Error on update for a 3 columns join table

I have a problem while updating a 3 keys join table. I've been on this
for quite a while now and have no idea how to fix it. In short, to be
able to get the new project working, I had to set these two habtm
declaration (in /models/project.rb):

    has_and_belongs_to_many :users,
        :join_table => "projects_roles_users",
        :foreign_key => 'property_id',
        :association_foreign_key => 'user_id',
        :insert_sql => 'INSERT INTO
                    projects_roles_users(project_id,role_id,user_id)
                    VALUES(#{id}, #{role_ids}, #{user_ids})',
        :delete_sql => 'DELETE FROM projects_roles_users
                        WHERE projects_roles_users.property_id =
#{id}'

      has_and_belongs_to_many :roles,
         :join_table => "projects_roles_users",
         :foreign_key => 'project_id',
         :association_foreign_key => 'role_id',
         :insert_sql => 'INSERT INTO
                    projects_roles_users(project_id,role_id,user_id)
                    VALUES(#{id}, #{role_ids}, #{user_ids})',
         :delete_sql => 'DELETE FROM projects_roles_users
                         WHERE projects_roles_users.project_id =
#{id}'

So the new project works just fine (although I get duplicates in
project_roles_users). However, when I want to update the role a user
has in a project, I get this error:

    Mysql::Error: You have an error in your SQL syntax; check the
manual that corresponds to your MySQL server version for the right
syntax to use near ' 1414)' at line 3: INSERT INTO
                    projects_roles_users(project_id,role_id,user_id)
                    VALUES(29, , )

Here's the controller:

      def update
        @project = Project.find(params[:id])

        respond_to do |format|
          if @project.update_attributes(params[:project])
            flash[:notice] = 'Project was successfully updated.'
            format.html { redirect_to user_path(current_user.id) }
            format.xml { head :ok }
          else
            format.html { render :action => "edit" }
            format.xml { render :xml => @project.errors, :status
=> :unprocessable_entity }
          end
        end
      end

And here's the view:

  <% form_for(@project,:html => { :multipart => true }) do |f| %>
    <%= f.error_messages %>

    <p>
    <%= f.label :name %><br />
    <%= f.text_field :name %>
    </p>

    <p>
    <% f.label :role, 'Role' %><br />
    <%= f.collection_select :role_ids, Role.find(:all, :order =>
'position'), :id, :role, {}, :multiple => false %>
    </p>

    <%= f.hidden_field :user_ids, :value => current_user.id %>

    <p>
    <%= f.submit 'Update' %>
    </p>
  <% end %>

  <%= link_to 'Show', @project %> |
  <%= link_to 'Back', projects_path %>
  <%= javascript_tag "$('address').focus()" %>

Finally, here's the log (maybe it'll help you figure it out...):

    Processing ProjectsController#update (for 127.0.0.1 at 2011-02-23
08:43:29) [PUT]
      Parameters: {"project"=>{"price"=>"12$", "city"=>"City",
"address"=>"452 Street st.", "user_ids"=>"14", "role_ids"=>["2"],
"postalcode"=>"12346", "description"=>"Description",
"state"=>"State"}, "commit"=>"Update",
"authenticity_token"=>"OKQaW4LPb3Xye6Kbh/W2EgxPWFe4aj26etIEVlCTKTg=",
"id"=>"29"}
      User Columns (0.9ms) SHOW FIELDS FROM `users`
      User Load (0.1ms) SELECT * FROM `users` WHERE (`users`.`id` =
14) LIMIT 1
      Page Load (0.1ms) SELECT * FROM `pages` WHERE (parent_id IS
NULL) ORDER BY position
      Project Columns (0.9ms) SHOW FIELDS FROM `projects`
      Project Load (0.1ms) SELECT * FROM `projects` WHERE
(`properties`.`id` = 29)
      SQL (0.1ms) BEGIN
      Role Columns (0.8ms) SHOW FIELDS FROM `roles`
      Role Load (0.1ms) SELECT * FROM `roles` WHERE (`roles`.`id` =
2)
      projects_roles_users Columns (0.5ms) SHOW FIELDS FROM
`projects_roles_users`
      Role Load (0.3ms) SELECT * FROM `roles` INNER JOIN
`projects_roles_users` ON `roles`.id = `projects_roles_users`.role_id
WHERE (`projects_roles_users`.project_id = 29 )
      User Load (0.3ms) SELECT `users`.id FROM `users` INNER JOIN
`projects_roles_users` ON `users`.id = `projects_roles_users`.user_id
WHERE (`projects_roles_users`.project_id = 29 )
      SQL (0.1ms) DELETE FROM projects_roles_users
     WHERE projects_roles_users.project_id = 29
      User Load (0.2ms) SELECT `users`.id FROM `users` INNER JOIN
`projects_roles_users` ON `users`.id =
`properties_roles_users`.user_id WHERE
(`projects_roles_users`.project_id = 29 )
      SQL (0.1ms) DELETE FROM projects_roles_users
     WHERE projects_roles_users.project_id = 29
      SQL (0.5ms) describe `projects_roles_users`
      User Load (0.3ms) SELECT `users`.id FROM `users` INNER JOIN
`projects_roles_users` ON `users`.id = `projects_roles_users`.user_id
WHERE (`projects_roles_users`.project_id = 29 )
      SQL (0.0ms) Mysql::Error: You have an error in your SQL
syntax; check the manual that corresponds to your MySQL server version
for the right syntax to use near ' 1414)' at line 3: INSERT INTO
     projects_roles_users(project_id,role_id,user_id)
     VALUES(29, , )
      SQL (0.1ms) ROLLBACK

    ActiveRecord::StatementInvalid (Mysql::Error: You have an error in
your SQL syntax; check the manual that corresponds to your MySQL
server version for the right syntax to use near ' 1414)' at line 3:
INSERT INTO
                    projects_roles_users(project_id,role_id,user_id)
                    VALUES(29, , )):
      app/controllers/properties_controller.rb:71:in `update'
      app/controllers/properties_controller.rb:70:in `update'

    Rendered rescues/_trace (136.5ms)
    Rendered rescues/_request_and_response (0.6ms)
    Rendering rescues/layout (internal_server_error)