NSComboBox to display unique items from a coredata entity binding of a property

Question

I have a typical Mac CoreData application which displays my entities in an NSTableView with an NSArrayController, and a panel on the right to edit/display the currently selected item in the NSTableView. I have an Entity (Pigeon) with an NSString property (colour) being displayed in an NSComboBox, so that the user can enter their own string for the colour or select a previously chosen colour from the drop down list. This is done by binding the Content Values of the NSComboBox to Pigeons.arrangedObjects.colour. This is sooooooo close to what I want, but because it's an array (and not a set) of all the colours that exist for the pigeons, duplicate values are listed in the drop down list of the NSComboBox. So if for example Blue is entered for the colour for one Pigeon, and it's also selected for a second Pigeon, then Blue is shown in the list twice.

Is there someway, somehow I can filter this to remove duplicate values? I've tried making a second NSArrayController which is bound to the Pigeons.arrangedObjects.colour and setting an NSPredicate to filter it, but I can't seem to figure out an NSPredicate for filtering out duplicate values. I want to use bindings for the values of the colours, so that as the application is running, if a colour is added or deleted (maybe a mistake was made in entering a colour), then the drop down list is updated to only have the currently entered colour values for Pigeons.

What is the best way to not showing these duplicate values?

Answer 1

I am on my phone and a little lazy, but I think you are looking for a keypath that looks like: @"@distinctunionofobects.someproperty"... Or google keypath operators.