:action => "/posts/#{@post.id}", :method => 'post'}
  %input{:type => 'hidden', :name => '_method', :value => 'put'}
  %p
    %label{:for => 'title'}Title: 
    %input{:name => 'title', :id => 'title', :value => @post.title}
  %p
    %label{:for => 'tags_list'}Tags:
    %input{:name => 'tags_list', :id => 'tags_list', :value => @post.tags_list}
  %p
    %label{:for => 'body'}Body:
    %br/
    = find_and_preserve do
      %textarea{:name => 'body', :id => 'body', :rows => '12', :cols => '40'}= @post.body
  %p
    %input{:type => 'submit'}
%a{:href => "/posts/#{@post.id}"} Back