implement needed methods and default values in model
implement needed methods and default values in model

cysjonathan/coursemology2,xzhflying/coursemology2,harryggg/coursemology2,Coursemology/coursemology2,cysjonathan/coursemology2,cysjonathan/coursemology2,harryggg/coursemology2,Coursemology/coursemology2,Coursemology/coursemology2,BenMQ/coursemology2,xzhflying/coursemology2,xzhflying/coursemology2,Coursemology/coursemology2,BenMQ/coursemology2,harryggg/coursemology2,Coursemology/coursemology2,Coursemology/coursemology2,BenMQ/coursemology2,Coursemology/coursemology2