# Adapter cookbook: from split_spec to native resamples

#### Selçuk Korkmaz

#### 2026-04-29

## 1 What this vignette is for

`splitGraph` ends at a `split_spec` object. It deliberately knows nothing about `rsample`, `tidymodels`, or any other resampling engine. The handoff contract is the `sample_data` table inside the spec plus a few scalar fields (`group_var`, `block_vars`, `time_var`, `ordering_required`, `recommended_resampling`).

This cookbook shows three small, self-contained adapters that turn a `split_spec` into something a downstream workflow can use:

1. A base-R adapter that returns a list of `(train, test)` row-index pairs — runnable here, no extra dependencies.
2. An `rsample::group_vfold_cv()` adapter for grouped cross-validation keyed to `group_id`.
3. An `rsample::rolling_origin()` adapter for ordered evaluation keyed to `order_rank`.

Adapters 2 and 3 show idiomatic glue but are not evaluated in this vignette so that `splitGraph` does not pick up `rsample` as a build-time dependency.

The same pattern works for any other resampling library you happen to use.

## 2 Build a split_spec to work with

    meta <- data.frame(
      sample_id    = c("S1", "S2", "S3", "S4", "S5", "S6"),
      subject_id   = c("P1", "P1", "P2", "P2", "P3", "P3"),
      batch_id     = c("B1", "B2", "B1", "B2", "B1", "B2"),
      timepoint_id = c("T0", "T1", "T0", "T1", "T0", "T1"),
      time_index   = c(0, 1, 0, 1, 0, 1),
      outcome_id   = c("ctrl", "case", "ctrl", "case", "case", "ctrl"),
      stringsAsFactors = FALSE
    )

    g <- graph_from_metadata(meta, graph_name = "cookbook")
    subject_constraint <- derive_split_constraints(g, mode = "subject")
    spec <- as_split_spec(subject_constraint, graph = g)
    spec
    #> <split_spec> subject
    #>   Samples: 6
    #>   Groups: 3
    #>   Recommended resampling: grouped_cv

The `sample_data` table is the contract:

    as.data.frame(spec)[, c("sample_id", "group_id", "batch_group", "order_rank")]
    #>   sample_id   group_id batch_group order_rank
    #> 1        S1 subject:P1          B1          1
    #> 2        S2 subject:P1          B2          2
    #> 3        S3 subject:P2          B1          1
    #> 4        S4 subject:P2          B2          2
    #> 5        S5 subject:P3          B1          1
    #> 6        S6 subject:P3          B2          2

## 3 Adapter 1 — base R: leave-one-group-out folds

This is the simplest meaningful adapter. It groups by whatever `split_spec$group_var` says is the split unit, and returns one held-out group per fold.

    logo_folds <- function(spec, observation_data, sample_id_col = "sample_id") {
      stopifnot(inherits(spec, "split_spec"))
      if (!sample_id_col %in% names(observation_data)) {
        stop("`observation_data` must contain a `", sample_id_col, "` column.")
      }

      joined <- merge(
        observation_data,
        spec$sample_data[, c("sample_id", spec$group_var)],
        by.x = sample_id_col, by.y = "sample_id", sort = FALSE
      )
      joined$.row <- seq_len(nrow(joined))
      groups <- split(joined$.row, joined[[spec$group_var]])

      lapply(names(groups), function(g) {
        list(
          group   = g,
          train   = unlist(groups[setdiff(names(groups), g)], use.names = FALSE),
          assess  = groups[[g]]
        )
      })
    }

    # Pretend we have an observation frame keyed by sample_id.
    obs <- data.frame(
      sample_id = meta$sample_id,
      x = rnorm(nrow(meta)),
      y = rbinom(nrow(meta), 1, 0.5)
    )

    folds <- logo_folds(spec, obs)
    length(folds)
    #> [1] 3
    folds[[1]]
    #> $group
    #> [1] "subject:P1"
    #>
    #> $train
    #> [1] 3 4 5 6
    #>
    #> $assess
    #> [1] 1 2

That is the entire downstream contract: take `spec`, take an observation frame, return train/assess index lists. Anything more complicated is specific to a resampling library.

