s4 objects in j groups are wrong

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
r
Domain
data

Research direction

Start by running the reproducible data.table and lubridate example in this issue, then compare the related discussions in #4131 and #1777. Trace grouped j evaluation and verify that each group preserves its own S4 object instead of reusing the days result.

Written by the indexing model from the issue text.

Description

bug non-atomic column

Another follow-up of #4131

j doesn't handle S4 objects properly:

library(data.table)
library(lubridate)
dt <- data.table(period_unit = c("days", "months", "weeks", "years"),
                 period_length = rep(1, times = 4))

dt[ , by = period_unit, do.call(.BY$period_unit, list(period_length))]
#    period_unit          V1
# 1:        days 1d 0H 0M 0S
# 2:      months 1d 0H 0M 0S
# 3:       weeks 1d 0H 0M 0S
# 4:       years 1d 0H 0M 0S

The days object has been cascaded to the other groups.

I think this is a duplicate of #1777, but I'm not sure; if so, I would close that one since I wrote this to be very direct & clear about the issue.

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