Question

The problem I have is that I have an NSOutlineView with Drag and Drop (see the bottom of post for code to make drag and drop work), it works, but when I drag one row into another row the dragged row becomes a child but it also stays where it was as a parent, and when I delete either row both of them get deleted.
To show you what I mean, I have recorded the problem, here is the link – http://dvlp.me/6kya9

And here is the code which makes drag and drop work in the outline view.

Header File.

@interface _NSControllerTreeProxy : NSObject 
{
    // opaque
}
//
// Number of objects at the root level.
//
- (unsigned int)count;

- (id)nodeAtIndexPath:(id)fp8;
- (id)objectAtIndexPath:(id)fp8;
@end

@interface _NSArrayControllerTreeNode : NSObject
{
    // opaque
}
- (unsigned int)count;
- (id)observedObject;
- (id)parentNode;
- (id)nodeAtIndexPath:(id)fp8;
- (id)subnodeAtIndex:(unsigned int)fp8;
- (BOOL)isLeaf;
- (id)indexPath;
- (id)objectAtIndexPath:(id)fp8;
@end

// some more detailed reverse engineering is available here
// http://www.blueboxmoon.com/wiki/?page=Binding%20Tree


@interface DragController : NSObject
{
    IBOutlet NSTreeController *groupTreeControl;
    IBOutlet NSOutlineView *treeTable;

    NSArray* dragType;

    _NSArrayControllerTreeNode* draggedNode;


}

- (void) resortGroups:(NSManagedObjectContext*)objectContext forParent:(NSManagedObject*)parent;
- (NSArray* ) getSubGroups:(NSManagedObjectContext*)objectContext forParent:(NSManagedObject*)parent;
- (BOOL) category:(NSManagedObject* )cat isSubCategoryOf:(NSManagedObject* ) possibleSub;

@end

#define INTERVAL 10

Implemetation File.

@implementation DragController
- (void)awakeFromNib {  

    dragType = [NSArray arrayWithObjects:   @"factorialDragType", nil];

    [ dragType retain ]; 

    [ treeTable registerForDraggedTypes:dragType ];
    NSSortDescriptor* sortDesc = [[NSSortDescriptor alloc] initWithKey:@"position" ascending:YES];
    [groupTreeControl setSortDescriptors:[NSArray arrayWithObject: sortDesc]];
    [ sortDesc release ];
}   


//------------------------------------
#pragma mark NSOutlineView datasource methods -- see NSOutlineViewDataSource
//---------------------------------------------------------------------------   
- (BOOL) outlineView : (NSOutlineView *) outlineView 
          writeItems : (NSArray*) items 
        toPasteboard : (NSPasteboard*) pboard {

    [ pboard declareTypes:dragType owner:self ];        
    // items is an array of _NSArrayControllerTreeNode see http://theocacao.com/document.page/130 for more info
    draggedNode = [ items objectAtIndex:0 ];

    return YES; 
}




- (BOOL)outlineView:(NSOutlineView *)outlineView acceptDrop:(id <NSDraggingInfo>)info item:(id)item childIndex:(int)index {

    _NSArrayControllerTreeNode* parentNode = item;
    _NSArrayControllerTreeNode* siblingNode;
    _NSControllerTreeProxy* proxy = [ groupTreeControl arrangedObjects ];

    NSManagedObject* draggedGroup = [ draggedNode observedObject ];

    BOOL draggingDown = NO;
    BOOL isRootLevelDrag = NO;

    // ----------------------
    // Setup comparison paths
    // -------------------------
    NSIndexPath* draggedPath = [ draggedNode indexPath ];
    NSIndexPath* siblingPath =  [ NSIndexPath indexPathWithIndex:  index  ];
    if ( parentNode == NULL ) {     
        isRootLevelDrag = YES;
    } else {
        // A non-root drag - the index value is relative to this parent's children
        siblingPath = [ [ parentNode indexPath ] indexPathByAddingIndex: index ];
    }

    // ----------------------
    // Compare paths - modify sibling path for down drags, exit for redundant drags
    // -----------------------------------------------------------------------------    
    switch ( [ draggedPath compare:siblingPath] ) {
        case NSOrderedAscending:  // reset path for down dragging
            if ( isRootLevelDrag ) {
                siblingPath = [ NSIndexPath indexPathWithIndex: index  - 1];                             
            } else {
                siblingPath = [ [ parentNode indexPath ] indexPathByAddingIndex: index - 1 ]; 
            }
            draggingDown = YES;
            break;

        case NSOrderedSame:
            return NO;
            break;               
    }

    siblingNode = [ proxy nodeAtIndexPath:siblingPath ];    

    //  NSLog(@"returning early");
    //  return NO;  // TODO robustify


    // ------------------------------------------------------------ 
    // SPECIAL CASE: Dragging to the bottom
    // ------------------------------------------------------------
    // - K                               - K                            - C                              - C
    // - - U                             - - C     OR     - U                                - F
    // - - C     ====>     - - F                    - F                              - K
    // - - F               - U              - K                              - U
    // ------------------------------------------------------------ 
    if ( isRootLevelDrag  && siblingNode == NULL ) {        
        draggingDown = YES;
        siblingPath = [ NSIndexPath indexPathWithIndex: [ proxy count ] - 1 ];          
        siblingNode = [ proxy nodeAtIndexPath:siblingPath ] ;
    }

