branch.rb

class Branch < ActiveRecord::Base
  belongs_to :tree
  has_many :leaves
  
  validates_presence_of :name, :message => "A branch without a name? WTF?"
  validates_presence_of :tree_id, :message => "was not there dude."
end

log

Parameters: {"commit"=>"Save changes", "authenticity_token"=>"YnGd4rKBCki4+r6sNcwfDlgR5Ur+O7emsQZkGvJu/zw=", "tree"=>{"name"=>"Hank the Tree", "branches_attributes"=>{"0"=>{"name"=>"Bob the Branch"}}}}

new.html.erb

New Tree
<br />
<% form_for setup_tree(@tree) do |tree_form| %>
  <%= tree_form.error_messages %>
  <%= tree_form.label :name, "Tree Name:" %>
  <%= tree_form.text_field :name %>
  <br /><br />
  Add a branch:
    <br />
  <% tree_form.fields_for :branches do |branch_form| %>
    <%= branch_form.label :name, "Branch Name:" %>
    <%= branch_form.text_field :name %>
  <% end %>
  <br />
  <%= tree_form.submit %>
<% end %>

theresult.html

<form action="/trees" class="new_tree" id="new_tree" method="post"><div style="margin:0;padding:0;display:inline"><input name="authenticity_token" type="hidden" value="YnGd4rKBCki4+r6sNcwfDlgR5Ur+O7emsQZkGvJu/zw=" /></div> 
  
  <label for="tree_name">Tree Name:</label> 
  <input id="tree_name" name="tree[name]" size="30" type="text" /> 
  <br /><br /> 
  Add a branch:
    <br /> 
  
    <label for="tree_branches_attributes_0_name">Branch Name:</label> 
    <input id="tree_branches_attributes_0_name" name="tree[branches_attributes][0][name]" size="30" type="text" /> 
  
  <br /> 
  <input id="tree_submit" name="commit" type="submit" value="Save changes" /> 
</form> 

tree.rb

class Tree < ActiveRecord::Base
  has_many :branches
  
  validates_presence_of :name
  accepts_nested_attributes_for :branches, :allow_destroy => true, :reject_if => :all_blank
  
end