Looping over repeating OBX values and updating the segment

I regularly need to update HL7 messages as the data flows between systems. Often, it's significantly cheaper to take the feed designed for another integration and massage the HL7 message before passing it onto your integration.  

With Integration Host you generally just remap using the Transformers and it's all very simple. But the other day I got a wee bit tangled when trying to update OBX values.

I wanted to loop over each OBX segment, look at the value in the OBX-3.4, then use some conditions and lookup tables (note: loving lookup tables right now) to alter the value before inserting it back into the OBX-3.2.

So I used a FOREACH transformer to loop over the OBX values, then put the OBX-3.4 into a variable that I then updated to the value I needed.  This was all easy, but the trick came when I tried to write the value back into the same OBX line the FOREACH was on.  My mapping just wrote each value into the first segment, overwriting it each time, leaving the last in place. This stumped me for a bit and I contacted HL7 Soup support.

It turns out that the loop is only looping over the incoming OBX values, not the outgoing ones, so you can’t use it to set the value in the destination message.  Rather you need to create a different HL7 path for each iteration and set that value.  Apparently, they looked at creating a feature in the UI to allow this several times, but it turns out that code is by far the easiest way:

//Update the OBX-3.2 on the same index as the for each loop.
IHL7Message destinationMessage = (IHL7Message)activityInstance.Message;
string path = $"OBX[{workflowInstance.GetVariable("ForEachIterator")}]-3.2";
destinationMessage.SetValueAtPath(path,  workflowInstance.GetVariable("UpdatedValueVariable"));

Above, you can see that you just construct the path we wish to write to using the forEachIterator variable (which is automatically populated by the FOREACH Transformer), then set the path in the destination message with the needed value - the variable "UpdatedValueVariable" in my case.