Storing a generic table column

[seanmrich]

Imagine supporting an undo feature that can undo/redo a single column change. Here's some non-working code that gets at what I'm trying to accomplish:

struct UndoInfo<Entity, Value> 
where Entity: PrimaryKeyedTable, Value: QueryBindable {
  let primaryKey: Entity.PrimaryKey
  let column: KeyPath<Entity.TableColumns, TableColumn<Entity, Value>>
  let before: Value
  let after: Value
}

When you modify a value, you'd generate an UndoInfo that could then be passed to a general-use undo/redo method:

func toggle<Entity, Value>(
  info: UndoInfo<Entity, Value>,
  undo: Bool,
  db: Database
) throws {
  let value = undo ? info.before : info.after
  try Entity
    .find(info.primaryKey)
    .update {
      $0[dynamicMember: info.column] = value
    }
    .execute(db)
}

This doesn't work, of course, but hopefully you can understand the intent. I think I'm mistakenly trying to assign a value rather than create the query fragment, but I can't seem to untangle the types involved.