ViewModel.cs

public class CertificateApiEditModel : IValidatableObject
    {
        [Required]
        public CertificateType? Type { get; set; }

        [Required]
        public CertificateRegionType? Region { get; set; }

        //[MustBeTrue]
        public bool SignedOff { get; set; }


        #region Customer details

        [Required]
        public CustomerDetailsWorkType? WorkType { get; set; }

        [RequiredIf("WorkType", CustomerDetailsWorkType.Commercial), StringLength(100)]
        public string BusinessName { get; set; }

        [RequiredIf("WorkType", CustomerDetailsWorkType.Commercial), StringLength(100)]
        public string ContactName { get; set; }

        [StringLength(20)]
        public string JobId { get; set; }

        [RequiredIf("WorkType", CustomerDetailsWorkType.Residential), StringLength(50)]
        public string FirstName { get; set; }

        [RequiredIf("WorkType", CustomerDetailsWorkType.Residential), StringLength(50)]
        public string LastName { get; set; }

        [Required, StringLength(500)]
        public string BillingAddress { get; set; }

        [Required, StringLength(500)]
        public string SiteAddress { get; set; }

        #endregion

        [Required]
        public WorkerApiEditModel Worker { get; set; }

        [Required]
        public ContractorApiEditModel Contractor { get; set; }


        #region TestResults

        [Required]
        public DateTime? TestDate { get; set; }

        [Required]
        public List<CircuitResultApiEditModel> CircuitResults { get; set; }
        #endregion

        #region added by trinvh
        public InstrumentApiEditModel Instrument { get; set; }
        [Required]
        public CertificateBussinessType BussinessType { get; set; }
        [Required]
        public List<BoardApiEditModel> Boards { get; set; }
        #endregion

        public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
        {
            var result = new List<ValidationResult>();

            if (CircuitResults != null && CircuitResults.Count == 0)
            {
                result.Add(new ValidationResult("At least one circut result must be added."));
            }

            return result;
        }
    }

    public class BoardApiEditModel
    {
        public bool IsMainboard { get; set; }
        public bool IsNew { get; set; }
        public bool IsMen { get; set; }

        [StringLength(50)]
        public string Location { get; set; }
        [StringLength(50)]
        public string Designation { get; set; }
        [StringLength(50)]
        public string SupplyVia { get; set; }

        public int NumOfPhases { get; set; }
        public float Rating { get; set; }
        public float PfcAtBoard { get; set; }
        public float NominalVoltage { get; set; }
        public float EarthContinuity { get; set; }
        public float Bonding { get; set; }
        [Required]
        public List<CircuitResultApiEditModel> CircuitResults { get; set; }
    }

    public class TestResultApiEditModel
    {
        [StringLength(50)]
        public string Name { get; set; }
        [StringLength(50)]
        public string Value { get; set; }

        public TestResultType Type { get; set; }
    }

    public class InstrumentApiEditModel
    {
        [StringLength(50)]
        public string Model { get; set; }
        [StringLength(100)]
        public string SerialNumber { get; set; }
    }

    public class CircuitResultApiEditModel : IValidatableObject
    {
        [Required, StringLength(50)]
        public string Name { get; set; }

        [StringLength(50)]
        public string EarthContinuity { get; set; }

        [StringLength(50)]
        public string Bonding { get; set; }

        [StringLength(50)]
        public string Polarity { get; set; }

        [StringLength(50)]
        public string InsulationResistance { get; set; }

        [StringLength(50)]
        public string FaultLoopImpedance { get; set; }

        [StringLength(4000)]
        public string Notes { get; set; }

        #region Added by trinvh
        public CircuitPhaseType Phase { get; set; }
        public string WiringType { get; set; }
        public float NumOfPoints { get; set; }
        public float CableActiveSize { get; set; }
        public float CableEarthSize { get; set; }
        public float Rating { get; set; }
        [StringLength(50)]
        public string Type { get; set; }

        public float ShortCapacity { get; set; }
        public float RCDOperationCurrent { get; set; }

        public List<TestResultApiEditModel> TestResults { get; set; }
        #endregion


        public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
        {
            var result = new List<ValidationResult>();

            if (String.IsNullOrEmpty(EarthContinuity) && String.IsNullOrEmpty(Bonding) &&  String.IsNullOrEmpty(Polarity) && String.IsNullOrEmpty(InsulationResistance) && String.IsNullOrEmpty(FaultLoopImpedance))
            {
                result.Add(new ValidationResult("At least one test must be included in a circuit test result."));
            }

            return result;
        }
    }