AutoMapper: profile-based mapping and ProjectTo for EF queries

Manually mapping domain objects to DTOs involves repetitive property assignment code that is easy to get wrong and hard to maintain across many types. Adding a new property means updating every mapping site.

Define mapping profiles and use ProjectTo for efficient database projections:

// Profile
public class OrderProfile : Profile
{
    public OrderProfile()
    {
        CreateMap<Order, OrderDto>()
            .ForMember(d => d.CustomerName,
                opt => opt.MapFrom(s => s.Customer.Name));

        CreateMap<CreateOrderRequest, Order>();
    }
}

// Registration
builder.Services.AddAutoMapper(typeof(OrderProfile).Assembly);

// Usage in service
public async Task<OrderDto> GetOrderAsync(Guid id)
{
    return await _db.Orders
        .ProjectTo<OrderDto>(_mapper.ConfigurationProvider) // SQL SELECT only needed columns
        .FirstOrDefaultAsync(o => o.Id == id)
        ?? throw new NotFoundException(id);
}

Add configuration validation at startup:

builder.Services.AddAutoMapper(cfg => cfg.AddProfile<OrderProfile>());
// AssertConfigurationIsValid() in tests

ProjectTo<T>() translates the AutoMapper configuration into an EF Core expression tree, producing a SELECT with only the columns needed for the DTO — avoiding loading full entities. _mapper.Map<T>() loads full entities first, then maps in memory.