Re-added ConstraintKeyObject which is consumed from the AbstractSource, fixes #3512
CS fixes
Merge pull request #5903 from ralphschindler/hotfix/db-metadata-missing-class-issue-3512

Re-added ConstraintKeyObject which is consumed from the AbstractSource w/ test (Fixes #3512)
[#5903] CS fixes

- License and class docblocks
Merge branch 'hotfix/5903'

Close #5903