在 Entity Framework 和 Sql Server 中创建对象计数器

Question

Note 1: I REPHRASED THE QUESTION. It now consists of Suppliers and Orders, instead of Cars and Parts.

Note 2: THIS PROBLEM IS HYPOTHETICAL. My goal is to understand how to create object counters.

For regulatory requirements, I need TO SEQUENTIALLY NUMBER EACH Order for each of the suppliers.

I'm Using 'Entity Framework` with Sql Server.

In my hypothetical example, I have a Supplier class and an Order class.

Each supplier has Orders . Each order has a product and a quantity. Meaning, it states which product was ordered from the supplier and how many of it.

I need to be able to create counters, like an auto incremented number, to count the orders FOR EACH supplier .

For regulatory reasons, each supplier must sequentially number its orders, in the order of creation, and using an integer only.

When we examine an Order , We should know by its OrderCountForSupplier column, what was its order of creation (a DateTime / TimeStamp column is insufficient by the regulatory authorities. They require such a counter).

For simplicity of this question, an order cannot be deleted (it's status can change, but it cannot be deleted).

It's very important for me to have a solution which includes the technical/programming way, not only theoretic way.

I've made a diagram in order to explain my problem in the most clear way possible:

---

I have a way that might work, and would be glad to hear feedback.

I'm thinking of an external table/tables, to hold the counters. Something like:

Supplier Order Counters Table

| SupplierId | OrderCountForSupplier
------------------------
| 54654         | 3
| 78787         | 2
| 99666         | 4

Would I need a trigger in order to increment the OrderCountForSupplier counter on each insertion, for each supplier?

If not - how can this incremental be done in a safe way ? (without for example, two processes in a race condition to get the next counter and increment it, which could eventually result in a duplicate Order Count).

And another note: Can this be done Entity Framework wise? if not - a Sql Server solution will be respected.

Answer 1

First answer , the example in the question has changed after it was written.

You say that is it OK to have gaps in the Part IDs, because "some parts might be deleted along the way".

So, what's the difference between your example:

Car      PartID
54654    1
54654    2
54654    3
78787    1
78787    2
99666    1
99666    2
99666    5
99666    7

And this variant:

Car      PartID
54654    1
54654    2
54654    3
78787    4
78787    5
99666    6
99666    7
99666    8
99666    9

In the second variant each part has some ID that is unique for each car (it is also globally unique as well, but it doesn't matter). In the second variant PartID specifies the order in which parts were inserted into the table, same as in the first variant.

So, I'd use a simple IDENTITY column:

Parts

PartID int IDENTITY NOT NULL (PRIMARY KEY)
CarLicenseNum int NOT NULL (FOREIGN KEY)
PartName varchar(255)

Update for Supplier-Order example

The most important bit in the updated question is "regulatory reasons" . It answers the question why would you want to do such unnatural thing. "Regulatory" and efficiency are often opposite.

Essentially, it means that you have to use serializable transaction isolation level when inserting a new row and calculating the next number in the sequence. It will hurt concurrency/throughput, but it will guarantee consistency and "be safe" in multi-user environment.

I don't know how to do it in Entity Framework, it should be possible. But, again, for "regulatory reasons" I'd put this logic in the stored procedure in the DB and make sure that ordinary users don't have write access to the Orders table directly, but have rights only to execute this dedicated stored procedure. You can replicate the logic of this stored procedure in the EF code, but the database itself will be open to changes done through other applications, which may not follow the regulatory requirements.

You can implement it using the separate table, which stores the latest sequence number for each supplier, or you can read the last maximum sequence number on the fly. If each supplier has only few orders, then this separate table with latest values of counters would be comparable to Orders table and you would not gain much. In any case, having a proper index is the key. Getting the latest counter value would be one seek in the index.

Here is an example of stored procedure without using an extra table.

Make sure that Orders table has unique index on (SupplierId, OrderCountForSupplier) . In fact, you must have this index even if you are using an extra table to enforce the constraint.

