MS Project 2013: display resources on summary tasks - ms-project

Question

Is there a way to configure MS Project 2013 so that it displays in a resource column (eg, resource initials) of the Gantt Chart view of summary tasks the UNION of all resources assigned to its leaf subtasks.
Say for example that I have a summary task S with 2 subtasks S1 & S2, S2 being itself divided into subsubtasks S21 & S22.
Say also that I have allocated resources R1,R2 to S1, resources R2,R3 to S21 and resource R4 to S22.
With my current configuration, the resource initial column of both S2 and S are left blank.
Instead, I would like the resource column of S2 to display R2, R3, R4 and the resource column of S to display R1, R2, R3, R4.
The idea is to be able to visualize all the resources allocated to a summary tasks even when its decomposition in subtasks is hidden.
Thank you very in advance for suggestions on how to achieve this.

Answer 1

Those resource fields exist at the summary level because you can directly assign resources to a summary task, so you can't use those fields for this purpose. However, here's a macro that aggregates the names of the resources assigned to the subtasks. The results are put in Text1 at the summary level. You can then modify the Gantt chart bar styles to show that text field.
Sub RollupResourceNames()
Dim tsk As Task
Dim list As String
Dim key As Variant
For Each tsk In ActiveProject.Tasks
If tsk.Summary Then
Dim col As New Collection
Set col = GetChildResourceAssignments(tsk)
list = vbNullString
For Each key In col
list = list & ", " & key
Next
If Len(list) > 2 Then
list = Mid$(list, 3)
End If
tsk.Text1 = list
End If
Next tsk
End Sub
Function GetChildResourceAssignments(parent As Task) As Collection
Dim col As New Collection
Dim child As Task
Dim asn As Assignment
For Each child In parent.OutlineChildren
If child.Summary Then
Dim col2 As New Collection
Set col2 = GetChildResourceAssignments(child)
Dim key As Variant
For Each key In col2
col.Add key, key
Next key
End If
For Each asn In child.Assignments
On Error Resume Next
col.Add asn.Resource.Name, asn.Resource.Name
On Error GoTo 0
Next asn
Next child
Set GetChildResourceAssignments = col
End Function

Answer 2

#Rachel Hettinger - Solution works great, except it will error out (error 457) if you have multiple levels of parent/child tasks and the same resource is present across different levels. It tries to add the resource name to the collection, but it already exists (since it was added earlier when the script checked the other set of tasks) and doesn't know what to do.
This is fixable by simply adding another "On Error Resume Next" line. Here is the revised macro, which works perfectly on my Project Plan. All credit to Rachel Hettinger here, I just added one line!
Sub RollupResourceNames()
Dim tsk As Task
Dim list As String
Dim key As Variant
For Each tsk In ActiveProject.Tasks
If tsk.Summary Then
Dim col As New Collection
Set col = GetChildResourceAssignments(tsk)
list = vbNullString
For Each key In col
list = list & ", " & key
Next
If Len(list) > 2 Then
list = Mid$(list, 3)
End If
tsk.Text1 = list
End If
Next tsk
End Sub
Function GetChildResourceAssignments(parent As Task) As Collection
Dim col As New Collection
Dim child As Task
Dim asn As Assignment
For Each child In parent.OutlineChildren
If child.Summary Then
Dim col2 As New Collection
Set col2 = GetChildResourceAssignments(child)
Dim key As Variant
For Each key In col2
On Error Resume Next
col.Add key, key
Next key
End If
For Each asn In child.Assignments
On Error Resume Next
col.Add asn.Resource.Name, asn.Resource.Name
On Error GoTo 0
Next asn
Next child
Set GetChildResourceAssignments = col
End Function