What are the hibernate annotations used to persist a Map with an enumerated type as a key?

I am having trouble getting the right hibernate annotations to use on a 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 startLet开发者_开发知识库ter?

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I found something that works on https://forum.hibernate.org/viewtopic.php?f=1&t=999270&start=0 although it is kind of ugly. If you assume that Letter is in the com.myexample package here are the annotations

@CollectionOfElements
@JoinTable(name = "Thing_letterFrequencies", joinColumns = @JoinColumn(name = "thingId"))
@MapKey(columns = @Column(name = "letter"),
    type = @Type(
      type="org.hibernate.type.EnumType",
      parameters = {@Parameter(name = "enumClass", value="com.myexample.Letter"), @Parameter(name="type", value="12")}
    ))
@Column(name = "count")
public Map<Letter,Double> letterCounts = new HashMap<Letter, Double>();

Note the @Parameter(name="type", value="12") Apparently the "value=12" maps the enumerated type to a varchar. Hopefully this helps someone else out but if anyone has a cleaner annotation without the use of magic numbers like 12 I want to hear it.

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If you use java6, you may try to use @MapKeyEnumerated (javax.persistence) annotation instead of @MapKey

@ElementCollection
    @JoinTable(name = "thing_letter_frequencies", joinColumns = @JoinColumn(name = "thing_id"))
    @MapKeyEnumerated(EnumType.STRING)
    @Column(name = "letter_count")
    public Map<Letter, Double> letterCount = new HashMap<Letter, Double>();

UPDATE:

Whole class code (using Project Lombok)

@Entity
public class Thing {

    @Getter
    @Setter
    @Id
    private String id;

    @Getter
    @Setter
    @Enumerated(EnumType.STRING)
    private Letter startLetter;

    @Getter
    @Setter
    @ElementCollection
    @JoinTable(name = "thing_letter_frequencies", joinColumns = @JoinColumn(name = "thing_id"))
    @MapKeyEnumerated(EnumType.STRING)
    @Column(name = "letter_count")
    public Map<Letter, Double> letterCount = new HashMap<Letter, Double>();
}

What hibernate generates in MySQL (create table statements):

CREATE TABLE `thing` (
    `id` VARCHAR(255) NOT NULL,
    `startLetter` VARCHAR(255) NULL DEFAULT NULL,
    PRIMARY KEY (`id`)
)
ENGINE=InnoDB
ROW_FORMAT=DEFAULT;

CREATE TABLE `thing_letter_frequencies` (
    `thing_id` VARCHAR(255) NOT NULL,
    `letter_count` DOUBLE NULL DEFAULT NULL,
    `letterCount_KEY` VARCHAR(255) NOT NULL DEFAULT '',
    PRIMARY KEY (`thing_id`, `letterCount_KEY`),
    INDEX `FKA0A7775246D72F41` (`thing_id`),
    CONSTRAINT `FKA0A7775246D72F41` FOREIGN KEY (`thing_id`) REFERENCES `thing` (`id`)
)
ENGINE=InnoDB
ROW_FORMAT=DEFAULT;