I want to have add more tags to the post by this way

post.tags.add(existedTag)
postRepo.save(post)

but It didn't work

Send here your Post JPA entity (that owns the relationship)

this is my user-role case, It have the same issue

@Entity
class Userr {
    @Id
    @GeneratedValue
    var id: Long? = null

    var name: String? = null

    @OneToMany(mappedBy = "user", cascade = [CascadeType.PERSIST])
    var roles: MutableList<Role> = mutableListOf()
}

@Entity
class Role {
    @Id
    @GeneratedValue
    var id: Long? = null

    var name: String? = null

    @ManyToOne
    @JsonIgnore
    var user: Userr? = null
}

does it work with

saveAndFlush

? Do you use

@Transactional

?

I don't use @Transactional let me try with saveAndFlush

I wouldn't use

@OneToMany

, because it's logically incorrect, a single post can have many tags and a tag itself can associate with multiple posts, this is

@ManyToMany

. But, try this:

<http://existedTag.post|existedTag.post> = post
post.tags.add(existedTag)
postRepo.save(post)

yes, you are right, I just demonstrate the issue. think about another example. User will have list of addresses how can I easy add new address to user

val newAddress = Address()
newAddress.user = user // redundant
user.addresses.add(newAddress)
userRepo.save(user)

but if this is the only way to work in kotlin, I'm ok BTW, thank you very much @Mikhail