AutoMapper throwing StackOverflowException when calling ProjectTo() on IQueryable

Question

I have created classes using EF Code First that have collections of each other. Entities:

public class Field
{
    public int Id { get; set; }
    public str         


        

Answer 1

You have self-referencing entities AND self-referencing DTOs. Generally speaking self-referencing DTOs are a bad idea. Especially when doing a projection - EF does not know how to join together and join together and join together a hierarchy of items.

You have two choices.

First, you can force a specific depth of hierarchy by explicitly modeling your DTOs with a hierarchy in mind:

public class FieldDTO
{ 
    public int Id { get; set; }
    public string Name { get; set; }
    public List Teachers { get; set; }
    public FieldDTO()
    {
        Teachers = new List();
    }
}

public class TeacherDTO 
{
    public int Id { get; set; }
    public string Email { get; set; }
    public string Password { get; set; }
    public string UserName => Email;
}

public class AppUserDTO : TeacherDTO
{
    public List Fields { get; set; }
    public AppUserDTO()
    {
         Fields = new List();
    }
}

This is the preferred way, as it's the most obvious and explicit.

The less obvious, less explicit way is to configure AutoMapper to have a maximum depth it will go to traverse hierarchical relationships:

CreateMap().MaxDepth(3);

I prefer to go #1 because it's the most easily understood, but #2 works as well.