Translate user picker property editor value to V14 compatible format (and back again)

src/Umbraco.Core/PropertyEditors/UserPickerPropertyEditor.cs

@@ -1,3 +1,11 @@
+using Umbraco.Cms.Core.IO;
+using Umbraco.Cms.Core.Models;
+using Umbraco.Cms.Core.Models.Editors;
+using Umbraco.Cms.Core.Serialization;
+using Umbraco.Cms.Core.Services;
+using Umbraco.Cms.Core.Strings;
+using Umbraco.Extensions;
+
 namespace Umbraco.Cms.Core.PropertyEditors;
 
 [DataEditor(
@@ -9,4 +17,41 @@ public class UserPickerPropertyEditor : DataEditor
     public UserPickerPropertyEditor(IDataValueEditorFactory dataValueEditorFactory)
         : base(dataValueEditorFactory)
         => SupportsReadOnly = true;
+
+    protected override IDataValueEditor CreateValueEditor() =>
+        DataValueEditorFactory.Create<UserPickerPropertyValueEditor>(Attribute!);
+
+    private class UserPickerPropertyValueEditor : DataValueEditor
+    {
+        private readonly IUserService _userService;
+
+        public UserPickerPropertyValueEditor(
+            IShortStringHelper shortStringHelper,
+            IJsonSerializer jsonSerializer,
+            IIOHelper ioHelper,
+            DataEditorAttribute attribute,
+            IUserService userService)
+            : base(shortStringHelper, jsonSerializer, ioHelper, attribute)
+            => _userService = userService;
+
+        public override object? ToEditor(IProperty property, string? culture = null, string? segment = null)
+        {
+            // the stored value is the user ID - need to transform this into the corresponding user key
+            var value = base.ToEditor(property, culture, segment);
+            if (value is not string stringValue || stringValue.IsNullOrWhiteSpace())
+            {
+                return value;
+            }
+
+            return int.TryParse(stringValue, out var userId)
+                ? _userService.GetUserById(userId)?.Key
+                : null;
+        }
+
+        // the editor value is expected to be the user key - store it as the user ID
+        public override object? FromEditor(ContentPropertyData editorValue, object? currentValue)
+            => editorValue.Value is string stringValue && Guid.TryParse(stringValue, out Guid userKey)
+                ? _userService.GetAsync(userKey).GetAwaiter().GetResult()?.Id
+                : null;
+    }
 }