Merge with multiple reference tables

I have two reference tables ,ref1 and ref2. and a dataset, df. All are data.tables

ref1 has variables "V1", "V2" and "x"
ref2 has variables "V1" and "x"
df has variables "V1" and "V2"

I want to extract the values of "x" from ref1 and ref2 based on the appropriate values for "V1" and "V2". There is no overlap between "V1" values in ref1 and ref2.

Some MWE data:

ref1 <- data.table(
  V1 = c("a","a","a","b","b","b","e","e"),
  V2 = c("1","2","3","4","5","6","10","11"),
  x = c(43,66,12,19,92,30,44,56)
)

ref2 <- data.table(
  V1 = c("c","d"),
  x = c(100,200)
)

df <- data.table(
  V1 = c("a","b","c","d","f"),
  V2 = c("2","4","7","9","14")
)

target <- data.table(
  V1 = c("a","b","c","d","f"),
  V2 = c("2","4","7","9","14"),
  x = c(66,19,100,200,NA)
)

Hi @MyKo101 ,
Nice MWE. I'm sure there's a simpler way, but two joins of ref1 and ref2 with df and then finding the non-NA values via coalesce works.

library("dplyr")
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
library("data.table")
#> 
#> Attaching package: 'data.table'
#> The following objects are masked from 'package:dplyr':
#> 
#>     between, first, last

ref1 <- data.table(
  V1 = c("a","a","a","b","b","b","e","e"),
  V2 = c("1","2","3","4","5","6","10","11"),
  x = c(43,66,12,19,92,30,44,56)
)

ref2 <- data.table(
  V1 = c("c","d"),
  x = c(100,200)
)

df <- data.table(
  V1 = c("a","b","c","d","f"),
  V2 = c("2","4","7","9","14")
)


df1 <- ref1[df, on = c("V1", "V2")]
df2 <- ref2[df, on = c("V1")]

(df3 <- df1[df2, on = c("V1", "V2")] %>%
  mutate(x = coalesce(x, i.x)) %>%
  select(V1, V2, x))
#>    V1 V2   x
#> 1:  a  2  66
#> 2:  b  4  19
#> 3:  c  7 100
#> 4:  d  9 200
#> 5:  f 14  NA

target <- data.table(
  V1 = c("a","b","c","d","f"),
  V2 = c("2","4","7","9","14"),
  x = c(66,19,100,200,NA)
)

identical(df3, target)
#> [1] TRUE

Created on 2021-09-29 by the reprex package (v2.0.1)