Does AutoMapper support Linq?

Question

I am very interested in Linq to SQL with Lazy load feature. And in my project I used AutoMapper to map DB Model to Domain Model (from DB_RoleInfo to DO_RoleInfo). In my repository code as below:

    public DO_RoleInfo SelectByKey(Guid Key)
    {
        return SelectAll().Where(x => x.Id == Key).SingleOrDefault();
    }

    public IQueryable<DO_RoleInfo> SelectAll()
    {
        Mapper.CreateMap<DB_RoleInfo, DO_RoleInfo>();
        return from role in _ctx.DB_RoleInfo
               select Mapper.Map<DB_RoleInfo, DO_RoleInfo>(role);
    }

SelectAll method is run well, but when I call SelectByKey, I get the error:

Method “RealMVC.Data.DO_RoleInfo MapDB_RoleInfo,DO_RoleInfo” could not translate to SQL.

Is it that Automapper doesn't support Linq completely?

Instead of Automapper, I tried the manual mapping code below:

public IQueryable<DO_RoleInfo> SelectAll()
{
    return from role in _ctx.DB_RoleInfo 
    select new DO_RoleInfo 
    {
        Id = role.id,
        name = role.name,
        code = role.code
    };
}

This method works the way I want it to.

Answer 1

While @Aaronaught's answer was correct at the time of writing, as often the world has changed and AutoMapper with it. In the mean time, QueryableExtensions were added to the code base which added support for projections that get translated into expressions and, finally, SQL.

The core extension method is ProjectTo¹. This is what your code could look like:

using AutoMapper.QueryableExtensions;

public IQueryable<DO_RoleInfo> SelectAll()
{
    Mapper.CreateMap<DB_RoleInfo, DO_RoleInfo>();
    return _ctx.DB_RoleInfo.ProjectTo<DO_RoleInfo>();
}

and it would behave like the manual mapping. (The CreateMap statement is here for demonstration purposes. Normally, you'd define mappings once at application startup).

Thus, only the columns that are required for the mapping are queried and the result is an IQueryable that still has the original query provider (linq-to-sql, linq-to-entities, whatever). So it is still composable and this will translate into a WHERE clause in SQL:

SelectAll().Where(x => x.Id == Key).SingleOrDefault();

---

¹ Project().To<T>() prior to v. 4.1.0