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https://github.com/steveklabnik/activerepository

A brain dump of a new persistence strategy for Rails.
https://github.com/steveklabnik/activerepository

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A brain dump of a new persistence strategy for Rails.

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# MOTIVATION:  As rails apps are growing, people are noticing the drawbacks

# of the ActiveRecord pattern.  Several apps I have seen, and several

# developers I have spoken to are looking towards other patterns for object

# persistence.  The major drawback with ActiveRecord is that the notion

# of the domain object is conflated with what it means to store/retrieve

# it in any given format (like sql, json, key/value, etc).

#

# This is an attempt to codify the Repository pattern in a way that would

# feel comfortable to beginner and seasoned Ruby developers alike.

#

# In the spirit of "make it look the way you want it to work, then make it

# work like that", here is a vision of the Repository pattern ala Ruby. I

# don't want to fault ActiveRecord for being an implementation of the

# ActiveRecord pattern; At RailsConf 2006, in his keynote, Martin Fowler

# said that ActiveRecord was "the most faithful implementation of the

# Activerecord pattern [he] had ever seen", and he was the one that

# documented that pattern in the first place.  So lets respect AR for what

# it is, and look at the repository pattern, done with Rails Flair.



# Person is a plain old Ruby object. It knows nothing about its persistence,

# but as a domain model, it knows something about its own attributes,

# relationships to other domain objects, as well as understands what it

# means to be 'valid'. Of course, it also has behavior.



class Person

  include ActiveModel::Relationships

  include ActiveModel::Validations

  

  has_many :hobbies

  belongs_to :fraternity

  

  attr_accessor :name

  attr_accessor :password

  attr_accessor :sex

  attr_accessor :born_on

  

  validates_presence_of :name, :password

  

  def favorite_drink

    if born_on < 7.years.ago

      "apple juice"

    elsif born_on < 18.years.ago

      "coke or pepsi"

    elsif < 21.years.ago

      "mountain dew"

    else

      "beer"

    end

  end

end



# conventions like _on and _at are still used.



# The person has no clue it is 'Persistent Capable'.  All of that is handled

# in the Repository.  Notice we could call the Repo anything we want.



class PersonStore < ActiveRepository::Base

  repository_for :person

  

  # other things that control the persistence can go here

  table :users

  encrypts_attribute :password

  map_attribute_to_column :sex, :gender

end



# I'm using inheritance here to enforce an opinion.  Of course this could

# be a mixin, but my current thoughts are that this Repository should only

# ever be a repository - after all, we are trying to get away from

# ActiveRecord's notion of "I'm the model and my own data store!".

# Inheritance would be an appropriate way to signal this *is a* repository.

# My fear is as a mixin, someone would think they are being clever by mixing

# the repository directly into the domain model, essentially recreating

# ActiveRecord and making this effort all for nothing.



# I would really like to have the ability to 'new' model objects as normal:



p = Person.new



# but it might be necessary to create them through the store, like:



p = PersonStore.create #, or

p = PersonStore.build



# saving is no longer done on the object, but through the repository



PersonStore.save(p)



# the save would be smart enough to call is_valid? if Validations were present.



# all the usual finder suspects are on the repository

p = PersonStore.find(5)

p = PersonStore.first

p = PersonStore.all

p = PersonStore.find_by_name("Chris")



# we could also create things like scopes, etc.



# Since Person is nothing special, you could easily swap in a different

# persistance Repository:



Class PersonStore < RedisRepository::Base

...

end



# or even:



Class PersonStore < RestfulResource::Repository

  repository_for :person

  base_uri "http://coolstuff.livingsocial.com"

end



# Swapping repositories might give you radically different methods (only an

# ActiveRepository would give you a find_by_sql method, for instance), but

# thats ok.  The "Persistant Capable" classes don't change with a change in

# the persistence mechanism. The "Persistent Aware" classes, like the

# controllers, would.  



# And it might even be possible to have multiple repositories in the same

# app...



Class PersonStore < ActiveRepository::Base

#...

end



# and



Class RemotePersonStore < RestfulResource::Repository

#...

end



# and then you could do stuff like:



p = RemotePersonStore.find(5)

PersonStore.save(p)



# and essentially use two repositories as an ETL engine.



# One nice thing we get from ActiveRecord would have to change slightly -

# the migrations.

# Actually, the migrations could work pretty much as they do now, but devs

# would have to make the corresponding attr_accessor declarations in their

# models.

#

# if an attr_accessor was declared that didn't have a corresponding column in

# the db, then it could warn on application init.  That warning for that field

# could be silenced in the repository with something like:



not_persisted :current_location



# and in reverse, if a migration added a column that couldn't map to an

# attr_accessor, it could warn unless the repo had a line like:



ignore_column :eye_color



# The generator could stick the attr_accessors in the class

# automatically if we wanted it to.  I wouldn't do anything 'magical' like

# have the persistence engine inject it with meta... that would make the

# attributes hard to discover, and could make multiple repositories in an

# app step on each other in nondeterministic ways.  By having attr_accessors,

# the model becomes the 'system of record' for what it means to be that

# kind of domain object.  I like that.



# (of course, nosql dbs may have their own requirements met with their own

# dsls).



# You could even have the store do something like:



synthetic_field :age, { Date.today - born_on }



# while you could do exactly the same thing by adding 'age' method to the

# model, having it in he Repository could be useful for an ETL task, for

# defining the transform step.  Imagine one database that has a born_on

# field, and another one that has an age field, and you are transforming

# data to go into the one with age...  in the store with the born_on,

# set the store to have a synthetic field :age.  In the other store, set

# it to ignore the born_on date. Then in the model you define attr_reader

# for :age.  Both stores see the domain model as exactly what it needs in

# order to make each database happy, and the domain model code stays clean.

#

# if you needed to map an attribute to a different column:



map_attribute_to_column :sex, :gender



# One potentially awesome tweak to the dsl is how this would

# handle polymorphic tables:



class PersonStore < ActiveRepository::Base

  repository_for :person

  repository_for :client

  repository_for :administrator

  polymorphic_model_column :type # would automatically store the class

                                 # type, just like AR polymorphism

end



# Given this pattern, I think relationship declarations go in the models,

# since there they can add the appropriate methods to return collections,

# etc, and since the repo knows what models it is supposed to manage, it can

# get access to the same knowledge to do whatever it needs to do.  If they

# were declared in the repo, it would be inappropriate for the model to

# introspect there, otherwise the model would become 'persistence aware'.

# They 'feel' a little attr_accessor-like things to me.



# Finally, while the model as code looks completely unaware of its storage,

# underneath the covers at runtime the repository could 'enhance' it with

# things like dirty flags for fields, is_dirty? & is_new? methods, etc.

# In fact, for years I used an object-oriented database in Java named

# Versant, and it had a 'bytecode enhancement' step that did exactly this

# during the compile - it modified the classes bytecode with dirty flags

# and other persistence helpers.