Refresh attributes of sfc column when subsetting from data.table

Start by reproducing the issue's data.table subsetting example with an sfc column, then compare its behavior with subsetting an sf object. Trace the data.table subsetting path and the sfc attributes shown in the report; done means supported subsets refresh metadata such as bbox without manual reconstruction.

I like to use the sf package and data.table together. Instead of using the sf class, which is built on top of the data.frame class, I use the data.table class with a column of class sfc which contains a vector of simple features with some metadata kept in the object's attributes.

However, when I subset some rows of my data.table, those attributes are not automatically updated, which can cause some problems. When subsetting from an sf, those attributes are automatically updated.

I have a non-elegant way to get around the problem by manually updating the attributes of the sfc column, but I'm wondering if there is a way to make it under the hood.

Let's say I have the following

library(sf)
library(data.table)

set.seed(20200130)

data <- data.table(
  id = 1:5,
  point = st_sfc(replicate(5, st_point(c(runif(1), runif(1))), simplify = FALSE))
)

data[]
##     id                        point
##  1:  1 POINT (0.09302893 0.6560987)
##  2:  2  POINT (0.4387638 0.7161379)
##  3:  3 POINT (0.8535522 0.08598417)
##  4:  4   POINT (0.927848 0.3534847)
##  5:  5 POINT (0.9615244 0.07300738)

Let's look at the attributes of the point column.

attributes(data$point)
##  $class
##  [1] "sfc_POINT" "sfc"      
##  
##  $precision
##  [1] 0
##  
##  $bbox
##        xmin       ymin       xmax       ymax 
##  0.09302893 0.07300738 0.96152443 0.71613790 
##  
##  $crs
##  Coordinate Reference System: NA
##  
##  $n_empty
##  [1] 0

If I subset some elements, the attributes won't update.

subset <- data[1:2]
attributes(subset$point)
##  $class
##  [1] "sfc_POINT" "sfc"      
##  
##  $precision
##  [1] 0
##  
##  $bbox
##        xmin       ymin       xmax       ymax 
##  0.09302893 0.07300738 0.96152443 0.71613790 
##  
##  $crs
##  Coordinate Reference System: NA
##  
##  $n_empty
##  [1] 0

However, if I use an sf object, they will silently update (you can see it easily with the bbox attribute).

set.seed(20200130)

data <- st_sf(
  id = 1:5,
  point = st_sfc(replicate(5, st_point(c(runif(1), runif(1))), simplify = FALSE))
)

subset <- data[1:2, ]

attributes(subset$point)
##  $class
##  [1] "sfc_POINT" "sfc"      
##  
##  $precision
##  [1] 0
##  
##  $bbox
##        xmin       ymin       xmax       ymax 
##  0.09302893 0.65609871 0.43876379 0.71613790 
##  
##  $crs
##  Coordinate Reference System: NA
##  
##  $n_empty
##  [1] 0

I don't understand everything behind how data.table works, but I'm wondering if there is a way you could update the attributes of any sfc column present in a data.table. An easy way of doing so would be to wrap a generalization of something like this.

set.seed(20200130)

data <- data.table(
  id = 1:5,
  point = st_sfc(replicate(5, st_point(c(runif(1), runif(1))), simplify = FALSE))
)

subset <- data[1:2]
subset$point <- st_sfc(lapply(subset$point, identity))

attributes(subset$point)
##  $class
##  [1] "sfc_POINT" "sfc"      
##  
##  $precision
##  [1] 0
##  
##  $bbox
##        xmin       ymin       xmax       ymax 
##  0.09302893 0.65609871 0.43876379 0.71613790 
##  
##  $crs
##  Coordinate Reference System: NA
##  
##  $n_empty
##  [1] 0

We would then achieve the same result than when using an sf object.

Like I said, I'm not familiar with the development of data.table, I'm really just a user who don't like to use any other data structure than data.table. You will excuse me if I'm missing something on why it is impossible to implement such feature.

sessionInfo()
##  R version 3.6.1 (2019-07-05)
##  Platform: x86_64-suse-linux-gnu (64-bit)
##  Running under: SUSE Linux Enterprise Server 12 SP1
##  
##  Matrix products: default
##  BLAS:   /usr/lib64/R/lib/libRblas.so
##  LAPACK: /usr/lib64/R/lib/libRlapack.so
##  
##  locale:
##   [1] LC_CTYPE=en_US.UTF-8          LC_NUMERIC=C                  LC_TIME=en_US.UTF-8          
##   [4] LC_COLLATE=en_US.UTF-8        LC_MONETARY=en_US.UTF-8       LC_MESSAGES=en_US.UTF-8      
##   [7] LC_PAPER=en_US.UTF-8          LC_NAME=en_US.UTF-8           LC_ADDRESS=en_US.UTF-8       
##  [10] LC_TELEPHONE=en_US.UTF-8      LC_MEASUREMENT=en_US.UTF-8    LC_IDENTIFICATION=en_US.UTF-8
##  
##  attached base packages:
##  [1] stats     graphics  grDevices utils     datasets  methods   base     
##  
##  other attached packages:
##  [1] leaflet_2.0.3       ggspatial_1.0.3     OpenStreetMap_0.3.4 ggplot2_3.2.1      
##  [5] data.table_1.12.6   sf_0.8-0            magrittr_1.5        dplyr_0.8.3        
##  [9] DBI_1.0.0          
##  
##  loaded via a namespace (and not attached):
##   [1] tidyselect_0.2.5        purrr_0.3.3             rJava_0.9-11           
##   [4] lattice_0.20-38         leaflet.providers_1.9.0 colorspace_1.4-1       
##   [7] vctrs_0.2.0             htmltools_0.4.0         segter_0.0.0.9000      
##  [10] yaml_2.2.0              blob_1.2.0              rlang_0.4.2            
##  [13] e1071_1.7-3             pillar_1.4.2            later_1.0.0            
##  [16] glue_1.3.1              withr_2.1.2             sp_1.3-2               
##  [19] bit64_0.9-7             dbplyr_1.4.2            lifecycle_0.1.0        
##  [22] munsell_0.5.0           gtable_0.3.0            raster_3.0-7           
##  [25] htmlwidgets_1.5.1       codetools_0.2-16        labeling_0.3           
##  [28] fastmap_1.0.1           httpuv_1.5.2            crosstalk_1.0.0        
##  [31] class_7.3-15            Rcpp_1.0.3              xtable_1.8-4           
##  [34] KernSmooth_2.23-16      scales_1.1.0            backports_1.1.5        
##  [37] promises_1.1.0          classInt_0.4-2          jsonlite_1.6           
##  [40] mime_0.7                farver_2.0.1            bit_1.1-14             
##  [43] hms_0.5.2               digest_0.6.23           shiny_1.4.0            
##  [46] grid_3.6.1              rgdal_1.4-8             odbc_1.2.1             
##  [49] tools_3.6.1             lazyeval_0.2.2          tibble_2.1.3           
##  [52] crayon_1.3.4            pkgconfig_2.0.3         zeallot_0.1.0          
##  [55] assertthat_0.2.1        rstudioapi_0.10         R6_2.4.1               
##  [58] units_0.6-5             compiler_3.6.1