attaching multiple files at a time on a has_many through

Hi     My models are

class MessageThread < ActiveRecord::Base   has_many :messages   accepts_nested_attributes_for :messages end

class Message < ActiveRecord::Base   has_many :message_participants   belongs_to :message_thread   has_many :document_messages   has_many :documents, :through => :document_messages   accepts_nested_attributes_for :documents end

class Document < ActiveRecord::Base   has_attached_file :attachment,                     :storage => :database   default_scope select_without_file_columns_for(:attachment)   has_many :document_messages   has_many :messages, :through => :document_messages end

class DocumentMessage < ActiveRecord::Base   belongs_to :message   belongs_to :document end

    I have to make an email compose interface. So my messages controller new and create(sample) action like

  def new     @message_thread = MessageThread.new     @message = @message_thread.messages.build   end

  def create      MessageThread.create(params[:message_thread])   end

    And the new form I tried like

  <%form_for @message_thread, :url => { :action => "create" },:html => {:multipart => true} do |f| %>     <p><%= text_area_tag 'mail_to',nil,:cols => 10, :rows => 3,:style => "height:30px;"%></p>     <p>       <% f.fields_for :messages_attributes do |message| %>         <%= message.label :subject,'Subject' %>         <%= message.text_field :subject,:maxlength => 255 %>       <%#=end %>     </p>     <div class="attachments">       <p><strong>Attachments:</strong></p>     <%message.fields_for :documents do |a| %>       <%=a.file_field "attachment"%>       <%=a.file_field "attachment"%>     <%end%>     </div>     <div class="two-buttons">       <button type="submit" class="form-button2 fixed"><span>Send</span></button>     </div>     <p>Or <a href="#">Cancel</a></p>   <%end%>

   And I am getting error

NoMethodError in MessagesController#create

undefined method `with_indifferent_access' for "":String

Please help me to correct this

Thanks in advance Tom