When using .includes on a belongs_to relation that has nil values the following exception is thrown:

Failure/Error: doc.set_relation(@metadata.name, element)

NoMethodError:
undefined method `set_relation' for nil:NilClass
Did you mean? set_trace_func

Checking for nil doc in Mongoid::Eager::Relations seems to resolve the issue:

def set_relation(doc, element)
doc.set_relation(@metadata.name, element) unless doc.blank?
end