data.table and parquet data.table have same internal.selfref but are not the same object

[TysonStanley]

While reviewing apache/arrow#45346, noticed this behavior in data.table. Not sure if intended to have same self ref but are treated as different objects.

dt_in <- data.table::data.table(x = 1:10)
attributes(dt_in)
#> $names
#> [1] "x"
#> 
#> $row.names
#>  [1]  1  2  3  4  5  6  7  8  9 10
#> 
#> $class
#> [1] "data.table" "data.frame"
#> 
#> $.internal.selfref
#> <pointer: 0x141813ee0>
arrow::write_parquet(dt_in, "test.parquet")
dt_out <- arrow::read_parquet("test.parquet")
attributes(dt_out)
#> $names
#> [1] "x"
#> 
#> $row.names
#>  [1]  1  2  3  4  5  6  7  8  9 10
#> 
#> $class
#> [1] "data.table" "data.frame"
data.table::setDT(dt_out)
attributes(dt_out)
#> $names
#> [1] "x"
#> 
#> $row.names
#>  [1]  1  2  3  4  5  6  7  8  9 10
#> 
#> $class
#> [1] "data.table" "data.frame"
#> 
#> $.internal.selfref
#> <pointer: 0x141813ee0>
dt_in[, y := 10:1]

attributes(dt_in)
#> $names
#> [1] "x" "y"
#> 
#> $row.names
#>  [1]  1  2  3  4  5  6  7  8  9 10
#> 
#> $class
#> [1] "data.table" "data.frame"
#> 
#> $.internal.selfref
#> <pointer: 0x141813ee0>
attributes(dt_out)
#> $names
#> [1] "x"
#> 
#> $row.names
#>  [1]  1  2  3  4  5  6  7  8  9 10
#> 
#> $class
#> [1] "data.table" "data.frame"
#> 
#> $.internal.selfref
#> <pointer: 0x141813ee0>

print(dt_in)
#>         x     y
#>     <int> <int>
#>  1:     1    10
#>  2:     2     9
#>  3:     3     8
#>  4:     4     7
#>  5:     5     6
#>  6:     6     5
#>  7:     7     4
#>  8:     8     3
#>  9:     9     2
#> 10:    10     1
print(dt_out)
#>         x
#>     <int>
#>  1:     1
#>  2:     2
#>  3:     3
#>  4:     4
#>  5:     5
#>  6:     6
#>  7:     7
#>  8:     8
#>  9:     9
#> 10:    10

^{Created on 2025-01-24 with reprex v2.1.1}

[MichaelChirico]

Is this a duplicate of #6737?

[TysonStanley]

Oh, yes, related to the same thing. I've linked the PR relating to it.