Add comparison operators for optional columns

[seanmrich]

Consider a table defined as:

@Table
struct Value {
  var a: Int?
}

If you want to fetch all Values with a < 10, you might try this:

Value
  .where { $0.a < 10 }

By adding the new overloads, this request now compiles.

[stephencelis]

@seanmrich Thanks for taking the time to explore and submit this. We've considered overloads here in the past but are a bit worried about type checking times and general number of overloads required. Here you've added the overloads for T? < T, but there's also T < T? to consider.

Our current workaround is to suggest an explicit #sql string:

Value
  .where { #sql("\($0.a) < 10") }

While slightly unfortunate, it does make it explicit that we are working with a comparison that would not work in the Swift type system, but we know what we're doing.

We'll think on it a bit more, though. Thanks for bringing it up!

[stephencelis]

@seanmrich We discussed this and for now are worried about the slippery slope nature of the thing, where more and more operators need optional overloads. We're open to revisit in the future, but want to hold off on such functionality for now. Thanks again for taking the time, though! You should be able to maintain these overloads in your own projects should you find them useful, and maybe with more real world use you can start a discussion here in the future with motivating examples.

[seanmrich]

Thanks for looking into it. I spent some time trying to work out a generic approach that improves the ergonomics here, but nothing practical came out of it.

For now, there's a couple ways to make it more readable.

.where { $0.a < Int?.some(10) } retains some type information that the #sql approach erases.

Another quickie that worked for me was this extension:

extension QueryExpression {
  public var optional: some QueryExpression<Optional<QueryValue>> {
    SQLQueryExpression(self.queryFragment, as: Optional<QueryValue>.self)
  }
}

This yields a more readable .where { $0.a < 10.optional }