test.R

# create two simple dt with same order
dt1 = data.table(k = c(1:10),
                 v1 = rnorm(10),
                 key = "k")

dt2 = data.table(k = c(1:10),
                 v2 = rnorm(10),
                 key = "k")

# change order of dt1
dt1 = dt1[order(v1)]

# make dt2 be of the same order as dt1
dt2[dt1[, .(k)]]