Allow Draft to have Optional values for columns with default values

[alephao]

I've been trying out the library and came across a use-case that seem to be generic enough that might be worth adding to the library:

Let's say I have a table with a default value:

CREATE TABLE my_table (
  created_at INTEGER DEFAULT(unixepoch()) NOT NULL
);

and the swift type would be:

@Table("my_table")
struct MyTable {
  @Column("created_at", as: Date.UnixTimeRepresentation.self)
  let createdAt: Date
}

The Draft API is a pretty nice way to insert a row on this table, but at the moment I need to pass a Date like so:

let draft = MyTable.Draft(createdAt: Date())

Would be great if I could annotate the property so the createdAt becomes optional on MyTable.Draft, similar to the @Column(primaryKey: true) macro. So I can use it like so:

let draft = MyTable.Draft()

One idea is to extend the @Column to accept a boolean and use it to decide if the generated property on Draft is optional or not, e.g.: hasDefault: Bool or optionalDraft: Bool. Then we can use it like so:

@Table("my_table")
struct MyTable {
  @Column("created_at", as: Date.UnixTimeRepresentation.self, hasDefault: true)
  let createdAt: Date
}

Let me know what you think, I'm happy to work on it and send a PR if you think that fits in the library scope.

[mbrandonw]

Hi @alephao, yeah this is a great thing we would love to support. We've been thinking that @Column could have a databaseInitialized: Bool option that allows one to omit that value from drafts. We're still thinking through all of the edge cases, but hopefully we will have something to share soon!

[alephao]

Awesome! Sounds like it's something in the works already, let me know if you need any help!

Thanks for the quick reply!

[stephencelis]

@alephao If you want to explore we'd be happy to help answer any questions! We don't know exactly when we'll find time to explore it ourselves, though it's definitely a feature we want.

[alephao]

created a PR here #36