linq2db as DTO layer

Hello. We’ve been using Linq2db in a project for a while and we got to a stage where we understood that working directly with generated classes is not enough. Is it worth creating a BO and DTO separately, using linq2db?

If i would like to introduce a field UserCreated in my Users table, which would automatically use Insert date, how could i achieve that using Linq2db without an extra layer, where i would manually write my Insert function?

If then i introduce an extra layer, then in order to use linq for my queries (lets say i have

var admins = db.Users.Where(usr => usr.IsAdmin).ToList();

I would have to reproduce the exact same query on my BO class, something like this

var admins = Users.All().Where(usr => usr.IsAdmin);

which makes me load the whole list, thus generate two separated queries, which is not nice.

What is the best approach to this structure?

Thank you!

[sdanyliv]

We need Expression Tree but not just object to object remapping, You have to write mapper only in the following way:

IQueryable<DTOUser> Map(this IQueryable<User> query)
{
   return query.Select(u => new DTOUser{
         UserId = u.USerId,
         ...
         UserCreated = u.InsertDate
      }
}

Or you can return Expresssion Tree for remapping:

Expression<Func<User, DTOUser>> Map()
{
    return u => new DTOUser{
         UserId = u.USerId,
         ...
         UserCreated = u.InsertDate
      }
}

And use in query

db.Users.Select(Map()).ToArray()

[sdanyliv]

Ineresting that i’m thinking about universal implementation for such needs. And i have proposed interfaces for implementing this https://github.com/linq2db/linq2db/issues/230#issuecomment-412290451 Thinking about implementation of some independed DTOEntityServices with registration of IEntityMapping. Almost everything can be done without changing linq2db code.