Merge pull request tidyverse#2859 from lionel-/fix-top-n-2648

Implement top_n() lazily
xkdog · Jun 14, 2017 · 81cef2e · 81cef2e
2 parents 08500a4 + bc56f73
commit 81cef2e
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diff --git a/NEWS.md b/NEWS.md
@@ -1,6 +1,10 @@
 # dplyr 0.7.0.9000
 
-* Fix undefined behaviour in `between()`, where `NA_REAL` were assigned instead of `NA_LOGICAL`. (#2855, @zeehio)
+* Fix undefined behaviour in `between()`, where `NA_REAL` were
+  assigned instead of `NA_LOGICAL`. (#2855, @zeehio)
+
+* `top_n()` now executes operations lazily for compatibility with
+  database backends (#2848).
 
 # dplyr 0.7.0
 

diff --git a/R/top-n.R b/R/top-n.R
@@ -44,20 +44,19 @@ top_n <- function(x, n, wt) {
 
   if (quo_is_missing(wt)) {
     vars <- tbl_vars(x)
-    inform(glue("Selecting by ", vars[length(vars)]))
-    wt <- pull(x, -1)
-  } else {
-    wt <- eval_tidy(wt, x)
+    wt_name <- vars[length(vars)]
+    inform(glue("Selecting by ", wt_name))
+    wt <- sym(wt_name)
   }
 
   if (!is_scalar_integerish(n)) {
     abort("`n` must be a scalar integer")
   }
 
   if (n > 0) {
-    quo <- quo(filter(x, min_rank(desc(!!wt)) <= !!n))
+    quo <- quo(filter(x, min_rank(desc(!! wt)) <= !! n))
   } else {
-    quo <- quo(filter(x, min_rank(!!wt) <= !!abs(n)))
+    quo <- quo(filter(x, min_rank(!! wt) <= !! abs(n)))
   }
 
   eval_tidy(quo)