has many through - Rails How to an association and data at once -

i m newbie on rails have user , attachment model , has_many through relationships. want add data,it can more one, third model intermediate form when create attachment data. can save attachment data don't know how save data third model.here files

attachments/new.html.erb

<%= form_for(@attachment) |f| %> <div class="field"> <%= f.label :filename, "file name" %><br> <%= f.text_field :filename %> </div><br /> <div class="field"> <%= f.label :file, "file" %><br> <%= f.file_field :file %> </div> <br />   <div class="actions"> <%= f.submit "dosya yükle" %> </div> <% end %> 

attachments_controller.rb

  def new   @attachment = attachment.new   end   def create   @attachment = attachment.new(file_params)   if @attachment.save      redirect_to some_path   else      render action: 'new'    end   end 

user.rb

  has_many :interforms   has_many :attachments, through: :interforms 

attachment.rb

  has_many :interforms   has_many :users, through: :interforms 

interform.rb

  belongs_to :user   belongs_to :attachment 

my question how add data interform when add attachment

edit : didn't have routes interforms right now