JPA and Hibernate: best way to mapping a primary key as UUID in MySQL 8.x

Proposal.kt

@Entity
class Proposal(
    val name: String,
    //
    @Column(columnDefinition = "binary(16)") // this works but uses a MySQL's specific type
    val customerId: UUID
) {

    @Id
    @GeneratedValue
    /**
     * By default Hibernate creates a column as BINARY(255) that works on INSERT but fails on SELECT
     * In fact, BINARY(16) works like a CHAR on MySQL:
     *  - https://dev.mysql.com/doc/refman/8.0/en/binary-varbinary.html
     */
    @Column(length = 16) // that works too and it's more compatible with other databases
    val id: UUID? = null

}

ProposalRepositoryTest.kt

@MicronautTest
class ProposalRepositoryTest(private val repository: ProposalRepository) {

    @Test
    fun `should find by id`() {
        // scenario
        val proposal = repository.save(Proposal(
            name = "p1",
            customerId = UUID.randomUUID()
        ))

        // action
        val found = repository.findById(proposal.id)

        // validation
        with(found) {
            assertTrue(isPresent)
            assertEquals(proposal.id, get().id)
            assertEquals(proposal.customerId, get().customerId)
        }
    }

    @Test
    fun `should find by id and customer id`() {
        // scenario
        val proposal = repository.save(Proposal(
            name = "p2",
            customerId = UUID.randomUUID()
        ))

        // action
        val found = repository.findByIdAndCustomerId(
            id = proposal.id!!,
            customerId = proposal.customerId
        )

        // validation
        with(found) {
            assertTrue(isPresent)
            assertEquals(proposal.id, get().id)
            assertEquals(proposal.customerId, get().customerId)
        }
    }
}

You can find more details and integration tests with Micronaut in this repository