__solution.rb

    def create
      @company = Company.new(params[:company])
      begin
        if @company.save
          redirect_to_index("Company was successfully created")
        else
          render :action => "new"
        end
      rescue ActiveRecord::RecordInvalid
        render :action => "new"
      end
    end

_address.rhtml

<% fields_for_model(:parent_model => @company, :child_model => address) do |address_form| %>
  <div class="address clear" id="<%= address.object_id -%>">
    <% @address = address %>
    <%= error_messages_for :address %>
    <div class="column span-8 first">
      <p>
        <%= address_form.label :address1 -%>
        <%= address_form.text_field :address1 -%>
      </p>
      <p>
        <%= address_form.label :address2 -%>
        <%= address_form.text_field :address2 -%>
      </p>
    </div>
    <div class="column span-5">
      <p>
        <%= address_form.label :town -%>
        <%= address_form.text_field :town -%>
      </p>
      <p>
        <%= address_form.label :county -%>
        <%= address_form.text_field :county -%>
      </p>
    </div>
    <div class="column span-5">
      <p>
        <%= address_form.label :postcode -%>
        <%= address_form.text_field :postcode -%>
      </p>
      <p>
        <%= address_form.label :main, "Main Address" -%>
        <%= check_box_tag "company[#{item_prefix(address)}_address_attributes][#{address.id}][main]", "1", check_if_attribute_is_set(:model_to_check => address, :attribute => "main"), {:id => "company_#{item_prefix(address)}_address_attributes_#{address.id}_main" } %>
      </p>
    </div>
    <div class="column span-5 last">
      <label> </label>
      <p><%= link_to_function "<img src=\"/stylesheets/plugins/buttons/icons/delete.png\" alt=\"Delete icon\"/> Delete address", "destroy(this)", :class => "button negative" %></p>
    </div>
  </div>
<% end %>

address.rb

class Address < ActiveRecord::Base
  has_many :address_linkings
  
  validates_presence_of :address1
  validates_uniqueness_of :postcode, :if => Proc.new { |address| !address.postcode.blank? }  
end

address_linking.rb

class AddressLinking < ActiveRecord::Base
  belongs_to :address
  belongs_to :address_linkable, :polymorphic => true
end

company.rb

class Company < ActiveRecord::Base
  has_many :address_linkings, :as => :address_linkable
  has_many :addresses, :through => :address_linkings
  
  after_update :save_addresses
  
  validates_associated :addresses

  def new_address_attributes=(address_attributes)
    address_attributes.each do |attributes|
      addresses.build(attributes)
    end
  end
  
  def existing_address_attributes=(address_attributes)
    addresses.reject(&:new_record?).each do |address|
      attributes = address_attributes[address.id.to_s]
      if attributes
        address.attributes = attributes
        address.main = "0" if attributes[:main].blank?
      else
        addresses.delete(address)
      end
    end
  end
  
  def save_addresses
    addresses.each do |address|
      address.save(false) # false so that it doesn't go through validation (we want to do that before)
    end
  end
end

company_controller.rb

class Admin::CompanyController < ApplicationController
    def new
      @company = Company.new
      @company.addresses.build
    end
    
    def create
      @company = Company.new(params[:company])
      if @company.save
        redirect_to_index("Company was successfully created")
      else
        render :action => "new"
      end
    end
  end

