Uni-Directional ManyToOne

Use the @ManyToOne annotation in conjunction with @JoinColumn to describe a many-to-one relationship.

Annotate the defining field in the owning entity to specify mapping details.

For a many-to-one, the many side of the relationship will always be the owner.
This is the entity for the table containing the foreign key column.

Like before, use the @JoinColumn annotation to specify the foreign key column name.

@Entity
public class Address {
     // ...
     @ManyToOne
     @JoinColumn(name="country_code")
     private Country country;
     // ...
}

For uni-directional, the non-owning side of the relationship remains unchanged.

Drill¶

Create a new Country entity for the country table.

Add a field called countryCode and annotate it as an @Id mapped to the country_code column.

Note this id is NOT a generated value.

Also create a field called name mapped to the country column.

The country table looks like this:
+--------------+-------------+------+-----+---------+-------+
| Field        | Type        | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+-------+
| country_code | char(2)     | NO   | PRI | NULL    |       |
| country      | varchar(50) | NO   |     | NULL    |       |
| formal_name  | varchar(60) | YES  |     | NULL    |       |
| sovereignty  | varchar(30) | YES  |     | NULL    |       |
| capital      | varchar(80) | YES  |     | NULL    |       |
| iso3_code    | char(3)     | YES  |     | NULL    |       |
| tld          | char(3)     | YES  |     | NULL    |       |
+--------------+-------------+------+-----+---------+-------+
Generate a no-arg constructor, gets/sets and the toString() method.

Create a JUnit test, CountryTest, to confirm JPA can retrieve the country France using the country code FR.

Create a uni-directional, many-to-one relationship between your Address and Country entities.

Modify Address, add a field of type Country annotated with @ManyToOne and @JoinColumn.

Also, generate get/set methods.

Update your AddressTest, adding a test named test_Address_Country_ManyToOne_relationship_mapping which checks that the appropriate Country entity is loaded for the Address.

The Address object with id = 1 has CA (Canada) as a country_code.

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