CREATE TABLE Countries (
  id INTEGER NOT NULL,
  iso_code CHAR(2) NOT NULL,
  PRIMARY KEY (id),
  UNIQUE (iso_code));

CREATE TABLE Zips (
  country_code CHAR(2) NOT NULL,
  code VARCHAR(10) NOT NULL,
  PRIMARY KEY (country_code, code),
  CONSTRAINT zips_countries_fk
    FOREIGN KEY (country_code)
    REFERENCES Countries (iso_code)
    ON DELETE NO ACTION
    ON UPDATE CASCADE);

@Entity
@Table(name = "Countries")
public class Country implements Serializable
{
    @Id
    @Column(name = "id")
    private Integer id;

    @Column(name = "iso_code")
    private String isoCode;

    ...
}

@Entity
@Table(name = "Zips")
public class Zip implements Serializable
{
    @EmbeddedId
    private ZipId embeddedId;

    @MapsId(value = "countryCode") // outcomment @MapsId to make JPA 1.0 @EmbeddedId work
    @ManyToOne
    @JoinColumn(name = "country_code", referencedColumnName = "iso_code")
    private Country country;
    
    ...
}

@Embeddable
public class ZipId implements Serializable
{
    @Column(name = "country_code", insertable = false, updatable = false)
    private String countryCode;

    @Column(name = "code")
    private String code;

    ...
}

Zip zi = em.find(Zip.class, new ZipId("DE", "64846"));

Entity manager created!

Loaded country = tld.geoareas.model.Country@5bf02b85[id=1,isoCode=AF]
Exception in thread "main" java.lang.IllegalArgumentException: You have provided an instance of an incorrect PK class for this find operation.  Class expected : class java.lang.Integer, Class received : class java.lang.String.
    at org.eclipse.persistence.internal.jpa.CMP3Policy.createPrimaryKeyFromId(CMP3Policy.java:215)
    at org.eclipse.persistence.internal.jpa.EntityManagerImpl.findInternal(EntityManagerImpl.java:695)
    at org.eclipse.persistence.internal.jpa.EntityManagerImpl.find(EntityManagerImpl.java:617)
    at org.eclipse.persistence.internal.jpa.EntityManagerImpl.find(EntityManagerImpl.java:496)
    at tld.geoareas.Main.main(Main.java:38)