handle many-to-many relations separately when process entities

test/models/author.hpp

@@ -3,10 +3,14 @@
 
 #include "matador/utils/access.hpp"
 
+#include "matador/sql/entity.hpp"
+
 #include <string>
 
 namespace matador::test {
 
+struct book;
+
 struct author
 {
   unsigned long id{};
@@ -27,7 +31,7 @@ struct author
     field::attribute(op, "date_of_birth", date_of_birth, 31);
     field::attribute(op, "year_of_birth", year_of_birth);
     field::attribute(op, "distinguished", distinguished);
-    field::has_many(op, "books", books, "author_id", "id", utils::fetch_type::LAZY);
+    field::has_many(op, books, "author_id", utils::fetch_type::LAZY);
   }
 };