there is no unique constraint matching given keys for referenced table "mail_message" code example

Example: there is no unique constraint matching given keys for referenced table

It's because the name column on the bar table does not have the UNIQUE constraint.

So imagine you have 2 rows on the bar table that contain the name 'ams' and you insert a row on baz with 'ams' on bar_fk, which row on bar would it be referring since there are two rows matching?