rails select tag with multiple values pre selected

I am trying to have a multiple select box. select box will contain all the stores in the DB but the ones that the user belongs to will be selected.

I'm half way there. I got a select box which has all the stores in the database. I'm unable to select the ones that the user belongs to.

I have the following:

<%= select_tag 'stores[]', options_fo开发者_开发技巧r_select(@stores.map {|s| [s.store_name, s.store_id]}, 
:selected => @user.stores.map {|j| [j.store_name, j.store_id]}), :multiple => true, :size => 
10 %>

I have a map with stores that a user belongs to. it is in:

@user.stores

after a fair amount of trial and error the following worked for me:

<%= select_tag 'stores[]', options_for_select(@stores.map { |s| [s.store_name, s.store_id] }, @user.stores.pluck(:id)), multiple: true, size: 10 %>

Another way of doing this would be to use the options_from_collection_for_select helper method. It will look something like this:

<%= select_tag 'stores[]', options_from_collection_for_select(@stores, :store_id, :store_name, [4,5,6]), multiple: true, size: '10%' %>