protect Lesson.get_from_sourceDB() from multiple versions with same sourceID
protect Lesson.get_from_sourceDB() from multiple versions with same sourceID

cjlee112/socraticqs2,derdmitry/socraticqs2,cjlee112/socraticqs2,raccoongang/socraticqs2,derdmitry/socraticqs2,raccoongang/socraticqs2,raccoongang/socraticqs2,derdmitry/socraticqs2,cjlee112/socraticqs2,raccoongang/socraticqs2,cjlee112/socraticqs2