Disallow moving relation endpoint if the relation after move already exists (#126)

Author: AlexeyMz

CHANGELOG.md

@@ -7,6 +7,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/) and this p
 #### 🐛 Fixed
 - Fix stale rendering (i.e. missing links after moving an endpoint until an element move) due to the lost scheduled layer updates.
 - Fix unable to scroll inside canvas components and templates when `requireCtrl` in `zoomOptions` is set `false`.
+- Disallow moving relation source or target to another entity in authoring mode if the relation after move already exists on the canvas.
 - Disable changing an entity IRI in a form when `MetadataProvider.canModifyEntity()` disallows it.
 - Move link vertices together with source and target elements if both are selected in `Selection` component.
 

src/widgets/visualAuthoring/dragEditLayer.tsx

@@ -113,11 +113,17 @@ export function DragEditLayer(props: DragEditLayerProps) {
                 return new EntityDragConnection([], workspace);
             }
 
+            const linkTypeId = link.data.linkTypeId === PlaceholderRelationType
+                ? undefined : link.data.linkTypeId;
+            if (target && linkTypeId && model.findLink(linkTypeId, source.id, target.id)) {
+                // Link already exists
+                return new EntityDragConnection([], workspace);
+            }
+
             const canConnect = await metadataProvider.canConnect(
                 source.data,
                 target?.data,
-                link.data.linkTypeId === PlaceholderRelationType
-                    ? undefined : link.data.linkTypeId,
+                linkTypeId,
                 {signal}
             );
             return new EntityDragConnection(canConnect, workspace);