MultiSelect in rails with Collection_select

Model1
    attr_accessible :model2s, :model2_ids
    has_many :model2s

Model2
   belongs_to :Model1



1. Have a model with attr_accessible :model2_ids

2. f.collection_select :model2_ids , Model2.all, :id, :name, {},{:multiple=>true,:size=>5}

The trick is "model2_ids"

Thats it !