Ciao, i have a Master and slave class based on MADescription definitions. Now in the master i need to have a description for manage a collection of slave instances (based on my MATimedIvaModel) , and control if in this collection there's another slave instance with same Date field based on MADateDescription. In the Master class i defined MAToManyRelationDescription for manage the collection of slave instance: descriptionCllTimedIva <magritteDescription> ^ MAToManyRelationDescription new classes: (Array with: MATimedIvaModel); label: 'Gestione Iva'; selectorAccessor: 'cllTimedIva'; comment: 'Gestione aliquota IVA per data validita'; priority: 2200; yourself It work fine but don't control if there's another slave instance with the same Date. How i can add this control ? Thanks for any considerations, Dario