Updating comments and state from the same form - ruby-on-rails

Question

I'm trying to build a small expense tracking app using Rails 4.1. When a user submits the expense request, it's state is marked as pending by default. The admin has to approve the request. I'm using state_machine gem to do this.
I just added comment functionality using acts_as_commentable gem, which works fine on its own. I wanted to combine the approval drop down and the comment box in the same form and used the following code in the expense show page:
<div class="row">
<div class="col-md-8">
<%= form_for [#expense, Comment.new] do |f| %>
<div class="form-group">
<%= f.label :state %><br />
<%= f.collection_select :state, #expense.state_transitions, :event, :human_to_name, :include_blank => #expense.human_state_name, class: "form-control" %>
</div>
<div class="form-group">
<%= f.label :comment %><br />
<%= f.text_area :comment, class: "form-control" %>
</div>
<%= f.submit "Submit", class: "btn btn-primary" %>
<% end %>
</div>
</div>
<br>
The problem is I get the "NoMethodError in Expenses#show - undefined method `state' for #". Is there a way I can update both the approval status and comment in one go?
The updated show page with nested attributes:
<div class="row">
<div class="col-md-8">
<%= nested_form_for (#expense) do |f| %>
<div class="form-group">
<%= f.label :state %><br />
<%= f.collection_select :state, #expense.state_transitions, :event, :human_to_name, :include_blank => #expense.human_state_name, class: "form-control" %>
</div>
<%= f.fields_for :comments do |comment| %>
<div class="form-group">
<%= comment.label :comment%>
<%= comment.text_area :comment, class: "form-control" %>
</div>
<% end %>
<%= f.submit "Submit", class: "btn btn-primary" %>
<% end %>
</div>
</div>

Answer 1

Nested attributes are your friend. They are so easy to implement I'd recommend avoiding acts_as_commentable to remove an unnecessary dependency and so you understand what's going on.
Use them like this:
# expense.rb
class Expense < ActiveRecord::Base
has_many :comments # << if polymorphic add `, as: :commentable`
accepts_nested_attributes_for :comments
end
# expenses_controller.rb
def new
#expense = Expense.new
#expense.comments.build
end
# expenses/new.html.haml
= form_for(#expense) do |f|
- # expense inputs...
= f.nested_fields_for(:comments) do |comment|
= comment.text_area(:body)
= f.submit
There are many options, so check the docs for more details (they're quite good).