Join of two data.tables fails

Question

I am trying to use a data table as a lookup table:

> (dt <- data.table(myid=rep(11:12,3),zz=1:6,key=c("myid","zz")))
   myid zz
1:   11  1
2:   11  3
3:   11  5
4:   12  2
5:   12  4
6:   12  6
> (id2name <- data.table(id=11:14,name=letters[1:4],key="id"))
   id name
1: 11    a
2: 12    b
3: 13    c
4: 14    d

What I want is

> (res <- data.table(myid=rep(11:12,3),zz=1:6,name=rep(letters[1:2],3),key=c("myid","zz")))
   myid zz name
1:   11  1    a
2:   11  3    a
3:   11  5    a
4:   12  2    b
5:   12  4    b
6:   12  6    b

however, the join I tried failed:

> dt[id2name]
Starting binary search ...done in 0 secs
Error in vecseq(f__, len__, if (allow.cartesian) NULL else as.integer(max(nrow(x),  : 
  Join results in 8 rows; more than 6 = max(nrow(x),nrow(i)). Check for duplicate key values in i, each of which join to the same group in x over and over again. If that's ok, try including `j` and dropping `by` (by-without-by) so that j runs for each group to avoid the large allocation. If you are sure you wish to proceed, rerun with allow.cartesian=TRUE. Otherwise, please search for this error message in the FAQ, Wiki, Stack Overflow and datatable-help for advice.
Calls: [ -> [.data.table -> vecseq

what am I doing wrong?

PS. I am amenable to any alternative way to get the results; what is the most idiomatic way to do what I want (dt must still be a data.table, but id2name can be anything mapping int to something else - as long as the int is not assumed to be a vector index).

Answer 1

> dt[id2name, allow.cartesian=T, nomatch=0]
   myid zz name
1:   11  1    a
2:   11  3    a
3:   11  5    a
4:   12  2    b
5:   12  4    b
6:   12  6    b

data.table is trying to save you from yourself in case you had an unintentional join on keys with duplicate values. Note that the error message (eventually) tells you what to do if you're sure you know what you're doing.

Alternatively:

> id2name[dt]
   id name zz
1: 11    a  1
2: 11    a  3
3: 11    a  5
4: 12    b  2
5: 12    b  4
6: 12    b  6