Feature request: for a `brms` model, when `draw_ids` are specified, `[add_]epred_draws` returns `draw_ids` in `.draw`.

[Fumeng-Yang]

When draw_ids are specified to get a subset of the draws, [add_]epred_draws still returns 1, 2, ..., row number in .draw. It would be useful to have draw_ids in .draw. Because we may want to split the draws and later combine them.

A minimal reproducible example

set.seed(1234)

# need a minimal model
m <- brm(
       x ~ 1, 
       data = tibble(x = rnorm(100, 0, 1)),  # generate some data
       backend = 'cmdstanr',
       refresh = 2000, # don't want lots of messages
       family = gaussian()
       )

add_epred_draws(
    m,
    newdata = tibble(x = 0),
    draw_ids = 100:102 # a subset of draws
  )

The output is

x	.row	.chain	.iteration	.draw	.epred
0	1	NA	NA	1	0.1354060
0	1	NA	NA	2	0.0083186
0	1	NA	NA	3	-0.0979545

Notice that .draw is still 1,2,3 while we specified 100:102.

[Fumeng-Yang]

FYI: It seems that brms's posterior_epred returns row names for an ordinal model. Not sure whether this is used in tidybayes.

A minimal reproducible example

set.seed(1234)

c <- brm(x ~ 1, 
       data = tibble(x = sample(1:3, 100, replace = TRUE)),  # need some data
       backend = 'cmdstanr',
       refresh = 2000, # don't want lots of messages
       family = cumulative('logit')
       )

df <- brms::posterior_epred(c,
                newdata = tibble(x = 1),
                draw_ids = 100:105 # a subset of draws 
                # but don't want to be confused with the categories 
          )

dimnames(df)

The output is

[[1]]
[1] "1" "2" "3" "4" "5" "6"

[[2]]
NULL

[[3]]
[1] "1" "2" "3"

where we see the first dimension has row numbers as row names.

[Fumeng-Yang]

BTW: these two features should be simple to add. I can probably figure out something after I understand the structure of tidybayes....