ActiveRecord: ignore column in creates and updates

I’m looking for a solution to the following problem: I have an
ActiveRecord entity that is backed by an updatable database view. This
view contains one column that is calculated from other columns. When a
new record is created for this entity, that field should not be set.
However, by default, it is set with the ‘default’ (NULL), which is
rejected by the database (there is not valid way to set such a field).

attr_readonly isn’t a solution, because that only excludes a column from
updates and not from creates.

attr_ignore, such as implemented by the ‘lincoln’ gem, is not a solution
either, because then the field is ignored entirely. However, the column
still needs to be read and be accessible. It’s even used as part of a
relation.

Does anyone know if there is a way in ActiveRecord to specify a column
as ‘never set this field’?