AutoMapper throwing StackOverflowException when calling ProjectTo<T>() on IQueryable

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<TeacherDTO> Teachers { get; set; }
    public FieldDTO()
    {
        Teachers = new List<TeacherDTO>();
    }
}

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<FieldDTO> Fields { get; set; }
    public AppUserDTO()
    {
         Fields = new List<FieldDTO>();
    }
}

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<AppUser, AppUserDTO>().MaxDepth(3);

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

Other option is using PreserveReferences() method.

CreateMap<AppUser, AppUserDTO>().PreserveReferences();