    // ------------------------------------------------------------ 
    // Give the dragged item a position relative to it's new sibling
    // ------------------------------------------------------------ 
    NSManagedObject* sibling = [ siblingNode observedObject ];   
    NSNumber* bystanderPosition = [ sibling valueForKey:@"position"];
    int newPos =   ( draggingDown ? [ bystanderPosition intValue ]  + 1 : [ bystanderPosition intValue ]  - 1 );
    [draggedGroup setValue:[ NSNumber numberWithInt:newPos ] forKey:@"position"];   

    // ----------------------------------------------------------------------------------------------
    // Set the new parent for the dragged item, resort the position attributes and refresh the tree
    // ----------------------------------------------------------------------------------------------    
    [ draggedGroup setValue:[ parentNode observedObject ] forKey:@"parent" ];
    [ self resortGroups:[draggedGroup managedObjectContext] forParent:[ parentNode observedObject ] ];          
    [ groupTreeControl rearrangeObjects ];  
    return YES;             
}






- (NSArray* ) getSubGroups:(NSManagedObjectContext*)objectContext forParent:(NSManagedObject*)parent {
    NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"projects" inManagedObjectContext:objectContext];

    [request setEntity:entity];
    NSSortDescriptor* aSortDesc = [[NSSortDescriptor alloc] initWithKey:@"position" ascending:YES];
    [request setSortDescriptors:[NSArray arrayWithObject: aSortDesc] ];
    [aSortDesc release];

    NSPredicate* validationPredicate = [NSPredicate predicateWithFormat:@"parent == %@", parent ];

    [ request setPredicate:validationPredicate ];

    NSError *error = nil;  // TODO - check the error bozo
    return [objectContext executeFetchRequest:request error:&error];    
}




- (void) resortGroups:(NSManagedObjectContext*)objectContext forParent:(NSManagedObject*)parent {

    NSArray *array = [ self getSubGroups:objectContext forParent:parent ];

    // Reset the indexes...
    NSEnumerator *enumerator = [array objectEnumerator];
    NSManagedObject* anObject;
    int index = 0;
    while (anObject = [enumerator nextObject]) {
        // Multiply index by 10 to make dragging code easier to implement ;) ....
        [anObject setValue:[ NSNumber numberWithInt:(index * INTERVAL ) ] forKey:@"position"];    
        index++;
    }   


}

- (NSDragOperation)outlineView:(NSOutlineView *)outlineView validateDrop:(id <NSDraggingInfo>)info proposedItem:(id)item proposedChildIndex:(int)index {

    _NSArrayControllerTreeNode* newParent = item;

    // drags to the root are always acceptable
    if ( newParent == NULL ) {  
        return  NSDragOperationGeneric; 
    }

    // Verify that we are not dragging a parent to one of it's ancestors
    // causes a parent loop where a group of nodes point to each other and disappear
    // from the control 
    NSManagedObject* dragged = [ draggedNode observedObject ];      
    NSManagedObject* newP = [ newParent observedObject ];

    if ( [ self category:dragged isSubCategoryOf:newP ] ) {
        return NO;
    }       

    return NSDragOperationGeneric;
}

- (BOOL) category:(NSManagedObject* )cat isSubCategoryOf:(NSManagedObject* ) possibleSub {

    // Depends on your interpretation of subCategory ....
    if ( cat == possibleSub ) { return YES; }

    NSManagedObject* possSubParent = [possibleSub valueForKey:@"parent"];   

    if ( possSubParent == NULL ) {  return NO; }

    while ( possSubParent != NULL ) {       
        if ( possSubParent == cat ) { return YES;   }

        // move up the tree
        possSubParent = [possSubParent valueForKey:@"parent"];          
    }   

    return NO;
}




// This method gets called by the framework but the values from bindings are used instead
- (id)outlineView:(NSOutlineView *)outlineView objectValueForTableColumn:(NSTableColumn *)tableColumn byItem:(id)item { 
    return NULL;
}

/* 
 The following are implemented as stubs because they are required when 
 implementing an NSOutlineViewDataSource. Because we use bindings on the
 table column these methods are never called. The NSLog statements have been
 included to prove that these methods are not called.
 */
- (int)outlineView:(NSOutlineView *)outlineView numberOfChildrenOfItem:(id)item {
    NSLog(@"numberOfChildrenOfItem");
    return 1;
}

- (BOOL)outlineView:(NSOutlineView *)outlineView isItemExpandable:(id)item {
    NSLog(@"isItemExpandable");
    return NO;
}

- (id)outlineView:(NSOutlineView *)outlineView child:(int)index ofItem:(id)item {
    NSLog(@"child of Item");
    return NULL;
}



@end

Answer 1

Just did a Bit of searching around, and found how to fix it.
In the Attributes for the NSTreeController set the fetch predicate as parent == nil.