Why are Rails model association results not naturally ActiveRecord::Relations?

问题:

I'm using Rails 3.2.0

Let's say I have:

class Comment < ActiveRecord::Base   has_many :articles end  c1 = Comment.last 

then

c1.articles.class # => Array  c1.articles.where('id NOT IN (999999)').class # => ActiveRecord::Relation     

Why is the result of an association not a type of ActiveRecord::Relation?

It clearly is / was at some point:

c1.articles.to_orig # undefined method `to_orig' for #<ActiveRecord::Relation:0x007fd820cc80a8>  c1.articles.class # => Array 

Certain evaluations act upon an ActiveRecord::Relation object, but inspecting the class gives a different type.

---

Particularly, this breaks building lazy-loaded queries when using merge to concat multiple queries.

回答1:

It is an ActiveRecord::Relation, but Rails is intentionally lying to you. You can see this already in the method calls, and continue to see it by calling ancestors, which includes a slew of ActiveRecord classes:

c1.articles.ancestors.select { |c| c.to_s =~ /ActiveRecord/ }.size  #=> 35 

which shows that it is very much not an Array.

This happens because what you’re getting back when calling c1.articles is an ActiveRecord::Associations::CollectionProxy*, which undefines class (along with many other methods). This means that class gets delegated via its method_missing, which sends it to target. As we can see, the class of target here is, in fact, Array:

c1.articles.target.class  #=> Array 

That is where c1.articles.class comes from. Nevertheless, it is an ActiveRecord::Relation.

_{* We can verify that it is indeed an ActiveRecord::Associations::CollectionProxy by calling Ruby’s original class method on the object in question: Object.instance_method(:class).bind(c1.articles).call. This is a nice trick to verify that the object is not trying to pretend to be of a different class.}

回答2:

Because when you define association, it places in your model:

def #{name}(*args)   association(:#{name}).reader(*args) end 

.reader() returns AssociationProxy, which removes the .class method and delegates unknown methods to @target through .method_missing.

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