Cosmos: Support unique keys

[AndriySvyryd]

https://docs.microsoft.com/en-us/azure/cosmos-db/unique-keys

For a unique key to be created in the container the EF model must have an alternate key on an entity type that includes the partition key and all entity types sharing the container must have the same key.

[jviau]

Will this feature also cover having the partition key be part of the lookup in DbSet<T>.Find(...)? I mean without specifically adding the partition key to be part of the complex key.

To be explicit:

public class MyModel
{
    public string Id { get; set; }

    public string PartitionKey { get; set; }
}

public class MyDbContext : DbContext 
{
    public DbSet<MyModel> MyModels { get; set; }
        
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<MyModel>()
                .HasPartitionKey(x => x.PartitionKey)
                .HasKey(x => x.Id);
    }
}


// Find model:
MyDbContext dbContext;
MyModel model = dbContext.MyModels.Find("partitionKeyValue", "idValue"); // not two separate values are passed in, but the "key" only has 1 value.

In cosmosdb, it would be preferable the PartitionKey is not part of the Id, as I believe cosmosdb would allow the same id under different partition keys already.

{
  "Id": "idValue",
  "PartitionKey": "partitionKeyValue",
   "Discriminator": "MyModel",
   "id": "MyModel|idValue"
}

Right now I am not sure how cosmos ef core would behave if I have the same id under different partition keys and try to execute Find("idValue")

[AndriySvyryd]

@jviau This is tracked by #17310