Migrating from Hibernate 3.6 to 4.2: Hibernate auto generates foreign keys

As the title suggests: I am currently migrating from Hibernate 3.6 to 4.2.

PROBLEM

After the upgrade, Hibernate started to automatically generate a foreign key that points to the same table. Now, when trying to persist a new customer, a ConstraintViolationException is thrown.

DETAILS

Here's a picture of what I mean:

Here is the code of the corresponding class:

@Entity
@Table(name = "customers")
@Inheritance(strategy = InheritanceType.JOINED)
public abstract class Customer extends AbstractModel {
public Customer(final Provider provider) {
this.provider = provider;
}
@Required
@ManyToOne(cascade = CascadeType.REFRESH)
public Provider provider;
@MaxSize(1023)
public String note;
public String getNote() { return note; }
...
}

As you can see, it extends from AbstractModel which is a @MappedSuperclass and contains the id property which serves as a primary key for all of our model classes. It's a @GeneratedValue.

It's a superclass itself. I don't know whether it's important so I am just going to include the code and schema of one of its child classes:

And the corresponding code:

@Entity
@Table(name="unregistered_customers")
public class UnregisteredCustomer extends Customer {
@MaxSize(MAX_SIZE_OF_NAMES_AND_IDENTIFIERS)
@Column(nullable = false, length = MAX_SIZE_OF_NAMES_AND_IDENTIFIERS)
public String userName;
@Email
@MaxSize(MAX_SIZE_OF_EMAIL_ADDRESSES)
@Column(unique = false, length = MAX_SIZE_OF_EMAIL_ADDRESSES)
public String email;
@MaxSize(MAX_SIZE_OF_NAMES_AND_IDENTIFIERS)
@Column(length = MAX_SIZE_OF_NAMES_AND_IDENTIFIERS)
public String mobile;
public UnregisteredCustomer(final Provider provider) {
super(provider);
}

RESEARCH

I already looked into the migration guide but nothing in there seems to be related to my issue.
I also looked into the Hibernate documentation especially into the section dealing with @Inheritance. Sadly, I didn't find anything concerning auto generated foreign keys.

QUESTION

How do I stop Hibernate from adding this self-referencing foreign key?

EDIT 1
As requested, the super class:

@MappedSuperclass
public abstract class AbstractModel extends AbstractBaseModel {
@Id
@GeneratedValue
public Long id;
public Long getId() {
return id;
}
@Override
public Object _key() {
return id;
}
}

Also, the super class of the super class:

@MappedSuperclass
public abstract class AbstractBaseModel extends GenericModel {
public static final int MAX_SIZE_OF_NAMES_AND_IDENTIFIERS = 80;
public static final int MAX_SIZE_OF_COMMENTS_AND_DESCRIPTIONS = 5000;
public static final int MAX_LIST_SIZE = 30;
public static final int MAX_SIZE_OF_EMAIL_ADDRESSES = 255;
public static final int MAX_SIZE_OF_JSON_CONTENT_FIELDS = 65535;
@Column(nullable = false, unique = true)
public String uuid;
@Column(nullable = false)
public Long created;
public DateTime getCreatedAsDate() {
return DateUtil.dateTimeWithSystemTimezone(created);
}
@Column(nullable = false)
public Long lastModified;
@PrePersist
protected void prePersist() {
final Long nowInMillis = DateUtil.dateTimeWithSystemTimezone().getMillis();
if (uuid == null) {
uuid = UuidUtil.newUUIDAsString();
Logger.trace("Created new Uuid for entity: %s", uuid);
}
if (created == null) {
created =  nowInMillis;
}
if (lastModified == null) {
lastModified = nowInMillis;
}
}
@PreUpdate
protected void preUpdate() {
lastModified = DateUtil.dateTimeWithSystemTimezone().getMillis();
}
@Override
public boolean equals(final Object obj) {
if (this == obj) {
return true;
}
if (obj == null) {
return false;
}
final AbstractBaseModel other = (AbstractBaseModel) obj;
return new EqualsBuilder().append(uuid, other.uuid).isEquals();
}
@Override
public int hashCode() {
return new HashCodeBuilder().append(uuid).toHashCode();
}
}

Put @Inheritance(strategy = InheritanceType.JOINED) to the parent entity (AbstractModel) not to the child. And customize the foreign key constraint using @PrimaryKeyJoinColumn at your child entity. Check out Part 4 (Joined Table) at https://www.baeldung.com/hibernate-inheritance to see it more clearly!