Why Spring data jpa 'findByEntityId' queries with join operation?-CodePudding

Let's say I have two entities.

@Entity
public class Foo {
   
    @Id (...)
    private Long id;

    @OneToOne(...)
    private Bar bar;

    // and other fields
}

@Entity
public class Bar {

    @Id(...)
    private Long id;

    // and other fields
}

When I create method FooRepository#findByBarId(Long barId) then the SQL created by Spring Data Jpa is like as belows

select ... 
from foo 
   left outer join bar where foo.id = bar.id 
where bar.id = ?

I expected this to be described as below.

select ...
from foo
where foo.bar_id = ?

I know that this can be solved by fixing method into FooRepository#findByBar(Bar bar). But I wanna know the reason for this.

CodePudding user response：

This is because inside foo entity there is a one to one relationship.By default one to one support eager fetch/loading.that mean jpa construct qurery to fetch match entity as well as the entities has relations for that entity.

CodePudding user response：

By nature of the method name you are asking hibernate to do a join. So event though there is a bar_id column in the foo table it will perform a join enabling access to the columns in the bar table

This is the required behaviour as you may be doing a similar method call but using a name field in bar where there is obviously no name column in the foo table

findByBarName