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Issue with Arrays in Downstream Mappers after a Custom Snap Transformation

New Contributor II

We built a custom snap that performs a transformation. You will notice in the output of the custom snap in the screenshot below that “medicalEnrollment” is a json array.


In a downstream mapper, we attempted a transformation on this array. We have tried (1) using the sl.ensureArray() method and (2) another downstream mapper performing a transformation in the array. However, we notice that in both the #1 and #2 attempts the mappers do not detect the array, thus causing issues with the transformation.

Below is a snippet of the preview of what we expect vs actual preview.


We also notice that chaining a Json Formatter and a Json Parser right after the custom snap does indeed makes the mappers work correctly. Pattern shown below:


So my question is, is there something hidden in the JSON parser snap that allows strong typing of the array that we are missing? Anything in between?

@robin , Tagging you since I was told you might be able to help out. Thanks!


Thanks. I installed the binary snap pack and set a breakpoint in the Mapper to see the Java types used in the input, which is the output of your snap. I can see that medicalEnrollment is still a HashSet, not an ArrayList:

Screen Shot 2021-09-09 at 4.43.41 PM

I haven’t studied your source code but I can see that you’re still using HashSet in a lot of places. Don’t use that type anywhere in your Document data. It’s not compatible with any of the JSON types. Use ArrayList everywhere you need an array.

View solution in original post


I’ve looked at the ticket and can see there’s a pipeline link. Can you please provide me with the permission to execute that pipeline? My username is

I see two different versions of the same custom snap pack in your /shared directory. They both appear to implement the same set of three snaps, including “Excel Structure”. One of them was updated today, but the other was last updated two days ago. I’m wondering if the pipeline you’re testing might be using the older one rather than the one you just updated to output an ArrayList. I suggest removing the older one to ensure it’s no longer being used if that’s your intention.

New Contributor II

@ptaylor , I have granted you access to the org and created a copy for you. Our newest snap is com-snapit-excelstructure_1-0001 and that’s the one that’s being used at the moment.

Thanks, but where can I find the copy I can access?

New Contributor II

@ptaylor Our organization is MercerDigitalDev. The pipeline is located in Workbook v3 - Projects/0000 - Snaplogic Testing. Either of the pipelines in that folder would work.