associations.rb

# Good:

class Post

  belongs_to :author, :class_name => User.to_s   # ActiveRecord
  belongs_to :author, User.to_s                  # DataMapper

  has_many :replies, :class_name => Comment.to_s # ActiveRecord
  has n,   :replies, Comment.to_s                # DataMapper

end

# Bad:

class Post

  belongs_to :author, :class_name => 'User' # ActiveRecord
  belongs_to :author, 'User'                # DataMapper

  has_many :replies, :class_name => 'Comment' # ActiveRecord
  has n,   :replies, 'Comment'                # DataMapper

end