You do not need to create or extend a constructor. All you need is a valid DBML file.
I recommend checking out DbLinq and the source code:
[DbLinq] allows you to use Linq, as well as DBML file generation using databases other than MS, such as MySQL, PostgreSQL, Oracle, Ingres, SQLite, FireBird and SQL Server
By the way, do you focus on a specific database server? (DbLinq is probably not the best choice in your case, as it does not seem to be supported since some time, and more and more LinqToSql / Entities providers are provided by providers ...)
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