Unique index generated for one-to-one relation causes errors when replacing entities.

[LukasKrc]

Bug Report

Q	A
BC Break	no
Version	2.6.3

Summary

A unique index error is caused when replacing entities (that is removing an existing relation and creating a new one) from the inverse side of a one-to-one relation.

Current behavior

When replacing an entity from the inverse side of a one-to-one relation, a unique index error is caused. This happens because the previously persisted related entity is not deleted before an attempt to insert the new one is made.

This was supposedly fixed in a previous issue: #2310. There is another issue which is partially related to this one: #6776.

How to reproduce

Please see this pull request: #7722 for an example test case.

Expected behavior

The expected behavior is that no unique index error is caused during the mentioned scenario.