连接到笛卡尔

我有两个DT。我想基于一列将DT1和DT2连接起来,并从DT2中获取一列。

DT1:
         id  place
     1: id1    A
     2: id2    B
     3: id3    B
     4: id4    C
     5: id5    C

DT2:

      place rank
   1:  A      3
   2:  B      2
   3:  C      1
   4:  D      3
   5:  E      2

Expected:

         id  place  rank
     1: id1    A       3
     2: id2    B       2
     3: id3    B       2
     4: id4    C       1 
     5: id5    C       1 

现在,我已经试过了-

dt1[dt2,on=c('place'),nomatch=0]

我认为这将基于中的值映射所有行 place 列并添加 rank 列添加到它。但是我得到一个错误,说笛卡尔坐标发生了。

Error in vecseq(f__, len__, if (allow.cartesian || notjoin || !anyDuplicated(f__,  :
  Join results in <> rows; more than <> = 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 by=.EACHI to run j for each group to avoid the large al
location. 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 data.table issue tracker for advice.

我的第一个问题是我不明白笛卡尔坐标系是怎么形成的? 是因为我的第一个DT在同一日期有多行吗?

第二,我如何正确实现这一点?