Update subset of data.table based on join

by

The easiest way I can think of is to key by id1 as well.
eg

setkey(DT1, id2,id1)
DT2[, id1 := 3]
setkey(DT2, id2, id1)

# use i.v1 to reference v1 from the i component
DT1[DT2, v1 := i.v1 ]


DT1
   id1 id2        v1
1:   2   e 0.7383247
2:   1   g 1.5952808
3:   2   j 0.3295078
4:   3   n 0.0000000
5:   3   s 0.5757814
6:   1   u 0.4874291

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