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Issues with merge factor attributes when merge all = TRUE

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r
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data

Rechercherichtung

Reproduce the two merge.data.table() examples and compare their factor attributes, then inspect the related rbind/rbindlist handling around src/rbindlist.c line 350. Check how unmatched rows and factor columns are stacked, and add regression coverage showing that source_field_name is retained for both full outer joins. Done means the custom attribute survives the unmatched-row case without changing factor levels or class.

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Beschreibung

[rbind|cbind|merge]list joins

merge.data.table() appears to drop custom attributes from factor columns when all = TRUE requires adding an unmatched row.

library('data.table')

d_x <- data.table(
  id = 1:2,
  value = structure(
    c(1L,2L),
    levels = c("No","Yes"),
    class = c('ordered','factor'),
    source_field_name = 'Q1'
  )
)

d_y1 <- data.table(id = 1:2)
d_y2 <- data.table(id = 1:3)

d_mrg1 <- merge(d_x, d_y1, by = 'id', all = TRUE)
d_mrg2 <- merge(d_x, d_y2, by = 'id', all = TRUE)

attributes(d_mrg1$value)
# $levels
# [1] "No"  "Yes"
#
# $class
# [1] "ordered" "factor"
#
# $source_field_name
# [1] "Q1"

attributes(d_mrg2$value)
# $levels
# [1] "No"  "Yes"
#
# $class
# [1] "ordered" "factor"

The only difference is that d_y2 contains an unmatched id = 3. When that row is present, the custom source_field_name attribute disappears.

I would expect the custom attribute to be retained in both cases. This seems to be specific to factor columns; custom attributes on other column types appear to survive the same operation.

I encountered this because two otherwise very similar full outer joins produced different attribute results depending on whether an unmatched row happened to be present.

From what I can tell this is related to the rbind rbindlist stacking factor issue I had expected rbindlist()..., and here.

d_x <- data.table(
  id = 1:2,
  z = structure(
    factor(c('No','Yes'), levels = c('No','Yes'), ordered = TRUE),
    source_field_name = 'Q1'
  )
)
attributes(d_x$z)

## Simple d_y with no z field. ##
d_y <- data.table(id = 3L)
d_z <- rbind(d_x, d_y, fill = TRUE)
attributes(d_z$z)

## d_y with Factor z field but not attribute. ##
d_y <- data.table(id = 3L, z = structure(
  factor(c('No'), levels = c('No','Yes'), ordered = TRUE)))
d_z <- rbind(d_x, d_y, fill = TRUE)
attributes(d_z$z)

## d_y with Factor z field with attribute. ##
d_y <- data.table(id = 3L, z = structure(
  factor(c('No'), levels = c('No','Yes'), ordered = TRUE),
  source_field_name = 'Q1'
))
d_z <- rbind(d_x, d_y, fill = TRUE)
attributes(d_z$z)

The funny thing is that I had written my own workaround version of rbindlist called rbl_safer that used my own preferences regarding how factors should be handled, but it is a lot harder to do for merge because of the suffix wrapping, etc. I could still write it, but I wanted to makes sure we knew there were secondary consequences. Handling rbindlist factor attributes is indeed "trickier than I initially thought", so I don't want to presume anything.

In the short term we could take off the factor restriction within rbindlist line L350, because the levels get establish at the end anyway.

> sessionInfo()
R version 4.6.1 (2026-06-24 ucrt)
Platform: x86_64-w64-mingw32/x64
Running under: Windows 11 x64 (build 26300)

Matrix products: default
  LAPACK version 3.12.1

locale:
[1] LC_COLLATE=English_United States.utf8  LC_CTYPE=English_United States.utf8    LC_MONETARY=English_United States.utf8 LC_NUMERIC=C                          
[5] LC_TIME=English_United States.utf8    

time zone: America/New_York
tzcode source: internal

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] data.table_1.18.99

loaded via a namespace (and not attached):
[1] compiler_4.6.1 tools_4.6.1
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