How do rails association methods work?
How do rails association methods work? Lets consider this example
class User < ActiveRecord::Base
has_many :articles
end
class Article < ActiveRecord::Base
belongs_to :user
end
Now I can do something like
@user = User.find(:first)
@user.articles
This fetches me articles belonging to that user. So far so good.
Now further I can go ahead and do a find on these articles with some conditions.
@user.articles.find(:all, :conditions => {:sector_id => 3})
Or simply declare and associations method like
class User < ActiveRecord::Base
has_many :articles do
def of_sector(sector_id)
find(:all, :conditions => {:sector_id => sector_id})
end
end
end
And do
@user.articles.of_sector(3)
Now my question is, how does this find work on the array of ActiveRecord objects fetched using the association method? Because if we implement our own User instance method called articles and write our own implementation that gives us the exact same results as that of the association method, the find on the fetch array of ActiveRecord objects wont work.
My guess is that the association methods attach certain properties to the array of fetched objects that enables further querying using find and other ActiveRecord methods. What is the sequence of code execution in this this case? How could I validate this?
Solution 1:
How it actually works is that the association object is a "proxy object". The specific class is AssociationProxy. If you look at line 52 of that file, you'll see:
instance_methods.each { |m| undef_method m unless m =~ /(^__|^nil\?$|^send$|proxy_|^object_id$)/ }
By doing this, methods like class no longer exist on this object. So if you call class on this object, you'll get method missing. So, there a method_missing implemented for the proxy object that forwards the method call to the "target":
def method_missing(method, *args)
if load_target
unless @target.respond_to?(method)
message = "undefined method `#{method.to_s}' for \"#{@target}\":#{@target.class.to_s}"
raise NoMethodError, message
end
if block_given?
@target.send(method, *args) { |*block_args| yield(*block_args) }
else
@target.send(method, *args)
end
end
end
The target is an Array, so when you call class on this object, it says it's an Array, but that's just because the target is an Array, the actual class is an AssociationProxy, but you can't see that anymore.
So all the methods that you add, such as of_sector, get added to the association proxy, so they get called directly. Methods like [] and class aren't defined on the association proxy, so they get sent to the target, which is an array.
To help you see how this is happening, add this to line 217 of that file in your local copy of association_proxy.rb:
Rails.logger.info "AssociationProxy forwarding call to `#{method.to_s}' method to \"#{@target}\":#{@target.class.to_s}"
If you don't know where that file is, the command gem which 'active_record/associations/association_proxy' will tell you. Now when you call class on a AssociationProxy, you will see a log message telling you it is sending that to the target, which should make it clearer what is happening. This is all for Rails 2.3.2 and could change in other versions.
Solution 2:
As already mentioned, the active record associations create a metric buttload of convenience methods. Sure, you could write your own methods to fetch everything. But that is not the Rails Way.
The Rails Way is the culmination of two mottos. DRY (Don't Repeat Yourself) and "Convention over Configuration". Essentially by naming things in a way that makes sense, some robust methods provided by the framework can abstract out all the common code. The code you place in your question is the perfect example of something that can be replaced by a single method call.
Where these convenience methods really shine are the more complex situations. The kind of thing involving join models, conditions, validations, etc.
To answer your question when you do something like @user.articles.find(:all, :conditions => ["created_at > ? ", tuesday]), Rails prepares two SQL queries and then merges them into one. where as your version just returns the list of objects. Named scopes do the same thing, but usually don't cross model boundaries.
You can validate it by checking the SQL queries in the development.log as you call these things in the console.
So lets talk about Named Scopes for a moment because they give a great example of how rails handles the SQL, and I think they're a simpler way to demonstrate what's going on behind the scenes, as they don't need any model associations to show off.
Named Scopes can be used to perform custom searches of a model. They can be chained together or even called through associations. You could easily create custom finders that return identical lists, but then you run into the same problems mentioned in the Question.
class Article < ActiveRecord::Base
belongs_to :user
has_many :comments
has_many :commentators, :through :comments, :class_name => "user"
named_scope :edited_scope, :conditions => {:edited => true}
named_scope :recent_scope, lambda do
{ :conditions => ["updated_at > ? ", DateTime.now - 7.days]}
def self.edited_method
self.find(:all, :conditions => {:edited => true})
end
def self.recent_method
self.find(:all, :conditions => ["updated_at > ?", DateTime.now - 7 days])
end
end
Article.edited_scope
=> # Array of articles that have been flagged as edited. 1 SQL query.
Article.edited_method
=> # Array of Articles that have been flagged as edited. 1 SQL query.
Array.edited_scope == Array.edited_method
=> true # return identical lists.
Article.recent_scope
=> # Array of articles that have been updated in the past 7 days.
1 SQL query.
Article.recent_method
=> # Array of Articles that have been updated in the past 7 days.
1 SQL query.
Array.recent_scope == Array.recent_method
=> true # return identical lists.
Here's where things change:
Article.edited_scope.recent_scope
=> # Array of articles that have both been edited and updated
in the past 7 days. 1 SQL query.
Article.edited_method.recent_method
=> # no method error recent_scope on Array
# Can't even mix and match.
Article.edited_scope.recent_method
=> # no method error
Article.recent_method.edited_scope
=> # no method error
# works even across associations.
@user.articles.edited.comments
=> # Array of comments belonging to Articles that are flagged as
edited and belong to @user. 1 SQL query.
Essentially each named scope creates an SQL fragment. Rails will skillfully merge with every other SQL fragment in the chain to produce a single query returing exactly what you want. The methods added by the association methods work the same way. Which is why they seamlessly integrate with named_scopes.
The reason for the mix & match didn't work is the same that the of_sector method defined in the question doeso't work. edited_methods returns an Array, where as edited_scope (as well as find and all other AR convenience methods called as part of a chain) pass their SQL fragment onward to the next thing in the chain. If it's the last in the chain it executes the query. Similarly, this won't work either.
@edited = Article.edited_scope
@edited.recent_scope
You tried to use this code. Here's the proper way to do it:
class User < ActiveRecord::Base
has_many :articles do
def of_sector(sector_id)
find(:all, :conditions => {:sector_id => sector_id})
end
end
end
To achieve this functionality you want to do this:
class Articles < ActiveRecord::Base
belongs_to :user
named_scope :of_sector, lambda do |*sectors|
{ :conditions => {:sector_id => sectors} }
end
end
class User < ActiveRecord::Base
has_many :articles
end
Then you can do things like this:
@user.articles.of_sector(4)
=> # articles belonging to @user and sector of 4
@user.articles.of_sector(5,6)
=> # articles belonging to @user and either sector 4 or 5
@user.articles.of_sector([1,2,3,])
=> # articles belonging to @user and either sector 1,2, or 3