Datamapper DataMapper

class Article
  include DataMapper::Resource
  property :id, Serial
  property :title, String, :nullable => false
  has n, :categories, :through => Resource
end

class Category
  include DataMapper::Resource
  property :id, Serial
  property :name, String, :nullable => false
  has n, :articles, :through => Resource
end
 
DataMapper.auto_migrate!
 
a1 = Article.create(:title => "foo1")
a2 = Article.create(:title => "foo2")

c1 = Category.create(:name => "cat1")
c2 = Category.create(:name => "cat2")
c3 = Category.create(:name => "cat3")

a1.categories = [Category.get(1), Category.get(2)]
a1.save

a1 = Article.get(1)
a1.categories = [Category.get(2), Category.get(3)]
a1.save

The last save fails:

INSERT INTO "articles_categories" ("article_id", "category_id") VALUES (1, 2)

error ~ columns article_id, category_id are not unique

I.e., it tries to insert the association with category 2 again, even though it already exists.

This is because the many-to-many proxy adds /new/ through_resources to the near_assocation.