fix(model): support getting discriminator by value when creating a ne…

…w model

Fix #7851
Re: #7586

lib/model.js

@@ -4576,12 +4576,16 @@ Model.compile = function compile(name, schema, collectionName, connection, base)
       }
       const discriminatorKey = model.schema.options.discriminatorKey;
 
+      if (model.discriminators == null || doc == null || doc[discriminatorKey] == null) {
+        Model.call(this, doc, fields, skipId);
+        return;
+      }
+
       // 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);
+      const Discriminator = model.discriminators[doc[discriminatorKey]] ||
+        getDiscriminatorByValue(model, doc[discriminatorKey]);
+      if (Discriminator != null) {
+        return new Discriminator(doc, fields, skipId);
       }
 
       // Otherwise, just use the top-level model