Translate [[ with numeric indexes

Fixes #520

NEWS.md

@@ -56,6 +56,8 @@
 
   * You can now use `::` in translations, so that (e.g.) `dbplyr::n()` is
     translated to `count(*)` (#207).
+
+  * `[[` can now also translate numeric indices (#520).
 
   * `%/%` now generates a clear error message; previously it was translated to
     `/` which is not correct (#108).

R/backend-.R

@@ -43,10 +43,14 @@ base_scalar <- sql_translator(
   `$`   = sql_infix(".", pad = FALSE),
   `[[`   = function(x, i) {
     i <- enexpr(i)
-    if (!is.character(i)) {
-      stop("Can only index with strings", call. = FALSE)
+    if (is.character(i)) {
+      build_sql(x, ".", ident(i))
+    } else if (is.numeric(i)) {
+      build_sql(x, "[", as.integer(i), "]")
+    } else {
+      stop("Can only index with strings and numbers", call. = FALSE)
     }
-    build_sql(x, ".", ident(i))
+
   },
   `[` = function(x, i) {
     build_sql("CASE WHEN (", i, ") THEN (", x, ") END")

tests/testthat/test-backend-.R

@@ -36,6 +36,14 @@ test_that("log becomes ln", {
   expect_equal(translate_sql(log(x)), sql('LN(`x`)'))
 })
 
+test_that("can translate subsetting", {
+  expect_equal(translate_sql(a$b), sql("`a`.`b`"))
+  expect_equal(translate_sql(a[["b"]]), sql("`a`.`b`"))
+
+  expect_equal(translate_sql(a[["b"]][[1]]), sql('`a`.`b`[1]'))
+})
+
+
 # binary/bitwise ---------------------------------------------------------------
 
 test_that("bitwise operations", {

tests/testthat/test-translate-sql.R

@@ -183,17 +183,6 @@ test_that("str_trim() translates correctly ", {
 
 # subsetting --------------------------------------------------------------
 
-test_that("$ and [[ index into nested fields", {
-  expect_equal(translate_sql(a$b), sql("`a`.`b`"))
-
-  expect_equal(translate_sql(a[["b"]]), sql("`a`.`b`"))
-})
-
-test_that("can only subset with strings", {
-  expect_error(translate_sql(a[[1]]), "index with strings")
-  expect_error(translate_sql(a[[x]]), "index with strings")
-})
-
 test_that("[ treated as if it is logical subsetting", {
   expect_equal(translate_sql(y[x == 0L]), sql("CASE WHEN (`x` = 0) THEN (`y`) END"))
 })