h2. Minimal project for reproduction
* [https://github.com/lwitzani/hibernate6_singleTableIssue|https://github.com/lwitzani/hibernate6_singleTableIssue|smart-link]
* 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}})
h2. 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
!uml.png|width=517,height=425!
h2. 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
h2. 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|http://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)
!hibernate5.png|width=1265,height=692!
h2. 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|http://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)
!hibernate6.png|width=1179,height=644!
h1. 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 |
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