error.txt

ActiveRecord::RecordInvalid in Admin/companyController#create

Validation failed: Postcode has already been taken

vendor/rails/activerecord/lib/active_record/validations.rb:1021:in `save_without_dirty!'
vendor/rails/activerecord/lib/active_record/dirty.rb:87:in `save_without_transactions!'
vendor/rails/activerecord/lib/active_record/transactions.rb:150:in `save!'
vendor/rails/activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb:66:in `transaction'
vendor/rails/activerecord/lib/active_record/transactions.rb:129:in `transaction'
vendor/rails/activerecord/lib/active_record/transactions.rb:138:in `transaction'
vendor/rails/activerecord/lib/active_record/transactions.rb:150:in `save!'
vendor/rails/activerecord/lib/active_record/transactions.rb:158:in `rollback_active_record_state!'
vendor/rails/activerecord/lib/active_record/transactions.rb:150:in `save!'
vendor/rails/activerecord/lib/active_record/associations/has_many_through_association.rb:53:in `insert_record'
vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:145:in `send'
vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:145:in `send'
vendor/rails/activerecord/lib/active_record/associations.rb:1391:in `after_create_or_update_associated_records_for_addresses'
vendor/rails/activerecord/lib/active_record/associations/association_collection.rb:359:in `method_missing_without_paginate'
vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:212:in `method_missing'
vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:212:in `each'
vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:212:in `send'
vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:212:in `method_missing'
vendor/rails/activerecord/lib/active_record/associations/association_collection.rb:359:in `method_missing_without_paginate'
/opt/local/lib/ruby/gems/1.8/gems/mislav-will_paginate-2.3.6/lib/will_paginate/finder.rb:167:in `method_missing'
vendor/rails/activerecord/lib/active_record/associations.rb:1391:in `after_create_or_update_associated_records_for_addresses'
vendor/rails/activesupport/lib/active_support/callbacks.rb:178:in `send'
vendor/rails/activesupport/lib/active_support/callbacks.rb:178:in `evaluate_method'
vendor/rails/activesupport/lib/active_support/callbacks.rb:166:in `call'
vendor/rails/activesupport/lib/active_support/callbacks.rb:93:in `run'
vendor/rails/activesupport/lib/active_support/callbacks.rb:92:in `each'
vendor/rails/activesupport/lib/active_support/callbacks.rb:92:in `send'
vendor/rails/activesupport/lib/active_support/callbacks.rb:92:in `run'
vendor/rails/activesupport/lib/active_support/callbacks.rb:277:in `run_callbacks'
vendor/rails/activerecord/lib/active_record/callbacks.rb:315:in `callback'
vendor/rails/activerecord/lib/active_record/callbacks.rb:238:in `create_without_timestamps'
vendor/rails/activerecord/lib/active_record/timestamp.rb:29:in `create'
vendor/rails/activerecord/lib/active_record/base.rb:2691:in `create_or_update_without_callbacks'
vendor/rails/activerecord/lib/active_record/callbacks.rb:222:in `create_or_update'
vendor/rails/activerecord/lib/active_record/base.rb:2375:in `save_without_validation'
vendor/rails/activerecord/lib/active_record/validations.rb:1009:in `save_without_dirty'
vendor/rails/activerecord/lib/active_record/dirty.rb:79:in `save_without_transactions'
vendor/rails/activerecord/lib/active_record/transactions.rb:179:in `send'
vendor/rails/activerecord/lib/active_record/transactions.rb:179:in `with_transaction_returning_status'
vendor/rails/activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb:66:in `transaction'
vendor/rails/activerecord/lib/active_record/transactions.rb:129:in `transaction'
vendor/rails/activerecord/lib/active_record/transactions.rb:138:in `transaction'
vendor/rails/activerecord/lib/active_record/transactions.rb:178:in `with_transaction_returning_status'
vendor/rails/activerecord/lib/active_record/transactions.rb:146:in `save'
vendor/rails/activerecord/lib/active_record/transactions.rb:158:in `rollback_active_record_state!'
vendor/rails/activerecord/lib/active_record/transactions.rb:146:in `save'
app/controllers/admin/company_controller.rb:26:in `create'


PARAMS

{"company"=>{"name"=>"New company",
 "new_address_attributes"=>[{"address1"=>"Address 1",
 "address2"=>"Address 2",
 "postcode"=>"A1 2BC",
 "county"=>"County",
 "town"=>"Town"},
 {"address1"=>"Another Address 1",
 "address2"=>"Another Address 2",
 "postcode"=>"A1 2BC",
 "county"=>"Another County",
 "town"=>"Another Town"}],
 "archived"=>"0",
 "fax"=>"Fax",
 "telephone"=>"Telephone"},
 "commit"=>"Create Client"}

helpers.rb

  def item_prefix(item)
    item.new_record? ? 'new' : 'existing'
  end

  def fields_for_model(options = {}, &block)
    parent_model = options[:parent_model]
    child_model = options[:child_model]

    fields_for("#{parent_model.class.name.downcase}[#{item_prefix(child_model)}_#{child_model.class.name.downcase}_attributes][]", child_model, &block)
  end