fix delete_layer default in write_sf.DBIObject

[etiennebr]

• The default is to delete a table if it exists (append = FALSE)
• Using append = TRUE will set delete_layer = FALSE and append to an existing table or create one
• Using append = TRUE and delete_layer = TRUE will raise an error using delete_layer rather than overwrite

fix #1599
related #1266

[etiennebr]

I'll have a look at the failures later today

[edzer]

regenerate docs.

[etiennebr]

Looks good now.

[edzer]

Thanks! Could you please double check this is still consistent with the current documentation?

#' @param append logical; should we append to an existing layer, or replace it?
#' if \code{TRUE} append, if \code{FALSE} replace; default \code{NA} will
#' raise an error if the layer exists. See also next two arguments.
#' @param delete_dsn logical; delete data source \code{dsn} before attempting
#' to write?
#' @param delete_layer logical; delete layer \code{layer} before attempting to
#' write?

[etiennebr]

Good point Edzer. I'll see what I can do to use NA to preserve a consistent behavior, but it might be tricky with the database.

[etiennebr]

I'll need to look at this more closely because I'm not sure what would make the most sense from a user perspective. In the sf1.Rmd vignette, it is mentioned that write_sf overwrites by default (which matches other write_* functions).

sf/vignettes/sf1.Rmd

Lines 477 to 485 in 61efbda

	```{r}
	st_write(nc, "nc.shp", delete_layer = TRUE)
	```
	or its quiet alternative that does this by default,
	```{r}
	write_sf(nc, "nc.shp") # silently overwrites
	```

However, what seems to be the agreement in #1266 (comment) is:

this commit renames update into append, deprecates update, raises an error if the layer exists but append has not been specified to either TRUE (do append) or FALSE (overwrite layer) .

I decided (for now) against a separate overwrite argument, as setting append=FALSE overwrites. In addition, we do have the delete_dsn and delete_layer logic of GDAL, which does different things to different drivers, which I think we want to keep.

Also As you mentioned:

sf/R/read.R

Lines 329 to 337 in 61efbda

	#' @param append logical; should we append to an existing layer, or replace it?
	#' if \code{TRUE} append, if \code{FALSE} replace; default \code{NA} will
	#' raise an error if the layer exists. See also next two arguments.
	#' @param delete_dsn logical; delete data source \code{dsn} before attempting
	#' to write?
	#' @param delete_layer logical; delete layer \code{layer} before attempting to
	#' write?
	#' @param fid_column_name character, name of column with feature IDs; if
	#' specified, this column is no longer written as feature attribute.

[etiennebr]

Sorry for the hesitation. I would like your approval on this @edzer.

I went back to original write_*() functions form readr, because if I remember correctly this is what the inspiration was for the behaviour.

append If FALSE, will overwrite existing file. If TRUE, will append to existing file. In both cases, if the file does not exist a new file is created.

The default is FALSE, so it overwrites by default.

If we decide to follow this behaviour for write_sf the strict implementation would be to set delete_layer=!append, but I think it would make sense to expose it in the arguments (as in a603d39). In that case, we could document the parameter as:

#' @param append logical; should we append to an existing layer, or replace it? 
#' if \code{TRUE} append, if \code{FALSE} replace. 
#' The default for \code{st_write} is \code{NA} which raises an error if the layer exists. 
#' The default for \code{write_sf} is \code{FASE}, which overwrites by default.
#' See also next two arguments for more control on overwrite behavior. 

Meanwhile I'll go with this version for the PR so you can merge it, but let me know if you would prefer another behaviour.

[etiennebr]

@edzer, let me know if there's anything missing to merge

[edzer]

Thanks a lot, Etienne!