Why does CALCULATETABLE not register as a single column when used as TREATAS

[Freece4802]

Hi all, 

I was hoping to understand why using a variable which uses calculate table and has only 1 column can't be used within a TREATAS function?  Its easy to overcome by using SELECTEDCOLUMN over a CALCULATETABLE but was just hoping to understand why this is required. See example code below:

VAR RewnewedPolicies =

        SELECTCOLUMNS(CALCULATETABLE(

            VALUES ( Fact_VT[PN] ),

            USERELATIONSHIP('Date Table'[Date],Fact_VT[StartDate])),

            "__PN", Fact_VT[PN])

RETURN

    CALCULATE ([Gross Incurred Claims],

        TREATAS ( RewnewedPolicies, Fact_Claims[PN] )

    )

[andrewsommer · Accepted solution]

When doing it without just CALCULATETABLE like this:

VAR RewnewedPolicies =
    CALCULATETABLE(VALUES(Fact_VT[PN]), ...)

you will get a single-column table, but the column retains its original metadata and name (Fact_VT[PN]).  TREATAS cannot reliably bind a source column with metadata tied to another table into a new context without ambiguity.

SELECTCOLUMNS flattens this ambiguity, you get a column with a defined name ("__PN")

So, effectively, SELECTCOLUMNS is your insurance policy that the shape of the table matches TREATAS' expectations.

Please mark this post as a solution if it helps you. Appreciate Kudos.

[Freece4802]

Amazing explanation, much appriciated and good to understand. 

Thanks, 

Will