如何在实体框架中启用IS标识?

我试图在代码中使用is identity来增加transactionid。但它显示以下错误:

SqlException:无法在中插入标识列的显式值 当identity_insert设置为off时的表“transactionhistories”。

我试着把 [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)] 句法。但没用。

public partial class TransactionHistories
    {
        [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
        public int TransactionId { get; set; }

        public DateTime? TransactionDate { get; set; }
        public long? CheckingAccountNumber { get; set; }
        public long? SavingAccountNumber { get; set; }
        [DisplayName("Transffered Amount")]
        public decimal? Amount { get; set; }
        public decimal? Transactionfee { get; set; }
        public long? TransactionTypeId { get; set; }
    }

            TransactionHistories = new TransactionHistories();
            TransactionTypes = new TransactionTypes();

            if (!ModelState.IsValid)
            {
                return Page();
            }

            SavingAccounts.Balance = SavingAccounts.Balance - Amount;
            CheckingAccounts.Balance = CheckingAccounts.Balance + Amount;

            //TransactionHistories.TransactionId = TransactionHistories.TransactionId + 1;
            TransactionHistories.TransactionDate = DateTime.Now;
            TransactionHistories.CheckingAccountNumber = CheckingAccounts.CheckingAccountNumber;
            TransactionHistories.SavingAccountNumber = SavingAccounts.SavingAccountNumber;
            TransactionHistories.Amount = Amount;
            TransactionHistories.Transactionfee = 0;
            TransactionHistories.TransactionType = "SavingToChecking";

上面是我的模型类,transactionid是主键,这是我试图使用is identity使其增加1的值。我怎样才能做到这一点呢?请给我任何建议和帮助。