a sa-weet-ass library for structuring rails applications using the 'data access object' design pattern. dao consists of two main data access objects, api objects and conducer objects. conducers combine the presenter pattern with the conductor pattern.


class Api < Dao::Api
  call('/posts') do
    get do
      data[:posts] = Post.all.map{|post| post.attributes}

    post do
      post = Post.new(params[:post])

      if post.save
        data[:post] = post.attributes
        status 420


  • TODO

wikipedia has this to say about dao in general

In computer software, a data access object (DAO) is an object that provides an abstract interface to some type of database or other persistence mechanism. By mapping application calls to the persistence layer, DAOs provide some specific data operations without exposing details of the database. This isolation supports the single responsibility principle. It separates what data accesses the application needs, in terms of domain-specific objects and data types (the public interface of the DAO), from how these needs can be satisfied with a specific DBMS, database schema, etc. (the implementation of the DAO).

-- http://en.wikipedia.org/wiki/Data_access_object

and this to say about the single responsibility principle

In object-oriented programming, the single responsibility principle states that every class should have a single responsibility, and that responsibility should be entirely encapsulated by the class. All its services should be narrowly aligned with that responsibility.

Responsibility [is defined] as a reason to change, and [single responsibility means] that a class or module should have one, and only one, reason to change. As an example, consider a module that compiles and prints a report. Such a module can be changed for two reasons. First, the content of the report can change. Second, the format of the report can change. These two things change for very different causes; one substantive, and one cosmetic. The single responsibility principle says that these two aspects of the problem are really two separate responsibilities, and should therefore be in separate classes or modules. It would be a bad design to couple two things that change for different reasons at different times.

-- http://en.wikipedia.org/wiki/Single_responsibility_principle

even though rails is the sweet, its ActiveRecord class violates (or, at least, encourages a programmer to violate) the single responsibility principle

this leads to obvious problems

Jim Weirich, at the end of his SOLID Ruby Talk at the 2009 Ruby Conference, asks the audience: "ActiveRecord objects implement a domain concept and a persistence concept. Does this violate the SRP (Single Responsibility Principle)?" The audience agrees that it does violate the SRP. Jim asks if this bothers them. Many members of the audience say yes. Why? It makes testing harder. It makes the persistence object a lot heavier.

-- http://programmers.stackexchange.com/questions/119352/does-the-activerecord-pattern-follow-encourage-the-solid-design-principles#comment293734_119352

and subtle yet sweeping consequences (as described by uncle bob)

The problem I have with ActiveRecord is that it creates confusion about ... two very different styles of programming. A database table is a data structure. It has exposed data and no behavior. But an ActiveRecord appears to be an object. It has “hidden” data, and exposed behavior. I put the word “hidden” in quotes because the data is, in fact, not hidden. Almost all ActiveRecord derivatives export the database columns through accessors and mutators. Indeed, the Active Record is meant to be used like a data structure.

On the other hand, many people put business rule methods in their ActiveRecord classes; which makes them appear to be objects. This leads to a dilemma. On which side of the line does the Active Record really fall? Is it an object? Or is it a data structure?

This dilemma is the basis for the oft-cited impedance mismatch between relational databases and object oriented languages. Tables are data structures, not classes. Objects are encapsulated behavior, not database rows.


The problem is that Active Records are data structures. Putting business rule methods in them doesn’t turn them into true objects. In the end, the algorithms that employ ActiveRecords are vulnerable to changes in schema, and changes in type. They are not immune to changes in type, the way algorithms that use objects are.


So applications built around ActiveRecord are applications built around data structures. And applications that are built around data structures are procedural—they are not object oriented. The opportunity we miss when we structure our applications around ActiveRecord is the opportunity to use object oriented design.

-- https://sites.google.com/site/unclebobconsultingllc/active-record-vs-objects

and a clear solution (again, uncle bob)

I am not recommending against the use of ActiveRecord. I think the pattern is very useful. What I am advocating is a separation between the application and ActiveRecord.

ActiveRecord belongs in the layer that separates the database from the application. It makes a very convenient halfway-house between the hard data structures of database tables, and the behavior exposing objects in the application.

Applications should be designed and structured around objects, not data structures. Those objects should expose business behaviors, and hide any vestige of the database.

-- https://sites.google.com/site/unclebobconsultingllc/active-record-vs-objects

welcome to the dao



applications that are written on dao look like this in ruby

  result = api.call('/posts/new', params)

and like this in javascript

  result = api.call('/posts/new', params)

in command-line applications they look like this

  result = api.call('/posts/new', params)

and in tests this syntax is used

  result = api.call('/posts/new', params)

when a developer wants to understand the interface of a dao application she does this

  vi app/api.rb

when a developer of a dao application wants to play with a dao application interactively she does

  (rails console)

  > api = Api.new result = api.call('/posts/new', params)

when a remote client wants to understand the api of a dao application she does

  curl --silent http://dao.app.com/api | less

this kind of brutally consistent interface is made possible by structuring access to data around the finest data structure of all time - the hash. in the case of dao the hash is a well structured and slightly clever hash, but a simple hash interface is the basis of every bit of goodness dao has to offer.

in dao, application developers do not bring models into controllers and, especially not into views. instead, a unified interface to application logic and data is used everywhere: in tests, in controllers, from the command-line, and also from javascript.

this seperation of concerns brings with it many, many desirable qualities:

  • total seperation of concerns between the front and back end of a web application. when developers are using dao changes to the data model have zero effect on controllers and views.

  • issues related to having models in controllers and views such as difficulty reasoning about caching and n+1 queries in views killing the db simply disappear.

  • bad programming practices like using quasi-global variables (current_user) or decorating models with view specific attributes (password_verification) are no longer needed.

  • developers are able to reason over the abilities of an application by reading only a few source files.

  • databases can be swapped, mixed, or alternate storage/caching mechanisms added at any time without affecting the application's controllers or views.

  • transition from form based views to semi-ajax ones to fully-ajax ones is direct.

  • forms and interfaces that involve dozens of models are as easy to deal with as simple ones.

  • code can be optimized at the interface.



gem 'dao', :path => File.expand_path('..') ### Gemfile
rails generate dao api
vim -o app/api.rb app/controllers/api_controller.rb
curl --silent
curl --silent



  • dao depends has tied itself to rails, for better or worse...
  • drop custom form encoding. just use a rack-like approach.
  • dao form parameter encoding has changed slightly to 'dao[/api/path][x,y,z]=42'
  • dao form paramters are now preparsed in a before filter