"Association expected, got String" with Many-to-Many association

Currently I'm creating something like a "sports-managing-and-results-gathering-app" in Rails 3. In this App I need to create several exercises which itself can have multiple 'resulttypes' (heartrate, distance in km, repititions, ...). And it should be possible to arrange the resulttypes in my prefered order. So, this is a classical many-to-many relationship.

I came up with the following migrations:

    class CreateExercises < ActiveRecord::Migration
      def self.up
        create_table :exercises do |t|
          t.integer :user_id
          t.string :name
          t.text :beschreibung
          t.integer :resulttype_id
          t.boolean :active, :default => true
          t.timestamps
        end
      end
      def self.down
        drop_table :exercises
      end
    end

    class CreateResulttypes < ActiveRecord::Migration
      def self.up
        create_table :resulttypes do |t|
          t.string :name
          t.string :einheit
          t.text :beschreibung

          t.timestamps
        end
      end

      def self.down
        drop_table :resulttypes
      end
    end

    class CreateExercisesResulttypesJoin < ActiveRecord::Migration
      def self.up
        create_table :exercises_resulttypes, :id => false do |t|
          t.integer "exercise_id"
          t.integer "resulttype_id"
        end
        add_index :exercises_resulttypes, ["exercise_id", "resulttype_id"]
      end

      def self.down
        drop_table :exercises_resulttypes
      end
    end

which works nicely with this formtastic code:

    <%= semantic_form_for(@exercise) do |f| %>        
      <%= f.inputs do %>
        <%= f.input :name %>
        <%= f.input :beschreibung %>
        <%= f.input :resulttype %>
        <%= f.input :active %>
      <% end %>
      <%= f.buttons %>
    <% end %>

The problem is here, that with this code I can not sort the resulttypes and each exercise can have each resulttype once only. So I changed the formtastic :resulttype form code to this (except the jquery code):

    <div id="conn_ctrl" class="float_left center">
            <div class="float_left center">
                <%= f.select :resulttypes, @resall.collect{|d| [d.name,d.id]}.sort, {}, { :multiple => true, :class => "conn_select" } %><br>
            </div>
            <div class="float_left center">
                <%= f.select :resulttypes, '', {}, { :multiple => true, :id => "exercise_resulttypes_save", :class => "conn_select" } %><br>
            </div>
            <div style="clear: both"></div>
            <%= tag("input", { :type => "button", :id => "remove", :class => "dualbuttons", :name => "", :value => "<<" }, false) %>
            <%= tag("input", { :type => "button", :id => "add", :class => "dualbuttons", :name => "", :value => 开发者_如何学Python">>" }, false) %>
            <%= tag("input", { :type => "button", :id => "up", :class => "dualbuttons", :name => "", :value => "Up" }, false) %>
            <%= tag("input", { :type => "button", :id => "down", :class => "dualbuttons", :name => "", :value => "Down" }, false) %><br>
        </div>
        <div style="clear: both"></div>

Now I have a "Dual Listbox" and I can add exercises to this specific workout multiple times and order it how I like it. BUT, when I submit the form, I get this error and I have no idea what that means:

Resulttype(#-614051528) expected, got String(#-608366078)

The submitted parameters are:

    {"commit"=>"Create Exercise",
     "authenticity_token"=>"v1l9zfxdxIJbdjZx6SsZ5tGuKMrlioBg+C9orSmVarA=",
     "utf8"=>"✓",
     "exercise"=>{"name"=>"dddd",
     "beschreibung"=>"ddddd",
     "resulttypes"=>["5",
     "3",
     "1"],
     "active"=>"1"}}

This currently drives me crazy a little bit. I hope you know why this happens...

Rails expect an array of objects for the method association=, use association_ids= instead:

<%= f.select :resulttypes_ids, .... %>