Q&A: Problems passing context in a Custom Rule Function (CRF)

Q: Why can I not access the properties of my CRF context parameter?

Consider the Attributes property of an Assertion Record.

The following rule returns a collection of key values when invoked from the Rule Tester:

Filter(Attributes.Keys, EndsWith(context, "Rule"))

But when the same command is attempted within the function below:

GetProbe(rulePrefix, attrDict)
Filter(attrDict.Keys, StartsWith(context, rulePrefix))

... an error is returned as follows:

Unable to calculate rule GetProbe("active Admin", Attributes) for object Unit Test Example (<guid>): 
Unable to find field or property rulePrefix on String, and String does not support indexing

Reducing the function body to the following:

StartsWith(context, rulePrefix)

... results in no error being raised, but no results are returned either.

How should a function be written to return a subset of keys from the supplied dictionary parameter?

A: Because to do this you need to specify parentContext when supplying a list to a CRF.

Consider the GetProbe function as specified above.

In each iteration of Filter, context is a string key, which does not have a rulePrefix property (which is from the parent context).

Instead you should specify the content of your CRF as follows:

Filter(attrDict.Keys, StartsWith(Child, Parent.rulePrefix), parentContext=context)

By passing the parentContext, each iteration of Filter applies to a structure that looks like this:

 { Child: <listitem>, Parent: <parentContext object> }

This behavior generalizes across all of the List/Enumerable processing functions.