Fix AssociationTypeMismatch

When you're getting this error, one possibility is that you've created a select field for an association instead of the associated object's id. Example:

Copyform.select :unit, Unit.for_select

will be expected to deliver a real Unit object, whereas

Copyform.select :unit_id, Unit.for_select

will make Rails typecast the String value from the select field to the unit's ID.