implement Instance.getConflictInstances
implement WorkGroup.checkConflict() and minor changes
implement assignment.checkConflict()
implement sort the workgroup
fix a small bug
some changes in Configuration model
combine importing two models
debug
Begin to use django style to name variables instead of camelCase in Assignment model and Configuration model. Also try to execute the raw sql query, it came out the performance can be improved dramatically.
implement getPreAssignmentCountByService using raw sql
minor changes