package domainmodel.core.accounts;

public class Account {
    private AccountId accountId;
    private String shortCode;
    public Account(AccountId accountId, String shortCode) {
        this.accountId = accountId;
        this.shortCode = shortCode;
    }
    public String getShortCode() {
        return shortCode;
    }
    public AccountId getAccountId() {
        return accountId;
    }
}

package domainmodel.core.accounts;

public class AccountId {
    private final int id;
    protected AccountId(int id) {
        this.id = id;
    }
    public int intValue() {
        return id;
    }
}

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="domainmodel.core.accounts" default-access="field">
  <class name="Account" table="accounts" >
     <composite-id name="accountId">
        <key-property name="id"/>
        <generator class="assigned" />
    </composite-id>
    <natural-id mutable="true">
        <property name="shortCode" column="short_code" not-null="true"/>
    </natural-id>
  </class>
</hibernate-mapping>

public Account findByCode(String shortCode) {
  ...
  return (Account)session.byNaturalId(Account.class).using("shortCode", shortCode).load();
}