fix: add backward compatibility

R/MsBackend.R

@@ -1119,7 +1119,9 @@ setMethod("dropNaSpectraVariables", "MsBackend", function(object) {
 #'
 #' @export
 setMethod("extractByIndex", c("MsBackend", "ANY"), function(object, i) {
-    stop("'extractByIndex' not implemented for ", class(object), ".")
+    if (existsMethod("[", class(object)[1L]))
+        object[i = i]
+    else stop("'extractByIndex' not implemented for ", class(object)[1L], ".")
 })
 
 #' @rdname MsBackend

tests/testthat/test_MsBackend.R

@@ -59,6 +59,30 @@ test_that("MsBackend methods throw errors", {
     expect_error(extractByIndex(dm, 1), "implemented for")
 })
 
+test_that("extractByIndex not implemented fallback", {
+    ## Backends that don't implement a dedicated `extractByIndex` method should
+    ## fall back to the [ method.
+    setClass("DummyBackend",
+             contains = "MsBackend",
+             slots = c(d = "integer"))
+    dm <- new("DummyBackend")
+    expect_error(extractByIndex(dm, 1L), "'extractByIndex' not implemented")
+
+    dm@d <- 1:4
+
+    ## Have an implementation for [ but not extractByIndex:
+    setMethod("[", "DummyBackend", function(x, i, j, ..., drop = FALSE) {
+        x@d <- x@d[i]
+        x
+    })
+
+    res <- dm[c(3, 1)]
+    expect_equal(res@d, c(3L, 1L))
+
+    res <- extractByIndex(dm, c(3, 1))
+    expect_equal(res@d, c(3L, 1L))
+})
+
 test_that("reset,MsBackend works", {
     res <- reset(sciex_mzr)
     expect_equal(res, sciex_mzr)