Ruby on Rails -- multiple selection in f.select

Question

After your { :prompt => "Please select"} add another hash with html options e.g.

<%= f.select(:TYPE, [['Type A', 'Type A'],
                                  ['Type B', 'Type B'],
                                  ['Type C', 'Type C'],
                                  ['Type D', 'Type D'],
                                  ['Type E', 'Type E']
                                 ],{ :prompt => "Please select"},
                                   { :multiple => true, :size => 5 }
                                 ) %>

Once you've done this you might want to move your :prompt option (keep the empty {} though so that html attributes don't get treated as Rails options.)

Also you'll need to ensure your controller code is correctly accepting and handling multiple values.

In case of collection, try

    <%= f.select(:TYPE, Categories.collect {|p| [ p.name, p.id ] }, 
                                           { :prompt => "Please select"}, 
                                           { :multiple => true, :size => 5 }) %>

I have a fully working example (including preselection when editing the object), when:

• Object is the considered object
• similar_ids is the key to relations, and is a string

In the form:

form_for(@object) do |f|
  = f.select :similar_ids, options_from_collection_for_select(Object.all, :id, :name, {:selected => @object.similar_ids.split(';')}), {}, {:multiple => true, :size => 4, :name => 'object[similar_ids][]'}

And in the Object.rb model:

class Object < ActiveRecord::Base
  before_save :handle_similars

  def handle_similars
    self.similar_ids = self.similar_ids.select(&:present?).join(';') 
    # .select(&:present?) is necessary to avoid empty objects to be stored
  end

  def similars
    self.class.find(self.similar_ids.split(';'))
  end

end

These posts helped me out:

• Select tag with multiple values pre-selected - Values inserted manually in database
• Ruby on Rails: Submitting an array in a form

Hope it helps

HTML

<%= form.select(:product_ids, Product.all.collect {|p| [ p.name, p.id ] }, 
                                                   { :prompt => "Please select"}, 
                                                   { :multiple => true, :size => 5  }) %>

Controller

@category = Category.new(category_params) 

def category_params
    params.require(:category).permit(:name, product_ids: [])
end