relations - data not saved


I've created 2 databases with a relation 1-N, for example 1 table
"tasks" and 1 table "categories", I've done the relation 1-N (in task.rb
belongs_to :category and in category.rb has_many :tasks) and I've
created a form to have the possibility to choose a category in a
dropdown list when I create a task. The creation of a new task is well
done but without the field "category_id" stays empty?

Does someone help me with this problem? I really don' understand
because another relation with the same principle works fine...

Thanks you