MONGOID-5034 When excluded from query, embeds many relations are returned as empty array instead of raising MissingAttributeError (#5196)

* MONGOID-5034 add test and potential implementation

* MONGOID-5034 add embedded condition with comment

* MONGOID-5034 only raise when not binding

* MONGOID-5034 update comments

* MONGOID-5034 dont raise during without_autobuild

* MONGOID-5034 update comments

Author: neilshweky

lib/mongoid/association/accessors.rb

@@ -105,6 +105,19 @@ def set_relation(name, relation)
       #
       # @return [ Proxy ] The association.
       def get_relation(name, association, object, reload = false)
+        field_name = database_field_name(name)
+
+        # As per the comments under MONGOID-5034, I've decided to only raise on
+        # embedded associations for a missing attribute. Rails does not raise
+        # for a missing attribute on referenced associations.
+        # We also don't want to raise if we're retrieving an association within
+        # the codebase. This is often done when retrieving the inverse association
+        # during binding or when cascading callbacks. Whenever we retrieve
+        # associations within the codebase, we use without_autobuild.
+        if !without_autobuild? && association.embedded? && attribute_missing?(field_name)
+          raise ActiveModel::MissingAttributeError, "Missing attribute: '#{field_name}'"
+        end
+
         if !reload && (value = ivar(name)) != false
           value
         else

spec/mongoid/association/embedded/embeds_many_query_spec.rb

@@ -47,5 +47,17 @@
         expect(patient.addresses.first.number).to eq(123)
       end
     end
+
+    context "when excluding the relation" do
+      let(:congress) do
+        EmmCongress.where(name: 'foo').only(:_id).first
+      end
+
+      it 'raises a MissingAttributeError' do
+        expect do
+          congress.legislators
+        end.to raise_error(ActiveModel::MissingAttributeError)
+      end
+    end
   end
 end