It's getting better :). Written that way would imply that insert_record is always called. If that is correct, I'd suggest:

"The through record (built with build_record) is temporarily cached, so that it
may be reused when insert_record is subsequently called."

And if it should be conditional, then:

"The through record (built with build_record) is temporarily cached, so that it
may be reused if insert_record is subsequently called."