Maintaining Many to Many relation with checkboxes (e.g. checkboxes for each category a post can be in) - ruby-on-rails-3

Question

We have a table with Categories containing unique categories. And our posts should link to any of these categories. I tried to add checkboxes for selecting the categories in the following manner:
Add the checkboxes containing the categories in a field called "post[categoriesss][]", containing as value the :id of the category.
Update the new and update method to take these these ids and collect the associated Categories with them, and then add each category to the categories attribute through the << operator as stated in the documentation.
We added the code to new and update to be sure not to have to do this in multiple controllers.
However we get the following error:
undefined property or relationship '<<' on Category
What are we doing wrong here? Is there another method to achieve our goal? Our code is below.
class Post
include DataMapper::Resource
property :id, Serial
has n, :categories, :through => Resource
def self.new(hash={})
if hash.key? :categoriesss
hash[:categoriesss].each do |category|
categories << Category.get(category)
end
hash.delete! :categoriesss
end
super hash
end
end
class Category
include DataMapper::Resource
property :id, Serial
property :name, String, :required => true
has n, :rekenservices, :through => Resource
end
<% Category.all.each do |category| %>
<label>
<input type="checkbox" name="post[categoriesss][]" value="<%= category.id %>" />
<%= category.name %>
</label>
<% end %>
UPDATE:
I now changed the names for the checkbox back to categories and updated the new and update methods to this:
def self.new(hash={})
if hash.key? :categories
hash[:categories].map! {|id| Category.get id}
end
super hash
end
def update(hash={})
if hash.key? :categories
hash[:categories].map! {|id| Category.get id}
end
super hash
end
Adding categories the first time works. When updating checking an extra category works great as well. Unchecking a category however yields an error message:
columns category_id, post_id are not unique