`complete` doesn't work well with grouped data frames.

[cimentadaj]

I just spent a few minutes figuring out why complete was generating all combination of values. This was happening because the data frame was grouped. Perhaps it would be useful to others to document this in the details section of complete.

library(dplyr, warn.conflicts = FALSE)
library(tidyr)
df <- tibble(
  group = c(1:2, 1),
  item_id = c(1:2, 2),
  item_name = c("a", "b", "b"),
  value1 = 1:3,
  value2 = 4:6
)

# Expected
df %>% complete(group, item_id, item_name)
#> # A tibble: 8 x 5
#>   group item_id item_name value1 value2
#>   <dbl>   <dbl> <chr>      <int>  <int>
#> 1     1       1 a              1      4
#> 2     1       1 b             NA     NA
#> 3     1       2 a             NA     NA
#> 4     1       2 b              3      6
#> 5     2       1 a             NA     NA
#> 6     2       1 b             NA     NA
#> 7     2       2 a             NA     NA
#> 8     2       2 b              2      5

# Not expected
df %>%
  group_by(group, item_id, item_name) %>%
  complete(group, item_id, item_name)
#> # A tibble: 3 x 5
#> # Groups:   group, item_id, item_name [3]
#>   group item_id item_name value1 value2
#>   <dbl>   <dbl> <chr>      <int>  <int>
#> 1     1       1 a              1      4
#> 2     1       2 b              3      6
#> 3     2       2 b              2      5

[Sibojang9]

I could not find clear documentation on the complete function, especially the use of grouping variables. Current official example showed only one grouping variable.

I tried complete(group,group2, nesting(item_id, item_name)) , which would run but ended up with surprising results. A more comprehensive guidance on complete is expected.

[monkeywithacupcake]

Related to #396

[DavisVaughan]

Slightly more minimal reprex:

library(dplyr, warn.conflicts = FALSE)
library(tidyr)

df <- tibble(
  group1 = 1:2,
  group2 = 3:4
)

df %>% 
  expand(group1, group2)
#> # A tibble: 4 × 2
#>   group1 group2
#>    <int>  <int>
#> 1      1      3
#> 2      1      4
#> 3      2      3
#> 4      2      4

df %>% 
  group_by(group1, group2) %>%
  expand(group1, group2)
#> # A tibble: 2 × 2
#> # Groups:   group1, group2 [2]
#>   group1 group2
#>    <int>  <int>
#> 1      1      3
#> 2      2      4

^{Created on 2021-12-20 by the reprex package (v2.0.1)}

[DavisVaughan]

This issue in particular is really a documentation issue. complete() on a grouped-df operates within each group. #1289 added an example with a grouped-df and a section in the docs detailing this