Microsoft Dynamics CRM 2011 Add Entity 1:N Relation

salesorderdetailId -> retrieve salesorderdetail
salesorderId -> retrieve salesorder

EntityReferenceCollection relatedEntities = new EntityReferenceCollection
{
new EntityReference("salesorderdetail", salesorderdetailId)
};

//relation
Relationship relationship = new Relationship("order_details");
wf.OrganizationService.Associate("salesorder", salesorderId , relationship, relatedEntities);