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kotlinlang

Hello everyone! I am beginning to start a new project with Kotlin 1.2.71 + Spring Boot 2.0.6 with JPA. I have written in Java before this time and I have questions about Kotlin (compile time and runtime features). There are features those surprised me a little bit.

I enable plugins for JPA (you know pure Kotlin couldn’t work with old frameworks for Java like Hibernate or Spring)
```
id 'org.jetbrains.kotlin.plugin.allopen' version '1.2.71'
id 'org.jetbrains.kotlin.plugin.noarg' version '1.2.71'
id 'org.jetbrains.kotlin.plugin.spring' version '1.2.71'
id 'org.jetbrains.kotlin.plugin.jpa' version '1.2.71'
```
Then I created entity instance and saved it in RDBMS
```
@Entity
data class User(
        @Id
        @SequenceGenerator(name = "users_id_generator", sequenceName = "users_id_seq")
        @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "users_id_generator")
        val id: Long = 0,
        val userId: String
}
```
```
    @Test
    fun `find by id and user id should return the saved User`() {
        val user = createUser(1)
        assertThat(user.id).isZero()

        val savedUser = userRepository.saveAndFlush(user)
        assertThat(user.id).isPositive()
        // ...
   }

    private fun createUser(userId: Long): User = User(userId = userId.toString())
```