class Category
  include DataMapper::Resource

  property :id,          Serial
  property :name,        String

  has n, :categories, :through => Resource, :constraint => :destroy!

end

class Category
  include DataMapper::Resource

  property :id,          Serial
  property :name,        String

  has n, :comments, :through => Resource, :constraint => :destroy!

end

# On MySQL
rake db:automigrate # First run works fine
rake db:automigrate # Run it again and it blows up with "Cannot delete or update a parent row: a foreign key constraint fails (code: 1217, sql state: 23000, query: DROP TABLE IF EXISTS "categories")"