Discrimination for embedded documents belongs on the EmbedOne or EmbedMany mapping, not the embedded document class (as described [here](http://docs.doctrine-project.org/projects/doctrine-mongodb-odm/en/latest/reference/embedded-mapping.html#mixing-document-types)). You'll want to add the following:
```
/** @ODM\EmbedOne(
 *    discriminatorField="type",
 *    discriminatorMap={"album"="GH501Album", "article"="GH501Article"}
 *  )
 */
```