Added a hint in the documention for using custom java types with a JSON mapping

[peter1123581321]

We lately run into a problem at my company when using @JdbcTypeCode(SqlTypes.JSON) on a custom java type that triggered many unnecessary update statements. The cause of the problem was that we missed to override the equals-method, in consequence, the dirty-checking mechanism didn't work as expected anymore and all of the managed entities get updated even if nothing has changed.

I found that other users faced the same or a similar problem, e.g.

• https://discourse.hibernate.org/t/extra-update-during-insert-of-entity-with-jsonfield-jdbctypecode-sqltypes-json-property/11908
• https://hibernate.atlassian.net/browse/HHH-17660

Hence, I think it's worth to add a small hint in the documentation.

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[beikov]

FWIW, we have HHH-17709 which would potentially solve this problem, although it would be a bit inefficient. Generally, I'd say @Embeddable is recommended, as that allows Hibernate ORM to peek into the objects and figure this all out more efficiently.