But in tidyr::nest() where nested variables are dropped if acting on a 'grouped' df

library(magrittr)
library(tibble)
library(tidyr)
#> 
#> Attaching package: 'tidyr'
#> The following object is masked from 'package:magrittr':
#> 
#>     extract
library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
# Proper nesting of cyl using group_by
df1 <- mtcars %>%
  group_by(cyl) %>%
  nest()
df1
#> # A tibble: 3 x 2
#>     cyl data              
#>   <dbl> <list>            
#> 1     6 <tibble [7 × 10]> 
#> 2     4 <tibble [11 × 10]>
#> 3     8 <tibble [14 × 10]>
names(df1$data[[1]])
#>  [1] "mpg"  "disp" "hp"   "drat" "wt"   "qsec" "vs"   "am"   "gear" "carb"

# Proper nesting of cyl using nest alone
df2 <- as_tibble(mtcars) %>%
  nest(-cyl)
df2
#> # A tibble: 3 x 2
#>     cyl data              
#>   <dbl> <list>            
#> 1     6 <tibble [7 × 10]> 
#> 2     4 <tibble [11 × 10]>
#> 3     8 <tibble [14 × 10]>
names(df2$data[[1]])
#>  [1] "mpg"  "disp" "hp"   "drat" "wt"   "qsec" "vs"   "am"   "gear" "carb"

# Now assume you did a group_by and forgot to 
# ungroup and then wanted to nest on gear
df3 <- mtcars %>%
  group_by(cyl) %>%
  #mutate(cyl_index = 1:n()) %>%
  nest(-gear)
df3
#> # A tibble: 3 x 2
#>     cyl data             
#>   <dbl> <list>           
#> 1     6 <tibble [7 × 9]> 
#> 2     4 <tibble [11 × 9]>
#> 3     8 <tibble [14 × 9]>
names(df3$data[[1]])
#> [1] "mpg"  "disp" "hp"   "drat" "wt"   "qsec" "vs"   "am"   "carb"
# In this case the group_by supersedes the nest argument.
# That's not much of a problem
# The problem is gear is now dropped from the nested data.frame

^{Created on 2019-01-30 by the reprex package (v0.2.1)}

Session info

devtools::session_info()
#> ─ Session info ──────────────────────────────────────────────────────────
#>  setting  value                       
#>  version  R version 3.5.2 (2018-12-20)
#>  os       macOS Mojave 10.14.3        
#>  system   x86_64, darwin15.6.0        
#>  ui       X11                         
#>  language (EN)                        
#>  collate  en_US.UTF-8                 
#>  ctype    en_US.UTF-8                 
#>  tz       America/Denver              
#>  date     2019-01-30                  
#> 
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#> 
#> [1] /Users/sfield/r_libs
#> [2] /Library/Frameworks/R.framework/Versions/3.5/Resources/library

stufield/nest_grouped.md