This isn’t how I would think of this. The ultimate consumer (the page containing this template) wants a list of completed items. It doesn’t want to worry about the fact that they need to be stitched together. So I would design this so that by the time these items get to the array, they are done. You could have two separate interfaces, one “heavy” one containing anotherField and one “light” one without it, or you could make anotherField optional, but either way you would be looking at something like the structure of the WombatService.