join retain key, roll branch only when non missing roll arg, closes #…

…3441

NEWS.md

@@ -32,6 +32,8 @@
 
 10. Quoted expression having `:=` and dot alias in RHS now works as expected. Thanks to @franknarf1 for raising up issue on [StackOverflow](https://stackoverflow.com/questions/41228076/using-data-tables-shortcut-in-quoted-expressions) and @jangorecki for PR.
 
+11. Key is properly removed in an edge case not handled before, when joining a *keyed* table to a 2+ length vector wrapped in list when only single row match. Thanks to @symbalex for reporting and @franknarf1 for tiny reproducible example. Closes [#3441](https://github.com/Rdatatable/data.table/issues/3441).
+
 #### NOTES
 
 1. When upgrading to 1.12.0 some Windows users might have seen `CdllVersion not found` in some circumstances. We found a way to catch that so the [helpful message](https://twitter.com/MattDowle/status/1084528873549705217) now occurs for those upgrading from versions prior to 1.12.0 too, as well as those upgrading from 1.12.0 to a later version. See item 1 in notes section of 1.12.0 below for more background.

R/data.table.R

@@ -1333,7 +1333,13 @@ chmatch2 <- function(x, table, nomatch=NA_integer_) {
           ## check key on i as well!
           ichk = is.data.table(i) && haskey(i) &&
                  identical(head(key(i), length(leftcols)), names(i)[leftcols]) # i has the correct key, #3061
-          if (keylen && (ichk || is.logical(i) || (.Call(CisOrderedSubset, irows, nrow(x)) && ((roll == FALSE) || length(irows) == 1L)))) # see #1010. don't set key when i has no key, but irows is ordered and roll != FALSE
+          if (keylen && (
+            ichk || is.logical(i) || (
+              .Call(CisOrderedSubset, irows, nrow(x)) && !missingroll && ( # !missingroll fix for #3441
+                (roll == FALSE) || length(irows) == 1L
+                )
+              )
+            )) # see #1010. don't set key when i has no key, but irows is ordered and roll != FALSE
             setattr(ans,"sorted",head(key(x),keylen))
         }
         setattr(ans, "class", class(x)) # fix for #5296

inst/tests/tests.Rraw

@@ -13618,6 +13618,14 @@ x = 5L
 test(1998.1, as.IDate(x), output = '1970-01-06')
 test(1998.2, class(x), 'integer')
 
+# Joining a keyed table on a non-keyed table is not working sometimes #3441
+dx = data.table(id = "A", key = "id")
+di = list(c("D", "A"))
+test(1999.1, key(dx[di]), NULL)
+dx = data.table(id = 1L, key = "id")
+di = list(z=c(2L, 1L))
+test(1999.2, key(dx[di]), NULL)
+
 
 ###################################
 #  Add new tests above this line  #