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