Rails has_many :through and collection_select with multiple - ruby-on-rails

Question

I have the following problem using a has_many :through many-to-many relation in a multi-select via collection_select :multiple => true. I have Suppliers who deliver many Ingredients which can be delivered by many Suppliers. Have a look:
The Ingredient model:
class Ingredient < ActiveRecord::Base
has_many :ingredient_suppliers
accepts_nested_attributes_for :ingredient_suppliers, :allow_destroy => true
has_many :suppliers, :through => :ingredient_suppliers
end
The Supplier model:
class Supplier < ActiveRecord::Base
has_many :ingredient_suppliers
has_many :ingredients, :through => :ingredient_suppliers
end
The relationship Entity:
class IngredientSupplier < ActiveRecord::Base
belongs_to :ingredient
belongs_to :supplier
end
And this is the form. Note that I could not get it to work without specifying the :name:
<%= form_for(#ingredient) do |f| %>
<%= f.fields_for :suppliers do |supplier_fields| %>
<%= supplier_fields.collection_select (:supplier_ids,
Supplier.all(:order=>"name ASC"),
:id, :name,
{:selected => #ingredient.supplier_ids,
:include_blank => true},
{:multiple => true,
:name => 'ingredient[supplier_ids]'}) %>
<% end %>
<% end %>
If I remove the :name, then I get this error message:
Supplier(#-617951108) expected, got Array(#-608411888)
Request
Parameters:
{"commit"=>"Anlegen",
"authenticity_token"=>"MuEYtngwThharmM1KaAbH8JD3bScXiDwj0ALMytxl7U=",
"_method"=>"put",
"utf8"=>"✓",
"id"=>"1",
"ingredient"=>{"name"=>"Ingredient 1",
"nr"=>"00100",
"unit"=>"kg",
"mol_per_unit"=>"2000,
00000",
"description"=>"",
"suppliers"=>{"supplier_ids"=>["1",
"2"]}}}
The problem now is, that the PUT parameters only contain one supplier_id instead of an array of supplier_ids:
"ingredient"=>{"name"=>"Rohstoff 3", "nr"=>"00300", "unit"=>"Stk.", "mol_per_unit"=>"0,00000", "description"=>"", "supplier_ids"=>"2"}

Answer 1

I've got the problem solved. In this case, using fields_for was the error. The solution is using a collection_select, like this:
<%= collection_select(:ingredient, :supplier_ids,
Supplier.all(:order=>"name ASC"),
:id, :name, {:selected => #ingredient.supplier_ids, :include_blank => true}, {:multiple => true}) %>