When you use a key (or set of keys) in the projection section of querybuilder you have no ability to choose any additional keys to use for the Query, Projectection, or Sort sections. You have to either remember the column name and type it in directly or open a new view of the same collection to get the name.

What would be awesome is if in the area "Drag field here or double-click to add field" you could get a drop-down list of keys to choose from. Or open up a pop-up list of fields like you get when you click on "Add Fields" in the "Add Index" section of the application