You can't really do that like this - bind variables as designed for
inserting literals. 'Normal' ruby interpolation might be the way
forward here (assuming that compare sign is coming from a trusted
source, if not sanitize it first).
You don't need to use the array to populate conditions, as long as the
"compare_sign" isn't user-supplied data; ie: you *know* that it isn't
going to contain something that will compromise your DB, because you
set it earlier.