Referring to this SO post from @juancentro:
dta <- data.table(idx=1:3, vala=4:6, fidx = c("a", "a", "b"), key="idx")
dtb <- data.table(idx=c(1,4), valb=c(10,11), key="idx")
# (1)
dta[dtb, sum(vala), by=fidx]
# fidx V1
#1: a 4
#2: NA NA
# (2)
dta[dtb, sum(valb), by=fidx]
# Error in `[.data.table`(dta, dtb, sum(valb), by = fidx) :
# object 'valb' not found
It would be nice to have case (2) returning the right result as well.
That is, as @juancentro and @eantonya mentioned:
dta[dtb, sum(valb), by=fidx] == dta[dtb][, sum(valb), by=fidx]
would be great!
Thanks to @juancentro for reporting on SO.
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