Is there a way to know if a @Table element is already stored (e.g. a computed isStored)?

[gorpello]

Thank you for your great work on this project!
I have a use case where I fetch a list of items from an external API, and the user can save or unsave items.

In my UI, I need to show different states depending on whether an item is already stored in the database or not (for example, show a “Save” button if not stored, or “Remove” if it already exists).

Is there already a mechanism in this library to expose whether a @table element is stored in the database (e.g. a computed isStored property)?
Or, if not, what would be the recommended way to model this use case?
I was wondering if something like this could make sense:

`@Table
struct Item {
var id: Int
var title: String

// maybe not persisted in DB, but useful in app logic
var isStored: Bool { 
    // true if this row already exists in DB
}

}`

Thanks a lot for your time.
Any advice or best practice would be greatly appreciated!

[mbrandonw]

Hi @gorpello, the library does not provide this tool directly because the only way to know that information is to query the database. A simple computed property of a value type cannot have knowledge of whether or not it is persisted do the database. But we do provide the tools that make it possible to figure this out by thinking about the problem a little differently.

To start with, if you have an Item value in your feature, then that should mean that it is a persisted item because the item holds onto its id, and ideally only the database assigns IDs. Then, to represent a value that could be or could not be persisted you can use the Item.Draft type that is generated for you by the @Table macro. It represents edits to an item that have not be persisted yet, and it's id field is optional.

That allows you to represent an item that has not yet been inserted into the database when id: nil:

@Table
struct Item {
  var id: Int
  var title = ""
}

struct ItemForm: View {
  @State var item: Item.Draft

  var body: some View {
    if item.id == nil {
      // New item
    } else {
      // Existing item
    }
  }
}

[gorpello]

Thank you very much for taking the time to reply.
I’ll run some tests on my side, and if I find a useful approach or pattern I’ll be happy to come back here and share my experience.

[mbrandonw]

No problem. One last trick to know is that you can turn an Item into an Item.Draft using a special initializer also generated by the macro: Item.Draft(item).