Introduce virtual_has_many through

This will automatically add the ruby deletate

It is assumed that a nil association is allowed, and a default of [] will be returned.

The delegate defined does still have logic for attributes (namely the has_attribute?) part.
Since there will be no attribute with this name, it will be ignored.

Not using traditional `delegate`, because that does not provide the `default` option

Author: kbrock

lib/active_record/virtual_attributes/virtual_reflections.rb

@@ -14,11 +14,12 @@ def virtual_has_one(name, uses: nil, **options)
           add_virtual_reflection(reflection, name, uses)
         end
 
-        def virtual_has_many(name, uses: nil, **options)
+        def virtual_has_many(name, uses: nil, source: nil, through: nil, **options)
           define_method(:"#{name.to_s.singularize}_ids") do
             records = send(name)
             records.respond_to?(:ids) ? records.ids : records.collect(&:id)
           end
+          define_delegate(name, source || name, :to => through, :allow_nil => true, :default => []) if through
           reflection = ActiveRecord::Associations::Builder::HasMany.build(self, name, nil, options)
           add_virtual_reflection(reflection, name, uses)
         end

spec/db/models.rb

@@ -135,6 +135,10 @@ class Book < VirtualTotalTestBase
   has_many :photos, :as => :imageable, :class_name => "Photo"
   has_one :current_photo, -> { all.merge(Photo.order(:id => :desc)) }, :as => :imageable, :class_name => "Photo"
 
+  virtual_has_many :author_books, :through => :author, :source => :books
+  # sorry. books.books doesn't totally make sense.
+  virtual_has_many :books, :through => :author
+
   scope :ordered,   -> { order(:created_on => :desc) }
   scope :published, -> { where(:published => true)  }
   scope :wip,       -> { where(:published => false) }

spec/virtual_delegates_spec.rb

@@ -300,6 +300,24 @@ def self.connection
     end
   end
 
+  describe "virtual_has_many with :through" do
+    it "with :source works" do
+      Author.create_with_books(2)
+      author = Author.create_with_books(3)
+      books = author.books.order(:id)
+
+      expect(books.first.author_books.order(:id)).to eq(books)
+    end
+
+    it "without :source works" do
+      Author.create_with_books(2)
+      author = Author.create_with_books(3)
+      books = author.books.order(:id)
+
+      expect(books.first.books.order(:id)).to eq(books)
+    end
+  end
+
   describe "#determine_method_names (private)" do
     it "works with column and to" do
       expect(determine_method_names("column", "relation", nil)).to eq([:column, :relation, :column])