fix(discriminator): use discriminator model when using `new BaseModel…

…()` with discriminator key

Fix #7586

lib/model.js

@@ -4586,6 +4586,17 @@ Model.compile = function compile(name, schema, collectionName, connection, base)
       if (!(this instanceof model)) {
         return new model(doc, fields, skipId);
       }
+      const discriminatorKey = model.schema.options.discriminatorKey;
+
+      // If discriminator key is set, use the discriminator instead (gh-7586)
+      if (model.discriminators != null &&
+          doc != null &&
+          doc[discriminatorKey] != null &&
+          model.discriminators[doc[discriminatorKey]] != null) {
+        return new model.discriminators[doc[discriminatorKey]](doc, fields, skipId);
+      }
+
+      // Otherwise, just use the top-level model
       Model.call(this, doc, fields, skipId);
     };
   }