Hibernate question: How to annotate a map of <object1, object2> in a m:m relationship

Hello!

I have two classes, the one is named Group, the other BaseFeature. now, i want that the class group is persisting several objects of the BaseFeature class as a map (list).

@Entity(name = "`group`")
public class Group {

private Map<String, BaseFeature> baseFeatures;

@ElementCollection
public Map<String, BaseFeature> getBaseFeatures() {
return baseFeatures;
}
}

If am adding now several baseFeatures to a group X and persisting group X, it works. but if i am adding then the same baseFeatures to group Y, an exception is thrown:

org.postgresql.util.PSQLException: ERROR: duplicate key value violates unique constraint "group_BaseFeature_basefeatures_id_key" Detail: Key (basefeatures_id)=(12) already exists.

Group_id [PK] bigint; basefeatures_id bigint; basefeatures_key [PK] character varying (255)
90;12;"Age"
90;10;"Emotion"
90;13;"Gender"
90;11;"Relationship"
90;14;"Wealth"
90;16;"WeatherDegree"
90;15;"WeatherType"

the generated table syntax looks like this, and i think that the problem is the last line, marked bold:

CREATE TABLE "group_BaseFeature"
(
"Group_id" bigint NOT NULL,
basefeatures_id bigint NOT NULL,
basefeatures_key character varying(255) NOT NULL,
CONSTRAINT "group_BaseFeature_pkey" PRIMARY KEY ("Group_id" , basefeatures_key ),
CONSTRAINT fkfe462505436784f6 FOREIGN KEY (basefeatures_id)
REFERENCES basefeature (id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE NO ACTION,
CONSTRAINT fkfe46250546aa625f FOREIGN KEY ("Group_id")
REFERENCES "group" (id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE NO ACTION,
CONSTRAINT "group_BaseFeature_basefeatures_id_key" UNIQUE (basefeatures_id )
)

I dont understand why group_BaseFeature_basefeatures_id_key is generated as UNIQUE, nor, how to solve my problem. thx for any help..

alex

solution;

@ManyToMany
public Map<String, BaseFeature> getBaseFeatures() {
return baseFeatures;
}
}