This posting might seem a little trivial, but having figured it out, I'm blogging it for my own reference. In fact, I was almost going to put it on the Tridion cookbook, but this is legacy stuff. There won't ever need to be a core service version of this, because in 2011, you can get the answer directly from Where Used.

But on older systems, say you wanted to update a schema, and wanted to figure out the impact on your templates. Which templates would you have to check for necessary updates, etc? (Imagine you were going to make a mandatory field optional, and wanted to check whether your templates would break if the user chose not to give a value.)

So you know which schema it is, and you want to know the component templates that have this as a related schema. I started to hack this out in Powershell using what are now for me fairly standard techniques. The trouble is that VBA collections are difficult to iterate over in the Powershell. Fortunately you can use the contains method on the RelatedSchemas collection to get the "where" clause you need. In most systems, you keep your templates, schemas etc, in a "system" folder, so the script simply starts at that location, and recursively grabs all the component templates it can find, If the schema of interest is in the related schemas, it will be listed.

$tdse = new-object -com TDS.TDSE
$interestingSchema = $tdse.Getobject("tcm:10-1234-8",1)
$systemFolder = $tdse.GetObject("tcm:11-123-2",1)
$rf = $tdse.CreateListRowFilter()
$rf.SetCondition("Recursive", $true)
$rf.SetCondition("ItemType", 32)
([xml]$systemFolder.GetListItems(3, $rf)).ListItems.Item | ?{$tdse.Getobject($_.ID,1).RelatedSchemas.Contains($interestingSchema)}