Is there a package for object-relational mapping in R?

(By object-relational mapping, I mean what is described here: Wikipedia: Object-relational mapping.)

Here is how I could imagine this work in R : a kind of "virtual data frame" is linked to a database, and returns the results of SQL queries when accessed. For instance, head(virtual_list) would actually return the results of (select * from mapped_table limit 5) on the mapped database.

I have found this post by John Myles White, but there seems to have been no progress in the last 3 years.

Is there a working package that implements this ?

If not,

1. Would it be useful ?
2. What would be the best way to implement it (S4 ?) ?

The very recent package dplyr is implementing this (amongst other amazing features).

Here are illustrations from the examples of function src_mysql():

# Connection basics ---------------------------------------------------------
# To connect to a database first create a src:
my_db <- src_mysql(host = "blah.com", user = "hadley",
  password = "pass")
# Then reference a tbl within that src
my_tbl <- tbl(my_db, "my_table")

# Methods -------------------------------------------------------------------
batting <- tbl(lahman_mysql(), "Batting")
dim(batting)
colnames(batting)
head(batting)