data.table: using setkey with a column name variable

I have a variable name saved into the string variable which_id.

W is a data.table. How do I call setkey on W with which_id ?

This is what I've tried

 > eval( paste( 'setkey(W,' , which_id , ')' ) )
[1] "setkey(W, customer_id_A )"

But a call to tables() shows that the customer_id_A key didn't take.

 > evalq( paste( 'setkey(W,' , which_id , ')' ) )
[1] "setkey(W, customer_id_A )"

customer_id_A key still didn't take.

 > setkeyv( W , cols=which_id )

and

 > setkeyv( W , cols=c( which_id ) )

--> same thing, customer_id_A key isn't there.

Any pointers?