PG::ForeignKeyViolation: ERROR: update or delete on table violates foreign key constraint
has_many :comments, dependent: :delete_all - just automatically delete them when the user is deleted
has_many :comments, dependent: :destroy - like above, but call #destroy on each comment instead of just deleting directly in the db
has_many :comments, dependent: :nullify - don't delete comments when the user is deleted, just null out their user_id column