Awaiting Clanad's opinion but from the Los Angeles Times...
"American experts say the typical procedure is for a flight attendant to use a food cart to block access to the cockpit when the pilot opens the door to leave. A flight attendant is supposed to remain in the cockpit and open the door for the pilot upon his or her return.
"It's a standard procedure" that Germanwings should have followed, Winn said. “That’s nothing secret. Everybody knows.”
If a member of the flight crew doesn't open the locked cockpit door from the inside, according to a manual for the Airbus A320 available online, the door can also be unlocked by the cabin crew outside the cockpit by entering a two- to seven-digit code, pre-programmed by the airline, on a keypad."
http://www.latimes.com/world/la-fg-german-jet-crash-door-20150325-story.html