Drop-Down-Menu for Many-to-Many relation in rails using nested attributes

Question

I have three tables via many-to-many-association: Supermarket, Product and Supply. Each Supermarket can hold many products and each product can be sold in many supermarkets.

Answer 1

In you products form you need to add this line...

<%= collection_select(:product, :supermarket_ids, SuperMarket.all, :id, :name, {}, { :multiple => true } )%>

You also shouldn't need to use an accepts_nested_attributes for this, the many to many association you already have set up should take care of the rest.