small optimization to check/assert_field_class()

instead of subsetting the data.table and then retrieving the columns'
classes, we now get the classes first and then subset. this change
speeds the function up by a second or so, which is quite noticeable when
we're making function calls in downstream packages (e.g. gtfstools) that
should be quite fast

R/checks.R

@@ -200,7 +200,8 @@ check_field_class <- function(x, file, fields, classes) {
 
   # checking - compare the desired classes to the actual classes
 
-  actual_classes <- vapply(x[[file]][, ..fields], class, character(1))
+  col_classes <- vapply(x[[file]], class, character(1))
+  actual_classes <- col_classes[fields]
 
   if (all(classes == actual_classes)) return(TRUE)
 
@@ -227,7 +228,8 @@ assert_field_class <- function(x, file, fields, classes) {
 
   # actual checking - compare the desired classes to the actual classes
 
-  actual_classes <- vapply(x[[file]][, ..fields], class, character(1))
+  col_classes <- vapply(x[[file]], class, character(1))
+  actual_classes <- col_classes[fields]
 
   if (all(classes == actual_classes)) return(invisible(x))