Auto-increment Id based on composite primary key - sql-server

Question

Note: Using Sql Azure & Entity Framework 6
Say I have the following table of a store's invoices (there are multiple stores in the DB)...
CREATE TABLE [dbo].[Invoice] (
[InvoiceId] INTEGER NOT NULL,
[StoreId] UNIQUEIDENTIFIER NOT NULL,
CONSTRAINT [PK_Invoice] PRIMARY KEY CLUSTERED ([InvoiceId] ASC, [StoreId] ASC)
);
Ideally, I would like the InvoiceId to increment consecutively for each StoreId rather than independent of each store...
InvoiceId | StoreId
-------------------
1 | 'A'
2 | 'A'
3 | 'A'
1 | 'B'
2 | 'B'
Question: What is the best way to get the [InvoiceId] to increment based on the [StoreId]?
Possible options:
a) Ideally a [InvoiceId] INTEGER NOT NULL IDENTITY_BASED_ON([StoreId]) parameter of some kind would be really helpful, but I doubt this exists...
b) A way to set the default from the return of a function based on another column? (AFAIK, you can't reference another column in a default)
CREATE FUNCTION [dbo].[NextInvoiceId]
(
#storeId UNIQUEIDENTIFIER
)
RETURNS INT
AS
BEGIN
DECLARE #nextId INT;
SELECT #nextId = MAX([InvoiceId])+1 FROM [Invoice] WHERE [StoreId] = #storeId;
IF (#nextId IS NULL)
RETURN 1;
RETURN #nextId;
END
CREATE TABLE [dbo].[Invoice] (
[InvoiceId] INTEGER NOT NULL DEFAULT NextInvoiceId([StoreId]),
[StoreId] UNIQUEIDENTIFIER NOT NULL,
CONSTRAINT [PK_Invoice] PRIMARY KEY CLUSTERED ([InvoiceId] ASC, [StoreId] ASC)
);
c) A way to handle this in Entity Framework (code first w/o migration) using DbContext.SaveChangesAsync override or by setting a custom insert query?
Note: I realize I could do it with a stored procedure to insert the invoice, but I'd prefer avoid that unless its the only option.

Answer 1

You should stick to an auto-incrementing integer primary key, this is much simpler than dealing with a composite primary key especially when relating things back to an Invoice.
In order to generate an InvoiceNumber for the sake of a user, which increments per-store, you can use a ROW_NUMBER function partitioned by StoreId and ordered by your auto-incrementing primary key.
This is demonstrated with the example below:
WITH TestData(InvoiceId, StoreId) AS
(
SELECT 1,'A'
UNION SELECT 2,'A'
UNION SELECT 3,'A'
UNION SELECT 4,'B'
UNION SELECT 5,'B'
)
Select InvoiceId,
StoreId,
ROW_NUMBER() OVER(PARTITION BY StoreId ORDER BY InvoiceId) AS InvoiceNumber
FROM TestData
Result:
InvoiceId | StoreId | InvoiceNumber
1 | A | 1
2 | A | 2
3 | A | 3
4 | B | 1
5 | B | 2

Answer 2

After playing around with the answer provided by #Jamiec in my solution I instead decided to go the TRIGGER route in order to persist the invoice number and better work with Entity Framework. Additionally, since ROW_NUMBER doesn't work in an INSERT (AFAIK) I am instead using MAX([InvoiceNumber])+1.
CREATE TABLE [dbo].[Invoice] (
[InvoiceId] INTEGER NOT NULL,
[StoreId] UNIQUEIDENTIFIER NOT NULL,
[InvoiceNumber] INTEGER NOT NULL,
CONSTRAINT [PK_Invoice] PRIMARY KEY CLUSTERED ([InvoiceId] ASC)
);
CREATE TRIGGER TGR_InvoiceNumber
ON [Invoice]
INSTEAD OF INSERT
AS
BEGIN
INSERT INTO [Invoice] ([InvoiceId], [StoreId], [InvoiceNumber])
SELECT [InvoiceId],
[StoreId],
ISNULL((SELECT MAX([InvoiceNumber]) + 1 FROM [Invoice] AS inv2 WHERE inv2.[StoreId] = inv1.[StoreId]), 1)
FROM inserted as inv1;
END;
This allows me to set up my EF class like:
public class Invoice
{
[Key]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int InvoiceId { get; set; }
public Guid StoreId { get; set; }
[DatabaseGenerated(DatabaseGeneratedOption.Computed)]
public int InvoiceNumber { get; set; }
}