I have a parent record case details which is having a one to many relationship with another record task details. When the data of caseDetails is loaded on first load of interface in rule input or local variable either using query record or process model, then I'm using count() or length() function to count no. of items in related record "task details" which is populating in relationship of caseDetails in same rule input or local variable.
Count or length both are returning count as 1 only. But, if I make any changes in loaded tasks list and it's updated on any user action then count is showing correct count of items in that relationship.
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I did a quick test using the following code, and it works as expected, returning 4.