Is leaf Inquiry

[lt79_Pax]

Dear Power BI Representative,
I am reviewing the DAX measure in chapter 11 figure 11-19 in the Definitive Guide to DAX and I do not understand what the PersonsAtParentLevel is trying to do at each personkey. 
I do not understand what Persons[ParentKey] = CurrentPersonKey means and how does it equate to 0?  You're insight on this measure would be greatly appreciated.
IsLeaf =
VAR CurrentPersonKey = Persons[PersonKey]
VAR PersonsAtParentLevel =
    CALCULATE (
        COUNTROWS ( Persons ),
        ALL ( Persons ),
        Persons[ParentKey] = CurrentPersonKey
    )
VAR Result = ( PersonsAtParentLevel = 0 )
RETURN
    Result

Thank you.

[amitchandak · Accepted solution]

@lt79_Pax , I am assuming this a column

This is value in a current row -Persons[PersonKey]

now you are counting does it exists as parent id

CALCULATE (
COUNTROWS ( Persons ),
ALL ( Persons ),
Persons[ParentKey] = CurrentPersonKey
)

Then you are checking if count = 0, which mean its does not exists as a parent , so it is a leaf node. 

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[lt79_Pax]

Thank you amitchandak.  You solved my problem.