UPDATE_FUNCTION_GET_A_POST

- [ ] create design
   - [ ] Spec file
   - [ ] class diagram
   - [ ] flow diagram
   - [ ] sequence diagram
- [ ] coding
   - [ ] get_a_post function in controller 
   - [ ] get_a_post function in model
- [ ] unit testing
   - [ ] test_get_a_post function in controller
   - [ ] test_get_a_post function in model