blend_predictions does not support gam models with a custom formula

[era127]

It does not seem to be possible to use a gam model with workflowset.

When using a gam model in a workflowset, the custom formula is specified using the formula argument of
workflowsets::update_workflow_model()

However stack_workflow() only uses extract_spec_parsnip() and extract_preprocessor() so it never saves the custom formula into the candidate.

library(magrittr)
# generate data
x <- data.frame(date = seq(from = as.POSIXct("2000-01-01"), to = as.POSIXct("2020-01-01"), by = "DSTday"))
x <- cbind(x, x = runif(n = NROW(x)), weekday = lubridate::wday(x$date))
x$y <- dplyr::lead(x$x, n = 1) + sin(as.numeric(x$weekday)) / 10
#
# recipe
p <- recipes::recipe(x, formula = y ~ date + x + weekday) %>%
  recipes::update_role(date, new_role = "date")
#
# sliding resampling
r <- rsample::sliding_period(data = x, index = date, period = "month", lookback = 120, assess_stop = 12, step = 12)
# workflow sets for lm and gam formula
m.lm <- parsnip::linear_reg()
m.gam <- parsnip::gen_additive_mod(mode = "regression")
ws <- workflowsets::workflow_set(preproc = list(p = p), models = list(lm = m.lm, gam = m.gam), cross = T) %>%
  workflowsets::update_workflow_model(id = "p_gam", spec = m.gam, formula = y ~ s(x) + s(weekday, bs = "cc", k = 7))
wfs <- workflowsets::workflow_map(
  object = ws,
  fn = "fit_resamples",
  resamples = r,
  control = tune::control_resamples(save_pred = T, save_workflow = T)
)

# stacking
s <- stacks::stacks() %>%
  stacks::add_candidates(wfs) %>%
  stacks::blend_predictions(penalty = 1e-8, mixture = 0, times = 2)

stacks::fit_members(s)
# Error in { :
#     task 2 failed - "`fit()` must be used with GAM models (due to its use of formulas)."

[simonpcouch]

Thank you for the issue! This is indeed a bug. I think we're due for a good push on mgcv docs and infrastructure, hopefully soon than later—a fix for this will come then.

[era127]

I was able to work around the issue by removing the call to stack_workflow(). That function appears to just create a 'minimal' workflow from an existing workflow, and I think it drops objects for the formula which are required.

[simonpcouch]

This is now addressed in the dev version of stacks! Thanks for bringing this up.

[github-actions]

This issue has been automatically locked. If you believe you have found a related problem, please file a new issue (with a reprex: https://reprex.tidyverse.org) and link to this issue.