Moving @ElementCollection to @Embeddable class not working

I am trying to move an @ElementCollection annotated field from my @Entity class into a different class that I annotated with @Embeddable. When the field is in the entity, Hibernate will issue the proper INSERT statements. When the field is moved to the @Embeddable class, no INSERT statements are generated.

So this works:

@Entity
public class MyEntity extends AbstractEntity<MyEntityId> {
  @ElementCollection
  @OrderColumn
  private List<Double> doubles;
}

But this does not:

@Entity
public class MyEntity extends AbstractEntity<MyEntityId> {
  @Embedded
  private Doubles doubles;
}

@Embeddable
public class Doubles {
  @ElementCollection
  @OrderColumn
  private List<Double> doubles;
}

There is no exception when saving a MyEntity instance (and flushing the EntityManager). If I disable creating the appropriate table in Flyway, the Hibernate validation does trigger a "missing table [my_entity_doubles]" error, so Hibernate does seem to correct interpret the setup.

Using Spring Boot 2.3.4 that uses Hibernate 5.4.21.

UPDATE: There is a reproducible testcase at https://github.com/wimdeblauwe/so-64336648

I don't know why. But assigning with a collection works.

@Embeddable
public class Doubles {
  @ElementCollection
  private List<Double> doubles = new ArrayList<>();
}

As it stated in the JPA specification (see 2.5 Embeddable Classes section):

> An embeddable class (including an embeddable class within another embeddable class) may contain a collection of a basic type or other embeddable class. Direct or indirect circular containment dependencies among embeddable classes are not permitted.

And simple example (based on Hibernate 5.4.21).

Assuming that we have the following schema:

create table T1_MY_ENTITY
(
   tm_id int not null,

   primary key (tm_id)
);

insert into T1_MY_ENTITY values (1);

create table T1_MY_COLL
(
   tc_tm_id int not null,
   tc_ord_id int not null,
   tc_code varchar(100),

   foreign key (tc_tm_id) references T1_MY_ENTITY(tm_id)
);

insert into T1_MY_COLL values (1, 1, 'CL1'), (1, 0, 'CL2');

and the following mapping:

@Entity
@Table(name = "T1_MY_ENTITY")
public class MyEntity
{
   @Id
   @Column(name = "tm_id")
   private Long id;

   @Embedded
   private Doubles dou;
   
   // constructor, getters, setters
}

@Embeddable
public class Doubles
{
   @OrderColumn(name = "tc_ord_id")
   @ElementCollection
   @CollectionTable(name = "T1_MY_COLL", joinColumns = @JoinColumn(name = "tc_tm_id"))
   @Column(name = "tc_code")
   private List<String> colls;

   // constructor, getters, setters
}

the following code:

MyEntity myEntity =  entityManager.find(MyEntity.class, 1L);
myEntity.getDou().getColls().add("CL5");
entityManager.merge(myEntity);

will generate insert:

19:51:29,612 DEBUG SQL:128 - 
    /* insert collection
        row com.my.hibernate.entities.MyEntity.dou.colls */ insert 
        into
            TEST_SCHEMA.T1_MY_COLL
            (tc_tm_id, tc_ord_id, tc_code) 
        values
            (?, ?, ?)
Hibernate: 
    /* insert collection
        row com.my.hibernate.entities.MyEntity.dou.colls */ insert 
        into
            TEST_SCHEMA.T1_MY_COLL
            (tc_tm_id, tc_ord_id, tc_code) 
        values
            (?, ?, ?)
19:51:29,612 TRACE ResourceRegistryStandardImpl:83 - Registering statement [/* insert collection row com.my.hibernate.entities.MyEntity.dou.colls */ insert into TEST_SCHEMA.T1_MY_COLL (tc_tm_id, tc_ord_id, tc_code) values (?, ?, ?)]
19:51:29,612 TRACE BasicBinder:64 - binding parameter [1] as [BIGINT] - [1]
19:51:29,613 TRACE BasicBinder:64 - binding parameter [2] as [INTEGER] - [2]
19:51:29,613 TRACE BasicBinder:64 - binding parameter [3] as [VARCHAR] - [CL5]

So, It looks like your problem lies in the other plane.

UPDATE: As for your test case, please try to correct it in the following way:

@Test
void test() {
   MyEntity2 entity = new MyEntity2();
   Doubles doubles = new Doubles();
   doubles.setDoubles(List.of(1.0, 1.5));

   entity.setDoubles(doubles); // It was absent !!!

   repository.save(entity);

   entityManager.flush();

   assertThat(jdbcTemplate.queryForObject("SELECT count(*) FROM my_entity2_doubles", Long.class)).isEqualTo(2);
}