Macro generate Table -> Draft convenience initializer.

[DandyLyons]

Currently @Table generates a Draft type and an initializer that makes it easy to convert a base Table into a Draft. It would be nice if there were a convenience initializer to convert a Draft into a table type. Specifically it would be nice if the macro generated something like this:

@Table MyTable {
  init(id: Self.ID, draft: Self.Draft) {
    // ...
  }
}

Use Case

In production this initializer isn't all that helpful. But for tests this can be quite helpful. For example, when using upsert(or:values:) you must provide a Draft. And when testing you want to assert that the write you performed caused the change you expected. But when you fetch, you can only fetch a Table, not a Draft. So now you need to convert a Draft into a Table so that you can make your assertion.

Workarounds

It's not difficult to make your own init(id:draft:) initializer which solves this problem. But this more code to maintain. Macros are perfect for this problem.

[stephencelis]

But when you fetch, you can only fetch a Table, not a Draft.

You actually can interact with Drafts just like you do Tables:

swift-structured-queries/Tests/StructuredQueriesTests/PrimaryKeyedTableTests.swift

Line 119 in ddaa35e

Reminder.Draft.find(1).select { ($0.id, $0.title) }

Does that help?

[DandyLyons]

Here's the test where I found myself wishing I had an init(id:draft:).

    @Test("Upsert New Flashcard")
    func upsertNewFlashcard() async throws {
      #expect(allFlashcards.count == 8)
      guard let firstDeck = fetchAllDecks.wrappedValue.first else {
        reportIssue("No decks available"); return
      }
      
      let newFlashcard = Flashcard.Draft(
        deckID: firstDeck.id,
        frontText: "New Front",
        backText: "New Back"
      )
      
      await withFetch($allFlashcards,
        performDBWrite: {
          DBMutator.upsertFlashcard(newFlashcard)
        },
        assert: {
          $0.append(Flashcard(id: UUID(8), draft: newFlashcard))
        }
      )
      #expect(allFlashcards.count == 9)
    }

(For context, I'm also using some test helpers here. withFetch is a TCA-style exhaustive assertion.)
In this test, I just want to upsert the draft, let the DB generate the ID, and assert that the ID and all the columns and all the rows match what I expected.