Automatically add more subclasses to tbl() inferred class?

[MichaelChirico]

dbplyr/R/src_dbi.R

Line 87 in 5726930

subclass <- class(src$con)[[1]] # prefix added by dplyr::make_tbl

We have a DBIConnection class that inherits from a virtual class:

is(con)
# [1] "MyDBIConnection"        "VirtualDBIConnection" "DBIConnection"          "DBIObject"

We had to write a tbl.MyDBIConnection in order to get the desired class structure of the output:

tbl.MyDBIConnection <- function(src, from, ...) {
  dbi_src <- src_dbi(src, auto_disconnect = FALSE)
  tbl <- tbl_sql(c("MyDBIConnection", "VirtualDBIConnection", "dbi"), src = dbi_src, from = from, ...)
  return(tbl)
}

It seems we could instead try to do this automatically:

tbl.src_dbi <- function(src, from, ...) {
  inheritance <- is(src$con)
  subclass <- head(inherits, match("DBIConnection", inheritance)-1L) # prefix added by dplyr::make_tbl
  tbl_sql(c(subclass, "dbi"), src = src, from = from, ...)
}

However I'm not sure the broader implications of this (do we require src$con to be S4? do we require it to inherit from DBIConnection?)

[MichaelChirico]

Actually this affects copy_to() as well, where this keeps only the first of src_MyDBIConnection/src_VirtualDBIConnection, so I think any fix would need to be a bit more sophisticated. (for now I've just retained our tbl.src_MyDBIConnection method).