## 4 Adapter 2 — `rsample::group_vfold_cv()`

Grouped CV keyed to `group_id`. The downstream package would typically ship something like this; the adapter is short enough that you can paste it into your own analysis script.

    spec_to_group_vfold <- function(spec, observation_data,
                                    v = NULL,
                                    sample_id_col = "sample_id") {
      stopifnot(inherits(spec, "split_spec"))
      if (!requireNamespace("rsample", quietly = TRUE)) {
        stop("Install rsample to use this adapter.")
      }

      joined <- merge(
        observation_data,
        spec$sample_data[, c("sample_id", spec$group_var)],
        by.x = sample_id_col, by.y = "sample_id", sort = FALSE
      )

      n_groups <- length(unique(joined[[spec$group_var]]))
      if (is.null(v)) v <- n_groups

      rsample::group_vfold_cv(
        data  = joined,
        group = !!spec$group_var,
        v     = v
      )
    }

`v = NULL` (the default above) gives leave-one-group-out, which is the right default when `splitGraph` has already grouped samples by their deepest leakage-relevant unit (e.g. subject). Pick a smaller `v` for k-fold-style grouped CV.

## 5 Adapter 3 — `rsample::rolling_origin()`

When `spec$ordering_required` is `TRUE` (or `spec$time_var` is set), the right downstream object is an ordered split rather than a grouped one.

    spec_to_rolling_origin <- function(spec, observation_data,
                                       sample_id_col = "sample_id",
                                       initial = NULL,
                                       assess = 1L) {
      stopifnot(inherits(spec, "split_spec"))
      if (is.null(spec$time_var)) {
        stop("This split_spec has no `time_var`; ordered evaluation is not available.")
      }
      if (!requireNamespace("rsample", quietly = TRUE)) {
        stop("Install rsample to use this adapter.")
      }

      joined <- merge(
        observation_data,
        spec$sample_data[, c("sample_id", spec$time_var)],
        by.x = sample_id_col, by.y = "sample_id", sort = FALSE
      )
      ordered <- joined[order(joined[[spec$time_var]]), , drop = FALSE]

      if (is.null(initial)) initial <- max(1L, floor(nrow(ordered) * 0.6))
      rsample::rolling_origin(ordered, initial = initial, assess = assess)
    }

The key idea: `splitGraph` puts ordering information on the spec; the adapter is just a thin shim that consumes it.

## 6 Going across language boundaries via JSON

If the downstream consumer is not in R, write the spec to JSON and let the consumer (Python, Julia, a CLI) interpret it.

    tmp <- tempfile(fileext = ".json")
    write_split_spec(spec, tmp)

    # Inspect the first ~30 lines so the on-disk format is visible.
    cat(readLines(tmp, n = 30), sep = "\n")
    #> {
    #>   "splitGraph_object": "split_spec",
    #>   "schema_version": "0.1.0",
    #>   "group_var": "group_id",
    #>   "block_vars": [
    #>     "batch_group"
    #>   ],
    #>   "time_var": "order_rank",
    #>   "ordering_required": false,
    #>   "constraint_mode": "subject",
    #>   "constraint_strategy": "subject",
    #>   "recommended_resampling": "grouped_cv",
    #>   "metadata": {
    #>     "graph_name": "cookbook",
    #>     "dataset_name": null,
    #>     "source_mode": "subject",
    #>     "source_strategy": "subject",
    #>     "relations_used": "sample_belongs_to_subject",
    #>     "n_samples": 6,
    #>     "n_groups": 3,
    #>     "warnings": [],
    #>     "enriched_from_graph": true
    #>   },
    #>   "sample_data": [
    #>     {
    #>       "sample_id": "S1",
    #>       "sample_node_id": "sample:S1",
    #>       "group_id": "subject:P1",
    #>       "primary_group": "subject:P1",
    #>       "batch_group": "B1",

    # And read it back exactly.
    spec2 <- read_split_spec(tmp)
    identical(spec$sample_data$group_id, spec2$sample_data$group_id)
    #> [1] TRUE

    unlink(tmp)

The same pair exists for `dependency_graph` (`write_dependency_graph()` / `read_dependency_graph()`). Both formats are documented under `?write_split_spec` and `?write_dependency_graph` and include a `schema_version` field so consumers can detect drift.

## 7 When you need a custom adapter

The only assumptions an adapter has to honor:

- `split_spec$sample_data` is keyed by `sample_id` (character).
- `split_spec$group_var` is the column that holds the splitting unit.
- `split_spec$block_vars` are present-but-coarser blocking columns.
- `split_spec$time_var`, when non-`NULL`, defines the ordering.
- `split_spec$recommended_resampling` is a hint, not a contract — your adapter is free to ignore it.

That is the whole interface. As long as those five fields are honored, anything is a valid downstream consumer.