How to set uniqueness at DB level for a one-to-many association?

My problem is simple but I could not find any GORM syntax for this.

Consider the following class:

class Article {
  String text

  static hasMany = [tags: String]

  static constraints= {
    tags(unique: true) //NOT WORKING
  }

}

I want to have one unique tag name per article defined in my constraints but I cannot make it with the above syntax. Clearly I need in DB schema something like:

create table article_tags (article_id bigint, tags_string varchar(255), unique (article_id , tags_string))

How can I do that?

PS: I am also stuck for setting constraints on tag minimum and maximum size

FYI, you can also use a custom validator in domain classes:

    static constraints = {
    tags(validator: { 
        def valid = tags == tags.unique()
        if (!valid) errors.rejectValue(
            "tags", "i18n.message.code", "default message")
        return valid
    })

At the database level, you can customize DDL generation by having the following code in grails-app/conf/hibernate/hibernate.cfg.xml:

<hibernate-mapping>
    <database-object>
        <create>
        ALTER TABLE article_tags
        ADD CONSTRAINT article_tags_unique_constraint 
        UNIQUE(article_id, tags_string);
    </create>
        <drop>
        ALTER TABLE article_tags 
        DROP CONSTRAINT article_tags_unique_constraint;
    </drop>
    </database-object>
</hibernate-mapping>