Minimal project for reproduction

• https://github.com/lwitzani/hibernate6_singleTableIssue
• project has two branches (one with hibernate 5 and one with hibernate 6)
• the only difference in the branches is the updated versions of spring boot and therefore hibernate
• i debugged very deep and think that it is a problem with hibernate at the location where the select statement is generated (LoaderSelectBuilder.createSelect)

Requisites

• entity ("Person") exists that has a field which is a set of other entities ("Set\<Leg>")
• these other entities are derived from an abstract super class ("Leg extends BodyPart")
• the abstract super class' inheritance strategy is SINGLE_TABLE

Reproduction

• execute the test PersonPersistenceTest.shouldUseSQLUpdateToChangeThePersonName
• the test basically
  • creates a Person (without Legs) and saves it using the repository
  • refetches the Person by findById
  • changes the Person's name
  • saves the changed Person again

Observations in Hibernate 5.6.14.Final

• when the second save is called (after changing the name) there will be a select statement which is used to check if the entity already exists
  • the body_part table is joined via left outer join
  • the where clause here only consists of "where person.id = \<id>"
• the entity is correctly found by the select statement so afterwards an update statement follows
• result: one entity exists (the changed one)

Observations in Hibernate 6.1.6.Final

• when the second save is called (after changing the name) there will be a different select statement now
  • the body_part table is joined via left join now (not outer)
  • the critical change here is that the where clause now suddenly consists of "where body_part.discriminator = 'LegBodyPart' and person.id = \<id>"
• although the entity exists, it is not found by the select statement so afterwards an insert statement follows which creates a second Person with a new id
• result: two entities exist (the original one and the changed one)

Summary

• after updating spring boot to 3.0.1 which also updates hibernate to 6.1.6.Final it does not seem to be possible to update an entity with this constellation
• the main difference is that an additional and unexpected where clause is added which prevents finding the correct entity in this case
• the result is that a new entity is persisted instead of the existing one being updated