Expose unique constrains metadata to schema API and T4 model

Hi, TT model generator sets correctly PrimaryKey attribute to properties of classes with attribute Table. Would it be possible to add also a new attribute, named AlternateKey(string alternateKeyName), to identify properties that partecipate to a unique constraint?

For example:

	[Table(Schema="DWUSER", Name="ST01_COMPONENT")]
	public partial class ST01Component
	{
		[Column("ID_COMPONENT"),     PrimaryKey,  NotNull] public decimal IdComponent     { get; set; } // NUMBER
		[Column("SITE"),             AlternateKey("ST01_NC_CODE_SITE_NC_CODE_UN"),  Nullable] public string  Site            { get; set; } // NVARCHAR2(18)
		[Column("SFC"),              AlternateKey("ST01_NC_CODE_SITE_NC_CODE_UN"),  Nullable] public string  Sfc             { get; set; } // NVARCHAR2(384)
		[Column("ITEM"),                Nullable         ] public string  Item            { get; set; } // NVARCHAR2(384)
		[Column("ITEM_DESCRIPTION"),    Nullable         ] public string  ItemDescription { get; set; } // NVARCHAR2(120)
}

The use case is the possibility to use this new attribute to generate new code based on it without the need to call the back end database another time.

Thank you!

Environment details

linq2db version: 3.0.0-preview.1 Database Server: Oracle Database Provider: Oracle.ManagedDataAccess.Core (3.0.0-preview.1) Operating system: Windows 10 .NET Framework:.NET Core 3.1

[MaceWindu]

You need to add code to your .tt file (not to linq2db t4 templates) between model load and generation steps https://linq2db.github.io/articles/T4.html#example-of-generation-process-customization

and then go through Tables collection and add additional attributes to table columns.

[sdanyliv]

You are right, a lack of time to implement. You can help us to retrieve Unique constraints by SchemaProvider. It can be good first PR.

Anyway HasUniqueKey helps to define unique constraint for the subquery, when you know which data returned by subquery.