code: func (a *hasManyAssociation) Constraint() (string, []interface{}) {
  	tn := flect.Underscore(a.ownerName)
  	condition := fmt.Sprintf("%s_id = ?", tn)
  	if a.fkID != "" {
  		condition = fmt.Sprintf("%s = ?", a.fkID)
  	}
  	return condition, []interface{}{a.ownerID}
  }
docs: // Constraint returns the content for a where clause, and the args
  // needed to execute it.