cubicweb-neuroimaging #3442397 Scan without actual data: avoid error in view [open]

Data is indeed optional: the cardinality of the has_data relation is '?1'.

Either change the cardinality to require data, or allow missing data in views.

The attached patch goes the second way. Which way to go can of course be discussed.

done in<not specified>
closed by<not specified>
patch[views] avoid error in case of Scan without actual data [applied]