nrow returns NULL for a simulated source?

[Michael Sumner]

I'm trying to understand tbl_lazy and op_vars behaviour. 

I expect that nrow() should return NA for a simulated source, but we get NULL

library(dbplyr)

library(dplyr)

## example data 

df <- tibble::tibble(apples = 1:3, oranges = c("a", "b", "c"))

## example SQLite db

db <- src_sqlite(tempfile(), create = TRUE)

## a real SQLite tbl

real_tbl <- copy_to(db, df)

## a simulated SQLite tbl

sim_tbl <- tbl_lazy(df, simulate_sqlite())

## NA, as expected

nrow(real_tbl)

## NULL, not expected

nrow(sim_tbl)

My question is, should nrow(real_tbl) return NA? How does that work though, I'm confused about the ops_ control here. 

The problem comes with the print: 

## this causes printing to fail in trunc_mat

print(sim_tbl)

#Error in if (is.na(rows) || rows > tibble_opt("print_max")) { : 

#    missing value where TRUE/FALSE needed

#  In addition: Warning message:

#    In is.na(rows) : is.na() applied to non-(list or vector) of type 'NULL'

## though other ops do work

str(op_base(real_tbl, "apples"))

str(op_base(sim_tbl, "apples"))

[Michael Sumner]

Oh, I forgot about the new site!

I'll ask on RStudio community, thanks

[Michael Sumner]

I figured it out, described here: https://community.rstudio.com/t/dbplyr-lazy-ops-nrow-returns-null-for-a-simulated-source/5442?u=mdsumner