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Original SO post: http://stackoverflow.com/questions/25145112/inconsistent-data-table-assignment-by-reference-behaviour
When assigning by reference with a data.table using a column from a second data.table, the results are inconsistent. When there are no matches by the key columns of both data.tables, it appears the assigment expression y := y is totally ignored - not even NAs are returned.
library(data.table)
dt1 <- data.table(id = 1:2, x = 3:4, key = "id")
dt2 <- data.table(id = 3:4, y = 5:6, key = "id")
print(dt1[dt2, y := y])
##    id x     # Would have also expected column:   y
## 1:  1 3     #                                   NA
## 2:  2 4     #                                   NA
However, when there is a partial match, non-matching columns have a placeholder NA.
dt2[, id := 2:3]
print(dt1[dt2, y := y])
##    id x  y
## 1:  1 3 NA    # <-- placeholder NA here
## 2:  2 4  5
This wreaks havoc on later code that assumes a y column exists in all cases. Otherwise I keep having to write cumbersome additional checks to take into account both cases.
Is there an elegant way around this inconsistency?