setDT method for tbl_df class

[jangorecki]

Output of many function in dplyr pkg is a tbl_df class objects. It is not handled correctly by setDT which works on regular df and lists very well. See below:

packageVersion("dplyr")
# [1] ‘0.4.1’
packageVersion("data.table")
# [1] ‘1.9.5’
library(dplyr) # latest CRAN
library(data.table) # latest dev

df <- data.frame(a = rnorm(1000), b = sample(1:5,1000,TRUE))
class(df)
# [1] "data.frame"
df2 <- as.tbl(df)
class(df2)
# [1] "tbl_df"     "tbl"        "data.frame"
setDT(df2)
class(df2)
# [1] "tbl_df"     "tbl"        "data.table" "data.frame"

Would be great to have setDT works on tbl_df and possibly also tbl classes.
Currently such object is not usable without using first setDT(as.data.frame(df2))[].

Edit
Or easier/faster operating directly on class attribute: setattr(df,"class",c("data.table","data.frame"))

[arunsrinivasan]

#1128 also seems related. Possible fix is as follows:

• If an object is already a data.table with additional classes, we retain in any and all operations possible (current behaviour as such need not be modified).
• If an object is converted from another object to data.table, then we simply do not retain classes.