Message Title

`@MapKey` always looks for a field in the entity that's used as the Map value. To quote the JPA 2.1 spec:

 
                                                                The MapKey annotation is used to specify the map key for associations of type java.util.Map
when the map key is itself the primary key or a persistent field or property of the entity that is the value
of the map.

So, the `inventoryCode` should be in `Book`.

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