CREATE PROCEDURE [dbo].[AddOrder]
    @ParamSupplierID int, 
    @ParamProductSerial varchar(10),
    @ParamQuantity int,
    @NewOrderID int OUTPUT
AS
BEGIN
    SET NOCOUNT ON; 
    SET XACT_ABORT ON;

    SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

    BEGIN TRANSACTION;
    BEGIN TRY

        DECLARE @VarMaxCounter int;

        SELECT TOP(1) @VarMaxCounter = OrderCountForSupplier
        FROM dbo.Orders
        WHERE SupplierID = @ParamSupplierID
        ORDER BY OrderCountForSupplier DESC;

        SET @VarMaxCounter = ISNULL(@VarMaxCounter, 0) + 1;

        INSERT INTO dbo.Orders
            (SupplierID
            ,OrderCountForSupplier
            ,ProductSerial
            ,Quantity)
        VALUES
            (@ParamSupplierID
            ,@VarMaxCounter
            ,@ParamProductSerial
            ,@ParamQuantity);

        SET @NewOrderID = SCOPE_IDENTITY();

        COMMIT TRANSACTION;
    END TRY
    BEGIN CATCH
        -- TODO: handle the error
        SET @NewOrderID = 0;
        ROLLBACK TRANSACTION;
    END CATCH;

END
GO

Answer 2

First things first: it is not OK to change your question entirely! Delete this question and create a new one. Having said that ...

Answer of the current question:

Answers to hypothetical questions are just oppinion based and/or too broad (there is actually a flag for this - Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. )!

My answer to the current question is: I do not see any benefit (or advantage or use) of the OrderCountForSupplier in the database ! Creating such counter in the database makes adding and maintenance (in a multi-threaded environment) very complicated and error-prone.

I think the problem can be solved more easily with the help of EF (move the creation of the counters in the code) and a different design of the database:

• in order to allow concurrent adding of Orders, create two columns - a GUID as the Order -PrimaryKey and a CreationDate of type DateTime . Filling those two columns from multiple threads is not a problem
• when retrieving all Orders for a specific SupplierId , sort the result list in ascending order by CreationDate
• when iterating over the result list using (for example) a for-loop, then the counter is the desired sequential counter
• as an alternative to the EF solution, the creation of the sequential counter can stay in SQL - create a view or stored procedure for the Order items and use ROW_NUMBER to create the artificial sequential count, after grouping the items over SupplierId and sorting on CreationDate

Reading the database from multiple threads (and creating the counter in every thread) is again not a problem any more.

---

Answer of the first question:

You are almost there. You need to normalize your data model a little bit more. This is a common scenario in which you want to minimize redundancy of the data and at the same time still maintain a meaningful relation (without the use of triggers).

One possible solution would be to create a Car_has_Part -Table in order to represent the relation between a Car and a Part entity:

| Car_has_Part |
----------------
| PartId       |
| CarId        |

The primary key of the Car_has_Part table is a composite primary key consisting of CarId + PartId which is unique and at the same time you avoid data duplication.

In your example in the Parts table the Doors part is repeated for every Car . Using this intermediate table the data is not duplicated and you have a proper relation.

Your new data model could look like this:

| Car  |    | Car_has_Part |    | Part   |
-------     ----------------    ----------
|CarId |    | PartId       |    | PartId |
|Model |    |              |    | Descr  |
| etc. |    | CarId        |    | etc.   |

This model allow resp. covers the specified requirements:

I need to be able to create a counter, like an auto incremented number, to count the parts for each car. Car 1, could have parts 1, 2, 3... and Car 2 would also have parts 1, 2, 5, 7... (some parts might be deleted along the way).

Select all PartId 's from the Car_has_Part table over CarId .

Each part HAS to be counted separately for its related car. That's the base requirement.

Same as above (without data duplication like in your example). Adding resp. removing a relation or modifying a part name has also become easier - you need to update only one row in the Parts table and the change is reflected for every car.

About the triggers question - you can only create a trigger with EF (using code first approach). Regarding execution - triggers are always executed in the database and EF can't control trigger execution (you can certainly enable/disable trigger using raw SQL queries , but if I understand your question correctly this is not what you want).

