requiredStatus on Qt 4.3 and PostgreSQL 8.3.3

[Auryn]

Some fields in one PostgreSQL table are configured with "NOT NULL".
Inside my Qt 4.3 application I try to detect this constraint, but I am trying with

if(my_model.requiredStatus == QSqlField::Required)
  // Warn user that this field must be filled

if(my_model.requiredStatus == QSqlField::Required)
  // Warn user that this field must be filled

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But it allways returns -1 (QSqlField::Unknown).
If this is not the method to follow, how can PostgreSQL tell to Qt when a field has some restriction (check, not null...)?