(Quick Reference)
                ignoreNotFound
Purpose
Specifies how foreign keys that reference missing rows are handled in many-to-one relationships.
Examples
class LegacyCdDomain {    String title
    Thumbnail thumbnail    static mapping = {
        thumbnail ignoreNotFound: true
    }
}class Thumbnail {
    byte[] data
}Description
Usage: 
association_name(ignoreNotFound: boolean)When the data in the database is corrupt and a foreign key references a non existent record, Hibernate will complain with a "No row with the given identifier exists" message. For example, a 
LegacyCdDomain record may have a reference to a 
Thumbnail record with an ID of 1234, but the database may no longer contain a 
Thumbnail with that ID.
One possible way to load such corrupt data is to use the 
ignoreNotFound mapping option. If you set it to 
true, Hibernate will treat a missing row as a 
null association. So in the above example, our 
LegacyCdDomain instance would load but its 
thumbnail property would be 
null. Specifying a value of 
false for 
ignoreNotFound will result in Hibernate throwing an 
org.hibernate.ObjectNotFoundException.
A dataset loaded with ignoreNotFound: true may throw an exception during save() because of the missing reference!
The default value for this setting is 
false. It maps to Hibernate's 
not-found property.
This settings can be useful if you have a legacy database with unusual behaviour when it comes to referential integrity.