library(arrow)          # version 18.1.0.1 
library(data.table)   # version 1.16.4

dt <- data.table(x = 1:3)

names(attributes(dt))

# returns
# "names"             "row.names"         "class"             ".internal.selfref"

#works, creating a new column by reference. 
dt[, y := letters[1:3]]

# save file using write_parquet
write_parquet(dt, "test.parquet")

# read file back in using read_parquet
dt_after_parquet <- read_parquet("test.parquet")

# this has stripped away the .internal.selfref attribute
names(attributes(dt_after_parquet))
# returns
# "names"             "row.names"         "class" 

# meaning that this works but with the following warning message.
dt_after_parquet[, z := 4:6]

# Warning message:
# In `[.data.table`(dt_after_parquet, , `:=`(z, 4:6)) :
#   Invalid .internal.selfref detected and fixed by taking a (shallow) copy of the 
# data.table so that := can add this new column by reference. At an earlier point, 
# this data.table has been copied by R (or was created manually using structure() 
# or similar). Avoid names<- and attr<- which in R currently (and oddly) may copy 
# the whole data.table. Use set* syntax instead to avoid copying: ?set, ?setnames 
# and ?setattr. If this message doesn't help, please report your use case to the 
# data.table issue tracker so the root cause can be fixed or this message improved.