24.1.40. @MapKey

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The [`@MapKey`](http://docs.oracle.com/javaee/7/api/javax/persistence/MapKey.html) annotation is used to specify the key of a `java.util.Map` association for which the key type is either the primary key or an attribute of the entity which represents the value of the map.

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See the [`@MapKey` mapping](#collections-map-unidirectional-example) section for more info.

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