MONGOID-4173 Fix nested eager loading

lib/mongoid/relations/eager/base.rb

@@ -48,7 +48,7 @@ def run
           @loaded = []
           while shift_metadata
             preload
-            @loaded << @docs.collect { |d| d.send(@metadata.name) }
+            @loaded << @docs.collect { |d| d.send(@metadata.name) if d.respond_to?(@metadata.name) }
           end
           @loaded.flatten
         end
@@ -99,12 +99,12 @@ def set_on_parent(id, element)
         # @example Return a hash with the current documents grouped by key.
         #   loader.grouped_docs
         #
-        # @return [ Hash ] hash with groupd documents.
+        # @return [ Hash ] hash with grouped documents.
         #
         # @since 4.0.0
         def grouped_docs
           @grouped_docs[@metadata.name] ||= @docs.group_by do |doc|
-            doc.send(group_by_key)
+            doc.send(group_by_key) if doc.respond_to?(group_by_key)
           end
         end
 

spec/mongoid/relations/eager_spec.rb

@@ -181,6 +181,46 @@
         expect(Mongoid::Relations::Eager::HasAndBelongsToMany).to receive(:new).with([houses_metadata], docs).once.and_call_original
         context.eager_load(docs)
       end
+
+      context 'when one of the eager loading definitions is nested' do
+
+        before do
+          class User
+            include Mongoid::Document
+          end
+
+          class Unit
+            include Mongoid::Document
+          end
+
+          class Booking
+            include Mongoid::Document
+            belongs_to :unit
+            has_many :vouchers
+          end
+
+          class Voucher
+            include Mongoid::Document
+            belongs_to :booking
+            belongs_to :created_by, class_name: 'User'
+          end
+        end
+
+        it 'successfully loads all relations' do
+          user = User.create
+          unit = Unit.create
+          booking = Booking.create(unit: unit)
+          Voucher.create(booking: booking, created_by: user)
+
+          vouchers = Voucher.includes(:created_by, booking: [:unit])
+
+          vouchers.each do |voucher|
+            expect(voucher.created_by).to eql(user)
+            expect(voucher.booking).to eql(booking)
+            expect(voucher.booking.unit).to eql(unit)
+          end
+        end
+      end
     end
 
     context "when including two of the same relation type" do