xmv() when using by

[qgeissmann]

query<- data.frame(experiment_id = "toy_experiment",
                        region_id = 1:10,
                        condition = c("A", "B")) 
dt <- toy_activity_data(query, 3)
#ok
dt[,unique(xmv(region_id))]
# errors
dt[,unique(xmv(region_id)) ,by=id]

We get :

Error in xmv(region_id) : 
  xmv can only be called from inside the `[]` of a behavr object

[qgeissmann]

I am not sure when this is actually useful to use by and metavariables in the same call. So maybe #wontfix. We should provide better error message though