笛卡尔滚动与data.table连接

Question

I'm not exactly sure how to describe this, but I'll gladly edit the title and/or post to reflect comments and answers.

Problem

I have two data.frames that I would like to merge with a combination of a left join, an outer join, and a rolling join.

• One of the key columns ( year ) is for the rolling join.
• Another key column ( cat ) is common to both data.frames . In the example below I've only supplied exemplary subsets of the full data, which has thousands of values for cat .
• The first data.frame , X , has an additional key column cnty (county), and the second data.frame , Y , has an additional key column pol (pollutant).
• For each group defined by cat and year , I would like the final result to contain a cartesian product of cnty and pol , with value columns emfac (from X ) and tput (from Y ). The goal is to be able to compute emfac * tput .

Here is an exemplary subset of X :

  cat year cnty  tput
1  29 2011  ALA 67852
2  29 2011   CC 33893
3  29 2011  MRN 11319

... and here is an exemplary subset of Y :

  cat year pol emfac
1  29 1975 TOG   2.4
2  29 1975  PM   5.3

Closest attempt so far

I can almost, but not quite, get the output I want:

X <- structure(list(
       cat = c(29L, 29L, 29L), 
       year = c(2011L, 2011L, 2011L), 
       cnty = c("ALA", "CC", "MRN"), 
       tput = c(67852, 33893, 11319)), 
       .Names = c("cat", "year", "cnty", "tput"), 
       class = c("data.frame"), row.names = c(NA, -3L))

Y <- structure(list(
       cat = c(29L, 29L), 
       year = c(1975, 1975), 
       pol = c("PM", "TOG"), 
       emfac = c(2.4, 5.3)),
       .Names = c("cat", "year", "pol", "emfac"), 
       class = c("data.frame"), row.names = c(NA, -2L))

library(data.table)
X <- data.table(X, key = c("cat", "cnty", "year"))
Y <- data.table(Y, key = c("cat", "pol", "year"))
Y[X, roll = TRUE]

   cat year pol emfac cnty  tput
1:  29 2011  PM   5.3  ALA 67852
2:  29 2011  PM   5.3   CC 33893
3:  29 2011  PM   5.3  MRN 11319

This is my "nearest miss". Most of my other attempts are much more wrong.

Expected result

   cat year pol emfac cnty  tput
1:  29 2011  PM   5.3  ALA 67852
2:  29 2011  PM   5.3   CC 33893
3:  29 2011  PM   5.3  MRN 11319
4:  29 2011 TOG   2.4  ALA 67852
5:  29 2011 TOG   2.4   CC 33893
6:  29 2011 TOG   2.4  MRN 11319

What am I doing wrong?