How to make a identity column for a 2-key primary key

Question

Here is my fictional tables:

Post{
    Post-Id int primary key identity,
    Post-another-columns blahblahblah
}

Lamp{
    Lamp-Id int primary key identity,
    Lamp-another-columns blahblahblah
}

Post-Lamp(n-n Relationship){
    Post-Id FOREIGN KEY REFERENCES Posts(Post-Id), 
    Lamp-Id FOREIGN KEY REFERENCES Lamp(Lamp-Id), 
    Lamp-Index int not null, --(describe the Lamp index that is in this Post)
    Post-Bar-another-columns blahblahblah,
    CONSTRAINT PK_Post_Lamp PRIMARY KEY NONCLUSTERED ([Post-Id], [Lamp-Id])
}

On my .Net code, every time that i put a Lamp into a Post, i insert a index based at the MAX index that a Post has of the Lamps. It's a foreach Lamp in Post, insert into Post-Lamp(postId, index, lampId) , but this seems very wrong. How can i make a incremental column for the Lamp-Index that respects each change of Post-Id?

Edit: .Net code

int postId = await _repositories.Post.Add(post);

List<LampPost> lamps = model.Lamp.Select((l, i) => new { Lamp = l, Index = i })
                .Select(item => new LampPost
                {
                    LampId= item.Lamp,
                    LampIndex = item.Index,
                    PostId = postId,
                }).ToList();

            await _repositories.Lamp.Add(lamps);

Answer 1

Your question title makes it sound like you want a composite primary key , but I think what you are trying to achieve by your example is just a table with 2 foreign keys like this:

create table Posts(
  PostId int, 
  OtherColumn varchar(50), 
  primary key (PostId)
)

create table Lamps(
  LampId int, 
  OtherColumn varchar(50), 
  primary key (LampId)
)

create table PostLamp(
  PostId int FOREIGN KEY REFERENCES Posts(PostId), 
  LampId int FOREIGN KEY REFERENCES Lamps(LampId), 
)

Answer 2

Hmm, maybe you can easily solve this by using a view.

First create the linking table with and identity column, eg:

CREATE TABLE postlamp_t
             (post integer,
              lamp integer,
              lampindex integer IDENTITY,
              PRIMARY KEY (post
                           lamp),
              FOREIGN KEY (post)
                          REFERENCES post
                                     (id),
              FOREIGN KEY (lamp)
                          REFERENCES lamp
                                     (id));

Now create a view using the row_number() window function to calculate the index per post.

CREATE VIEW postlamp
AS
SELECT post,
       lamp,
       row_number() OVER (PARTITION BY post
                          ORDER BY lampindex) lampindex
       FROM postlamp_t;

As long as you don't try to update the calculated column, this view is updatable and you can use it like if it was the actual linking table ( INSERT , UPDATE , DELETE , SELECT ).

Find a demonstration at db<>fiddle .

(Side note: If a lamp shouldn't be in two different posts at a time, which seems realistic if we're talking actual real world lamps, consider a unique constraint on the lamp in the linking table.)

Answer 3

Whats the difference between
Lamp-Id FOREIGN KEY REFERENCES Lamp(Lamp-Id) and Lamp-Index int not null ??

I'm little confused with the title, but seems like you should check out SCOPE_IDENTITY(). It returns the last identity value inserted into an identity column in the same scope. In your stored proc that inserts values in Lamp table set a variable with Scope_Identity() after inserting a lamp and then us that variable in Lamp-Post insert query..