Multi-Source Mapping

The [AdditionalSource] attribute lets a DTO combine properties from multiple source types into a single DTO.

Usage

public class Employee
{
    public int Id { get; set; }
    public string FirstName { get; set; } = "";
    public string LastName { get; set; } = "";
    // ...
}

public class EmployeeProfile
{
    public string Bio { get; set; } = "";
    public string PhotoUrl { get; set; } = "";
}

[Facette(typeof(Employee), "SocialSecurityNumber",
    NestedDtos = new[] { typeof(AddressDto) },
    GenerateProjection = false)]
[AdditionalSource(typeof(EmployeeProfile), "Profile")]
public partial record EmployeeDetailDto;

How it works

The [AdditionalSource] attribute takes two parameters:

• sourceType — the additional type to map from
• prefix — a string prefix added to properties from this source

With the prefix "Profile", the properties from EmployeeProfile are generated as:

• ProfileBio (from Bio)
• ProfilePhotoUrl (from PhotoUrl)

Generated FromSource

The generated FromSource method accepts multiple source parameters:

public static EmployeeDetailDto FromSource(Employee source, EmployeeProfile source1)
{
    return new EmployeeDetailDto
    {
        Id = source.Id,
        FirstName = source.FirstName,
        // ... Employee properties ...
        ProfileBio = source1.Bio,
        ProfilePhotoUrl = source1.PhotoUrl,
    };
}

Usage

var employee = new Employee { /* ... */ };
var profile = new EmployeeProfile
{
    Bio = "Senior engineer with 10 years of experience",
    PhotoUrl = "https://example.com/photos/alice.jpg"
};

var dto = EmployeeDetailDto.FromSource(employee, profile);
Console.WriteLine(dto.ProfileBio);      // "Senior engineer with 10 years of experience"
Console.WriteLine(dto.ProfilePhotoUrl); // "https://example.com/photos/alice.jpg"

Notes

• Projection is typically disabled (GenerateProjection = false) for multi-source DTOs, since LINQ projections work with a single IQueryable<T> source
• Multiple [AdditionalSource] attributes can be stacked for more than two sources
• The prefix helps avoid property name collisions between sources
• An empty prefix ("") means properties are added without a prefix — only safe if names don't collide