Question

What will let me know if I am going to get a Relation, Array, or some other type back from an ActiveRecord call? I know I can type .class in the console and figure it out, but is there something in the call itself that will let me know what I am asking for?

Answer 1

You know, Rails sometimes lies to you — all magicians do 🙂

Rails allows you to build complex queries by chaining your has_many associations. The core of this functionality is a bunch of XXXAssocation (like HasManyAssociation) classes.
When you call .class on a has_many association your call is applied in fact for HasManyAssociation instance. But here’s the magic starts:

# collection_proxy.rb
instance_methods.each { |m| undef_method m unless m.to_s =~ /^(?:nil\?|send|object_id|to_a)$|^__|^respond_to|proxy_/ }

Rails undefs (hides) methods of HasManyAssociation instance (except the few, as you can see in the regular expression) and then uses delegation and method_missing to pass your call to some underlying array (if you’re trying to fetch records) or to association itself (if you’re chaining your association):

  delegate :group, :order, :limit, :joins, :where, :preload, :eager_load, :includes, :from,
           :lock, :readonly, :having, :pluck, :to => :scoped 

  delegate :target, :load_target, :loaded?, :to => :@association

  delegate :select, :find, :first, :last,
           :build, :create, :create!,
           :concat, :replace, :delete_all, :destroy_all, :delete, :destroy, :uniq,
           :sum, :count, :size, :length, :empty?,
           :any?, :many?, :include?,
           :to => :@association

  def method_missing(method, *args, &block)
    match = DynamicFinderMatch.match(method)
    if match && match.instantiator?
      send(:find_or_instantiator_by_attributes, match, match.attribute_names, *args) do |r|
        proxy_association.send :set_owner_attributes, r
        proxy_association.send :add_to_target, r
        yield(r) if block_given?
      end
    end

    if target.respond_to?(method) || (!proxy_association.klass.respond_to?(method) && Class.respond_to?(method))
      if load_target
        if target.respond_to?(method)
          target.send(method, *args, &block)
        else
          begin
            super
          rescue NoMethodError => e
            raise e, e.message.sub(/ for #<.*$/, " via proxy for #{target}")
          end
        end
      end

    else
      scoped.readonly(nil).send(method, *args, &block)
    end
  end

So, HasManyAssociation instance decides what to handle by itself and what needs to be accomplished via hidden array (class method isn’t what HasManyAssociation interested in so it will be called on this hidden array. The result, of course, will be Array, which is a little deception).