I am having trouble getting the right hibernate annotations to use on a value typed Map with an enumerated class as a key. Here is a simplified (and extremely contrived) example.

public class Thing {
    public String id;
    public Letter startLetter;
    public Map<Letter,Double> letterCounts = new HashMap<Letter, Double>(); 
}


public enum Letter {
    A,
    B,
    C,
    D
}

Here are my current annotations on Thing

@Entity
public class Thing {

    @Id
    public String id;

    @Enumerated(EnumType.STRING)
    public Letter startLetter;

    @CollectionOfElements
    @JoinTable(name = "Thing_letterFrequencies", joinColumns = @JoinColumn(name = "thingId"))
    @MapKey(columns = @Column(name = "letter", nullable = false))
    @Column(name = "count")
    public Map<Letter,Double> letterCounts = new HashMap<Letter, Double>();

}

Hibernate generates the following DDL to create the tables for my MySql database

create table Thing (id varchar(255) not null, startLetter varchar(255), primary key (id)) type=InnoDB;
create table Thing_letterFrequencies (thingId varchar(255) not null, count double precision, letter tinyblob not null, primary key (thingId, letter)) type=InnoDB;

Notice that hibernate tries to define letter (my map key) as a tinyblob, however it defines startLetter as a varchar(255) even though both are of the enumerated type Letter. When I try to create the tables I see the following error

BLOB/TEXT column 'letter' used in key specification without a key length

I googled this error and it appears that MySql has issues when you try to make a tinyblob column part of a primary key, which is what hibernate needs to do with the Thing_letterFrequencies table. So I would rather have letter mapped to a varchar(255) the way startLetter is.

Unfortunately, I've been fussing with the MapKey annotation for a while now and haven't been able to make this work. I've also tried @MapKeyManyToMany(targetEntity=Product.class) without success. Can anyone tell me what are the correct annotations for my letterCounts map so that hibernate will treat the letterCounts map key the same way it does startLetter?