Improve singular association creation

[lazaronixon]

Motivation / Background

Replacing a has_one association nowadays is very confusing...

• create_association calls INSERT then DELETE in different transactions whilerecord.assoctiation = new_record calls DELETE then INSERT in a single transaction.
• create_association raises an exception if your association is backed by a unique index, since it first try to create the new record before delete the old one.
• create_association is prone to create duplicated rows if your association is not backed by a unique index, even though it has a logic to delete the old record later.

Fixes #45920

Detail

This pull request improves the create_association method to work similarly to record.assoctiation = new_record, so when replacing an association, a single transaction is created and the old record is deleted before a new one is created, making the method safer with a unique index or not and less confusing.

Before:

# begin transaction
INSERT new_record;
# commit transaction

# begin transaction
DELETE old_record;
# commit transaction

After:

# begin transaction
DELETE old_record;
INSERT new_record;
# commit transaction

Additional information

https://andycroll.com/ruby/be-careful-assigning-to-has-one-relations

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.
• CI is passing.