writing SQL query WHERE.. IN.. anArray ?

ex : @dpt_ids = ["43", "48", "49", "50", "51"]
I try to insert it into my sql query to be used in a 'find_by_sql'

     query = "SELECT * , #{kms} AS km FROM cities"
     query << " WHERE department_id IN ( #{dpt_ids} ) "

but this transform my array into a unique string : ' 4348495051'

I also tried

query << " WHERE department_id IN ( #{dpt_ids.each {|d| p d }} ) "

same..

How could I get the following resulting query :
WHERE department_id IN ( "43", "48", "49", "50", "51" ) ???

query << " WHERE department_id IN ( "#{dpt_ids.join('","')}" )

Just make sure that dpt_ids really contains integers only and not something malicious though.