Add column mapping defaults functionality and example JSON file

Start-SPMigration.ps1

@@ -32,6 +32,8 @@ param(
 Set-StrictMode -Version Latest
 $ErrorActionPreference = "Stop"
 $script:SkippedImportFieldWarnings = @{}
+$script:ListImportTrackingFieldInternalName = "StartSPMigrationSourceUniqueId"
+$script:ListImportTrackingFieldDisplayName = "Start-SPMigration Source UniqueId"
 
 function Initialize-SharePointPowerShell {
     if($null -eq $(Get-Module SharePointServer)) {
@@ -2704,6 +2706,68 @@ function Import-SPDocumentLibraries {
     }
 }
 
+function Ensure-SPMigrationListTrackingField {
+    param(
+        [Parameter(Mandatory = $true)]
+        [Microsoft.SharePoint.SPList]$List
+    )
+
+    $field = Resolve-SPField -Fields $List.Fields -InternalName $script:ListImportTrackingFieldInternalName
+    if ($null -ne $field) {
+        return $field
+    }
+
+    [void]$List.Fields.Add($script:ListImportTrackingFieldInternalName, [Microsoft.SharePoint.SPFieldType]::Text, $false)
+    $List.Update()
+
+    $field = Resolve-SPField -Fields $List.Fields -InternalName $script:ListImportTrackingFieldInternalName
+    if ($null -eq $field) {
+        throw ("Konnte Tracking-Feld fuer idempotenten Listenimport nicht anlegen: {0}" -f $List.Title)
+    }
+
+    $field.Title = $script:ListImportTrackingFieldDisplayName
+    $field.Hidden = $true
+
+    try {
+        $field.Indexed = $true
+    }
+    catch {
+        Write-Warning ("Tracking-Feld konnte nicht indiziert werden: {0}. {1}" -f $List.Title, $_.Exception.Message)
+    }
+
+    $field.Update()
+    $List.Update()
+
+    return $field
+}
+
+function Get-SPListItemByMigrationSourceUniqueId {
+    param(
+        [Parameter(Mandatory = $true)]
+        [Microsoft.SharePoint.SPList]$List,
+
+        [Parameter(Mandatory = $true)]
+        [string]$FieldInternalName,
+
+        [string]$SourceUniqueId
+    )
+
+    if ([string]::IsNullOrWhiteSpace($SourceUniqueId)) {
+        return $null
+    }
+
+    $escapedValue = [System.Security.SecurityElement]::Escape($SourceUniqueId)
+    $query = New-Object Microsoft.SharePoint.SPQuery
+    $query.RowLimit = 1
+    $query.Query = "<Where><Eq><FieldRef Name='$FieldInternalName' /><Value Type='Text'>$escapedValue</Value></Eq></Where>"
+
+    $matchingItems = $List.GetItems($query)
+    if ($matchingItems.Count -eq 0) {
+        return $null
+    }
+
+    return $matchingItems[0]
+}
 function Import-SPLists {
     param(
         [Parameter(Mandatory = $true)]
@@ -2755,10 +2819,12 @@ function Import-SPLists {
 
         Write-Host ("Importiere Liste: {0}" -f $targetListTitle)
         $fieldMapping = Get-FieldMappingForContainer -MigrationMappingTable $MigrationMappingTable -ObjectType "List" -SourceTitle $sourceListTitle
+        $trackingField = Ensure-SPMigrationListTrackingField -List $targetList
 
         foreach ($sourceItem in $items) {
             $sourceItemId = [string](Get-ObjectPropertyValue -Object $sourceItem -PropertyName "Id")
             $sourceItemTitle = [string](Get-ObjectPropertyValue -Object $sourceItem -PropertyName "Title")
+            $sourceItemUniqueId = [string](Get-ObjectPropertyValue -Object $sourceItem -PropertyName "UniqueId")
             $fileSystemObjectType = [string](Get-ObjectPropertyValue -Object $sourceItem -PropertyName "FileSystemObjectType")
             if ($fileSystemObjectType -eq "Folder") {
                 Write-Warning ("Ordner in normalen Listen werden in dieser Version uebersprungen: {0}" -f $targetListTitle)
@@ -2770,8 +2836,22 @@ function Import-SPLists {
             }
 
             try {
-                $targetItem = $targetList.Items.Add()
+                $targetItem = Get-SPListItemByMigrationSourceUniqueId -List $targetList -FieldInternalName $trackingField.InternalName -SourceUniqueId $sourceItemUniqueId
+                $isExistingItem = $null -ne $targetItem
+
+                if ($isExistingItem) {
+                    Write-Host ("Aktualisiere vorhandenes Listenelement: Liste '{0}', Quell-ItemId '{1}'" -f $targetListTitle, $sourceItemId)
+                }
+                else {
+                    $targetItem = $targetList.Items.Add()
+                }
+
                 Apply-FieldMappingToItem -Item $targetItem -FieldMapping $fieldMapping -SourceFieldValues (Get-ObjectPropertyValue -Object $sourceItem -PropertyName "FieldValues") -SourceFieldTextValues (Get-ObjectPropertyValue -Object $sourceItem -PropertyName "FieldTextValues")
+
+                if (-not [string]::IsNullOrWhiteSpace($sourceItemUniqueId)) {
+                    $targetItem[$trackingField.InternalName] = $sourceItemUniqueId
+                }
+
                 Save-SPListItem -Item $targetItem -Context ("Liste '{0}', Quell-ItemId '{1}', Titel '{2}'" -f $targetListTitle, $sourceItemId, $sourceItemTitle)
             }
             catch {