fix(update): use discriminator schema to cast update if discriminator…

… key specified in filter

Fix #7843

lib/query.js

@@ -4333,7 +4333,20 @@ Query.prototype._castUpdate = function _castUpdate(obj, overwrite) {
     useNestedStrict = this.options.useNestedStrict;
   }
 
-  return castUpdate(this.schema, obj, {
+  let schema = this.schema;
+  const filter = this._conditions;
+  if (schema != null &&
+      utils.hasUserDefinedProperty(filter, schema.options.discriminatorKey) &&
+      typeof filter[schema.options.discriminatorKey] !== 'object' &&
+      schema.discriminators != null) {
+    const discriminatorValue = filter[schema.options.discriminatorKey];
+    const byValue = helpers.getDiscriminatorByValue(this.model, discriminatorValue);
+    schema = schema.discriminators[discriminatorValue] ||
+      (byValue && byValue.schema) ||
+      schema;
+  }
+
+  return castUpdate(schema, obj, {
     overwrite: overwrite,
     strict: strict,
     omitUndefined,