Issue with MINX DAX Calculation

[kdesai]

Hi,

I am trying to calculate minimum start_date as IPST for a customer as below-

IPST = VAR Per=SELECTEDVALUE(Patient_Population2[Person_id])
RETURN MINX(FILTER(Patient_Population2,Patient_Population2[Person_id]=Per),Patient_Population2[start_date].[Date])

I am using a table visual.
Patient_Population2 table lists all the start_dates for a customer.

But what I am getting is a value similar to start_date column for IPST, see screen shot below

I am expecting the output to be this

[d_gosbell]

The problem here is that in the call to Filter() the reference to Patient_Population2 brings in the value of the start_date as part of the filter context. So when you calcuate the first row it's filtering for the min of start_date where start_date = 11/5/2018. 

One way to fix this would be to wrap the table reference in the ALL() function 

IPST = VAR Per=SELECTEDVALUE(Patient_Population2[Person_id])
RETURN MINX(FILTER( ALL( Patient_Population2 ),Patient_Population2[Person_id]=Per),Patient_Population2[start_date].[Date])

A different pattern which should achieve the same effect and is slightly simpler is the following

IPST = VAR Per=SELECTEDVALUE(Patient_Population2[Person_id])
RETURN CALCULATE( MIN(Patient_Population2[start_date].[Date]), Patient_Population2[Person_id] = Per)