Hi all. I am using data class for my database enti...

# getting-started

[Gasan]

Hi all. I am using data class for my database entities. I have a parent child relationship, where child has a reference to a parent and a parent has a collection of its children. So a parent references a child and the child references the parent. A circular reference. How to create parent and child, taking into account that they are immutable?

[Gasan]

@Ruckus rethink, but how? My database entity has a reference to a parent (foreign key).

[Gasan]

Btw. Thanks a lot for the code snippet btw. I think I got it. So Parent has a list of children. I create a parent with an empty list and then add children there. Thank you so much.

[Gasan]

Well, if I look at my database structure. The parent column is nullable. Btw parent and child is the same entity represented by the same type in the application code.

[Gasan]

Could you advise anything to read on the topic? Books, etc?

[Gasan]

Thanks a lot. Very interesting read.

[Goetz Markgraf]

You could start with one of the links empty and then create a changed version (.copy()) that contains the second link.

[Gasan]

@Goetz Markgraf
Immutable properties do not work for circular references:

parent = Parent()
child = Child(parent)
parent.copy(child = child)

The child of the parent has a link to the old parent without children, not the new one.

For circular references some form of mutability is required. See options in previous replies.

[Goetz Markgraf]

Sorry, you‘re absolutely right

[Gasan]

hi all. I finally went back to implement this in my code.
I got this result:

data class Entity(val name, val parent: Entity?, var children: List<Entity>?)

1. parent and children is a part of the data class because I want to copy() them
2. I must override equals(), hashCode() and toString() otherwise I get stack overflow error.