[5.0.x] Don't leave unknown FK values when principal is known but has not-set, non-generated, value #24178
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Port of 6.0 fix #23875 to 5.0 release.
Fixes #23730
Description
An exception is thrown when no key value is set for a non-generated key of an owned type. Normally this is a negative case since non-generated key values must be explicitly set. However, this can works when the non-generated value is part of a composite key for which other parts of the key are generated. In this case, the non-generated part can have the same default value for multiple inserts without violating the primary key constraint.
Customer Impact
This is a regression for the case described above. There is no reasonable workaround.
(We already fixed this for EF Core 6.0, but decided not to patch since it seemed to be a regression only in a negative case. Since then other customers have reported the issue and one customer outlined the scenario above where it is a regression in working code.)
How found
Reported by multiple customers.
Test coverage
Test coverage for this case has been added in this PR.
Regression?
Yes, from EF Core 3.1.
Risk
Low. The fix is already in EF Core 6.0 and is targetted to this case. Also quirked.