Answer 3

This is not a real world example. That's why you are struggling. For an example, A real world parts entity is lot more complicated than that. A real world part will have a ManufacturerId (BMW, Audi etc), PartNumber(B4-773284-YT), VehicleModelId (AUDI A4 etc), Description, ManufacturerYear so on and so forth. Usually when it comes to parts entities, we use a concatanated primary key on ManufacturerId and PartNumber.

Same with your car table. It's not a real world example too. Car entity should have a VIN number, which is unique. When you say each part is specific, you are not talking about Part entity. You are talking about PartInventory entity. PartInventory has a unique serial number (barcode) for each part. So every single part can be identified uniquely. When you attach a part to a vehicle, you are not just attaching a Part, you are actually attaching a PartInventory item, which is recognizable by a unique serial number.

Once the partInventory item is attached to a vehicle, it becomes a fitted part item of the vehicle. Which means the part gets transferred to VehicleParts table.

Unfortunately I see a lot of gaps in your vehicle industry domain knowledge. We develop systems to address real world problems. When you try to address hypothetical problems, you run in to this kind of issues. That leads to wasting lot of other peoples time who are trying to help you out.

Answer 4

After investigating some possible approaches (see links at the bottom), I've came out with a very basic solution, with the help of @Vladimir Baranov.

I've ruled out using SqlServer triggers / Stored Procedures . They seemed hard to implement in conjunction with Entity Framework, and they seem to me like an Over-Kill in this scenario.

I've also ruled out the Optimistic Concurrency approach (using a concurrency token), because in this scenario, the counters cannot be updated simultaneously. They only get updated after a successful insertion to the orders table.

My orders table looks like that. I've added a unique constraint on the OrderId , SupplierId and OrderCountForSupplier trio, so insertion of the same order count for a supplier would fail.

I've indeed used a counters table, from which I can take the latest counter - for each of the suppliers.

Supplier Order Counters Table

| SupplierId | OrderCountForSupplier
------------------------
| 54654         | 3
| 78787         | 2
| 99666         | 4

These are the steps:

1. Get the current supplier orders counter.
2. Try insert a new order for the supplier, using the current counter + 1.

3. If the insertion goes ok => Increase the orders counter for this supplier, on the supplier counters table.

   If insertion goes wrong, and we get an error stating the has been a violation of the constraint (same order count, which already exists): Try 2 more times to get the current counter, and try inserting the order again.

The Code:

   public class SupplierRepository
   {
        private MyContext _context;
        private Supplier _supplier;

        public SupplierRepository(int supplierId)
        {
            _context = new MyContext();
            _supplier = context.Suppliers.Single(x => x.SupplierId == supplierId);
        }

        // Retrieve the latest counter for a supplier
        public SupplierCounter GetCounter()
        {
            var counterEntity = _context.SupplierCounters.Single(x => x.SupplierId == _supplier.SupplierId);
            return counterEntity;
        }

        // Adding a supplier
        public void AddSupplier(Order order)
        {
            int retries = 3;

            while (retries > 0)
            {
                SupplierCounter currentCounter = GetCounter();
                try
                {
                    // Set the current counter into the order object
                    _order.OrderCountForSupplier = currentCounter.OrderCountForSupplier;
                    _context.Add(order);                        
                    // Success! update the counter (+1) and then break out of the while loop.
                    currentCounter.OrderCountForSupplier += 1;  
                    // I'M CALLING `SAVECHANGES` AFTER ADDING AN ORDER AND INCREASING THE COUNTER, SO THEY WOULD  BE IN THE SAME TRANSACTION. 
                    // THIS WOULD PREVENT A SCENARIO WHERE THE ORDER IS ADDED AND THE COUNTER IS NOT INCREMENTED.
                    _context.SaveChanges();
                    break;
                }
                catch (SqlException ex)
                {
                    if (ex.Number == 2627) // Violating unique constraint
                    {
                        --retries;
                    }
                }
            }
        }
    }

Some useful links:

• SQL Server Unique Composite Key of Two Field With Second Field Auto-Increment
• Atomic Increment with Entity Framework
• how to inc/dec multi user safe in entity framework 5