Select with :multiple => true not saving values

Question

Im trying to save multiple selected values form a multiple select field generated by the rails helper select.

  <div class="form-group">
    <%= f.label :available_type, "Available in category" %><br>
    <%= f.select :available_type, options_for_select(Setting.subscription_type, @terminal.available_type), { }, { class: "form-control", :multiple => true, :size => 5    } %>
  </div>

This renders like this (the selected value is from a previous attempt without the ":multiple => true" attribute that works perfectly):

<select class="form-control" id="terminal_available_type" multiple="multiple" name="terminal[available_type][]" size="5">
<option value="Postpaid">Postpaid</option>
<option value="MBB">MBB</option>
<option selected="selected" value="Prepaid">Prepaid</option>
</select>

Any help is appreciated. :)

Edit: I have tried putting serialize :available_type in my Terminal model, dident change anything. :-/

Edit 2: I noticed that the multiple selcted field does not mark options as selected when i mark them. if i add Selected attributes manualy i get these parametres:

{"utf8"=>"✓", "_method"=>"patch", "authenticity_token"=>"RrwWlKk8XlGeC+dTu/w6oSM68e9LcbUFJWTI+eRS9mI=", "terminal"=>{"inndate"=>"2015-01-13", "outdate"=>"", "brand_id"=>"2", "name"=>"iPhone 5c", "available_type"=>["", "MBB", "Prepaid"], "product_number"=>"3r2342", "ean_code"=>"", "navision_nb"=>"324234", "cost_price_map"=>"3200.0", "manual_price"=>"", "sales_info"=>"Just sell!"}, "commit"=>"Submit", "action"=>"update", "controller"=>"terminals", "id"=>"2"}

The available_type field has the value "available_type"=>["", "MBB", "Prepaid"]

Im using rails 4.0.2 and here are my strong parametres:

# Never trust parameters from the scary internet, only allow the white list through.
def terminal_params
  params.require(:terminal).permit(:inndate, :outdate, :brand_id, :name, :product_number, :navision_nb, :cost_price_map, :manual_price, :sales_info, :available_type)
end

Answer 1

Finaly found the answer!

This problem was a combo of PG and Rails 4.

First i needed to convert the column from a string to a text column marked as array like so:

class ChangeAvailableTypeOnTerminals < ActiveRecord::Migration
  def up
    change_column :terminals, :available_type, :text, array: true, default: []
  end

def down
    change_column :terminals, :available_type, :string
  end
end

Then i needed to treat the strong parametre as an array in the terminals controller like so:

# Never trust parameters from the scary internet, only allow the white list through.
def terminal_params
  params.require(:terminal).permit(:inndate, :outdate, {:available_type => []}, :brand_id, :name, :product_number, :navision_nb, :cost_price_map, :manual_price, :sales_info)
end

To be more spesific:

Change from :available_type to {:available_type => []}

This solved the issue for me. :)