如何使用Spring Data从不同的表中获取额外的列？

Question

So lets imagine following situation. I have an entity such as this:

@Entity
public class Price {

    @Id
    private int id;

    @Column
    private int amount;

    private String currency;

}

And I have two tables:

CREATE TABLE currency (
id integer not null primary key,
name varchar
);
CREATE TABLE price (
    id integer not null primary key,
    amount integer,
    currency_id integer references currency(id)
);

I want to tell Spring that when I access Price.getCurrency() I want to have whatever is stored in column "name" of the "currency" table. In other words, I want to connect two tables in one entity.

I can make currency a separate class, annotate the property with @OneTo... and get it like price.getCurrency().getName(). But I don't want a separate class, I just need this specific column.

I tried adding it via @SecondaryTable annotation like this:

@SecondaryTable(name = "currency",
            pkJoinColumns = @PrimaryKeyJoinColumn(name = "id", referencedColumnName = "currency_id"))

But in this case Spring connect two tables by it's ids like this:

SELECT * FROM price LEFT JOIN price ON price.id = currency.id

And of course it is not working. So how do I do this? Is @SecondaryTable a correct way and if so how do I connect it through non-primary key column?

Answer 1

Yes, you can use @SecondaryTable :

@Entity
@Table(name = "price")
@SecondaryTable(
    name = "currency",
    pkJoinColumns = {
        @PrimaryKeyJoinColumn(name = "id", referencedColumnName = "currency_id")
    })
public class Price {

    @Id
    private int id;

    @Column
    private int amount;

    @Column(table = "currency", name = "name")
    private String currency;

}