Add test for example where $over(..., order_by = ...) is useful

[etiennebacher]

See examples added in pola-rs/r-polars#1147

[etiennebacher]

I think not possible because I can't join several $over() in the same expression: pola-rs/polars#12051

library(polars)
library(dplyr, warn.conflicts = FALSE)
options(polars.do_not_repeat_call = TRUE)

df = pl$DataFrame(
  g = c(1, 1, 1, 1, 2, 2, 2, 2),
  t = c(1, 2, 3, 4, 4, 1, 2, 3),
  x = c(10, 20, 30, 40, 10, 20, 30, 40)
)

df$with_columns(
  x_lag = pl$col("x")$shift(1)$over("g", order_by = "t")
)
#> shape: (8, 4)
#> ┌─────┬─────┬──────┬───────┐
#> │ g   ┆ t   ┆ x    ┆ x_lag │
#> │ --- ┆ --- ┆ ---  ┆ ---   │
#> │ f64 ┆ f64 ┆ f64  ┆ f64   │
#> ╞═════╪═════╪══════╪═══════╡
#> │ 1.0 ┆ 1.0 ┆ 10.0 ┆ null  │
#> │ 1.0 ┆ 2.0 ┆ 20.0 ┆ 10.0  │
#> │ 1.0 ┆ 3.0 ┆ 30.0 ┆ 20.0  │
#> │ 1.0 ┆ 4.0 ┆ 40.0 ┆ 30.0  │
#> │ 2.0 ┆ 4.0 ┆ 10.0 ┆ 40.0  │
#> │ 2.0 ┆ 1.0 ┆ 20.0 ┆ null  │
#> │ 2.0 ┆ 2.0 ┆ 30.0 ┆ 20.0  │
#> │ 2.0 ┆ 3.0 ┆ 40.0 ┆ 30.0  │
#> └─────┴─────┴──────┴───────┘

df$with_columns(
  x_lag = (pl$col("x")$shift(1)$over(order_by = "t"))$over("g")
)
#> Error: Execution halted with the following contexts
#>    0: In R: in $with_columns()
#>    1: Encountered the following error in Rust-Polars:
#>          invalid operation: window expression not allowed in aggregation

df |> 
  as_tibble() |> 
  mutate(
    x_lag = lag(x, order_by = t),
    .by = g
  )
#> # A tibble: 8 × 4
#>       g     t     x x_lag
#>   <dbl> <dbl> <dbl> <dbl>
#> 1     1     1    10    NA
#> 2     1     2    20    10
#> 3     1     3    30    20
#> 4     1     4    40    30
#> 5     2     4    10    40
#> 6     2     1    20    NA
#> 7     2     2    30    20
#> 8     2     3    40    30