fix: preload polymorphic associations to improve performance (#731)

Author: nicolasalexandre9

app/serializers/forest_liana/serializer_factory.rb

@@ -139,10 +139,10 @@ def has_one_relationships
 
             next if !should_include_attr?(attribute_name, attr_data)
 
-            unless relation.polymorphic?
+            unless relation.nil? || (relation.respond_to?(:polymorphic?) && relation.polymorphic?)
               relation_class_name = ForestLiana.name_for(relation.klass).demodulize
 
-              if object.send(relation.foreign_key.to_sym) &&
+              if object.respond_to?(relation.foreign_key.to_sym) &&
                 @options[:fields][relation_class_name]&.size == 1 &&
                 @options[:fields][relation_class_name]&.include?(relation.klass.primary_key.to_sym)
 

app/services/forest_liana/resources_getter.rb

@@ -56,10 +56,22 @@ def query_for_batch
 
     def records
       records = @records.offset(offset).limit(limit).to_a
-
       polymorphic_association, preload_loads = analyze_associations(@resource)
+
       if polymorphic_association && Rails::VERSION::MAJOR >= 7
-        # TODO
+        preloader = ActiveRecord::Associations::Preloader.new(records: records, associations: polymorphic_association)
+        preloader.loaders
+        preloader.branches.each do |branch|
+          branch.loaders.each do |loader|
+            records_by_owner = loader.records_by_owner
+            records_by_owner.each do |record, association|
+              record_index =  records.find_index { |r| r.id == record.id }
+              records[record_index].define_singleton_method(branch.association) do
+                association.first
+              end
+            end
+          end
+        end
       end
 
       preload_cross_database_associations(records, preload_loads)