[R] Create a field ref to a field in a struct

[asfimport]

See also ARROW-11259. This probably needs to be a $ and [[ method on Expression

Reporter: Neal Richardson / @nealrichardson

Related issues:

• [C++] Support nested FieldRefs (is blocked by)

_{Note: This issue was originally created as ARROW-13858. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
It turns out this doesn't work in C++. Created ARROW-13987 for that.

[asfimport]

Dewey Dunnington / @paleolimbot:
Just doing some exploring:

Where the field reference is created in from /R: https://github.com/apache/arrow/blob/master/r/R/expression.R#L170-L173

Where the field reference is created in /src: https://github.com/apache/arrow/blob/master/r/src/expression.cpp#L72-L74

Example of creating a nested field reference from a test (but it appears it wasn't implemented?) https://github.com/lidavidm/arrow/blob/master/cpp/src/arrow/compute/exec/expression_test.cc#L506-L509

You can do this kind of thing in dplyr using $:

library(arrow, warn.conflicts = FALSE)
library(dplyr, warn.conflicts = FALSE)

RecordBatch$create(df_col = tibble(a = 1)) %>% 
  mutate(df_col_a = df_col$a) %>% 
  collect()
#> # A tibble: 1 × 1
#>   df_col$a
#>      <dbl>
#> 1        1

tibble(df_col = tibble(a = 1)) %>% 
  mutate(df_col_a = df_col$a)
#> # A tibble: 1 × 2
#>   df_col$a df_col_a
#>      <dbl>    <dbl>
#> 1        1        1