当一个引用的pk引用另一个引用的pk时，如何映射GORM中的实体

CREATE TABLE `table_a` (
`a_id` int(11) NOT NULL AUTO_INCREMENT, 
PRIMARY KEY (`a_id`)
)

CREATE TABLE `table_b` (
`b_id` int(11) NOT NULL AUTO_INCREMENT, 
 PRIMARY KEY (`b_id`), 
 KEY `b_id_fk` (`b_id`), 
 CONSTRAINT `b_id_fk` FOREIGN KEY (`b_id`) REFERENCES `table_a\' (`a_id`)
 )

class TableB {

 TableA tableA
 static belongsTo = [TableA]

 static mapping = {
    id column:\"b_id\"
    version false
    tableA column:\"b_id\"
 }

 static constraints = {
    tableA unique: true
 }
}

TableB

TableA

class TableA {
    static mapping = {
        id column: \'a_id\'
        table \'table_a\'
        version false
        tablePerHierarchy false
    }
}

class TableB extends TableA {
    static mapping = {
        id column: \'b_id\'
        table \'table_b\'
        version false
    }
}

TableA

TableB

TableA

table_a