How express `SELECT1 UNION ALL SELECT 2` sub-query in Linq2Db

I have a query which uses IN clause:

var query = dataContext.ExecuteObject<Card>()
                .Where(x => x.Version == workingVersion);
                .Where(x => cities.Contains(x.CityId));

which is not very effective, because cities collection is quite big (and provided by user request). I would like to replace IN clause with INNER JOIN like that:

[x1].CityId IN (1, 2, 3, 4)

INNER JOIN (SELECT 1 AS Id UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4) [c] ON [c].Id = [x1].[CityId]

How can I express such SELECT UNION in Linq2Db?

[sdanyliv]

@NickAb, you can create temp table using linq2db, bulk copy data to that table and use inner join. It sholuld be more effective.

[ili]

Solution needed, or we can close an issue?