@SOARt_of_Lost Appreciate the response. I have since figured out exactly what we want to achieve. The key to achieving it was figuring out how the value is passed to the filter. The DJANGO 'in' filter expects a comma even if just one value is found for the custom field So the python script in the custom function looks at /rest/artifacts?_filter_cef__<our_custom_field>__in="a","b","c","d"&page_size=0 for multiple values & /rest/artifacts?_filter_cef__<our_custom_field>__in="a",&page_size=0 when a single value is found. As for the filter outputs to restrict fields, we eventually achieved that in the function output. The plan was to restrict values/volume of data return but oh well, wasn't working any which way! so function output was the way to go.
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