Incorrectly raising `ReadonlyAttributeError` when autosaving `has_one` association with readonly foreign key about rails

[joshuay03]

This is an interesting one! It looks like because updater_id on the nested record is changing, an autosave is triggered (this is always enabled for nested attributes), causing the foreign key on the nested record to be set here despite it not changing:

rails/activerecord/lib/active_record/autosave_association.rb

Line 461 in 351a8d9

record[foreign_key] = _read_attribute(primary_key)

Interestingly, this isn't an issue on belongs_to which already only sets the nested record's foreign key if it's changing:

rails/activerecord/lib/active_record/autosave_association.rb

Line 524 in 351a8d9

self[foreign_key] = association_id unless self[foreign_key] == association_id

I've got a fix passing against your script, just adding some coverage before I get a PR up.

[joshuay03]

Got this up: #50901

[Earlopain]

Thank you, that looks good and gets all my actual tests passing. This seems to be mirroring what #46759 did for belongs_to, I found #46758 while searching for similar issues but didn't connect the dots.

[joshuay03]

Thank you, that looks good and gets all my actual tests passing. This seems to be mirroring what #46759 did for belongs_to, I found #46758 while searching for similar issues but didn't connect the dots.

Ah nice find, I'll reference it in my PR, thanks!