NullReferenceException on join with Postgres EF Provider - postgresql

Question

I have a postgres database and using asp.net core mvc (+ ef). The database is created correctly. I have two tables 'Module' and 'ModuleMenu'. I want to get all the menu's for a given module but I keep on failing to create the linq query.
Situation
Model: Module.cs
namespace project.Model
{
public class Module
{
[Required]
public string ID { get; set; }
[Required]
public string Name { get; set; }
[Required]
public string Description { get; set; }
}
}
Model: ModuleMenu.cs
namespace project.Models
{
public class ModuleMenu
{
[Required]
public string ID { get; set; }
public int ModuleID { get; set; }
[Required]
public string Title { get; set; }
[ForeignKey("ModuleID")]
public virtual Module Module { get; set; }
}
}
ApplicationDbContext.cs
namespace project.Data
{
public class ApplicationDbContext
{
public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options)
: base(options)
{
}
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
base.OnModelCreating(modelBuilder);
}
public DbSet<Module> Modules { get; set; }
public DbSet<ModuleMenu> ModuleMenus { get; set; }
}
}
Query
public List<ModuleMenu> GetModuleMenus(){
var query = from m in _dbContext.ModuleMenus
join mod in _dbContext.Modules on
m.ModuleID equals mod.ID
select m;
return query.ToList();
}
Error
fail: Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddleware[0]
An exception was thrown attempting to execute the error handler.
System.NullReferenceException: Object reference not set to an instance of an object.
Can anyone help me to correctly create the query?

Answer 1

Is this part correct in your code?
public int ModuleID { get; set; }
It seems that you might have had an error in the type used for the fk.
Below I changed the type to be string rather than int.
public string ModuleID { get; set; }
based on that update, the query could look like this.
public ModuleMenu[] GetModuleMenusForModule(string moduleId)
{
return _dbContext.ModuleMenus.Where(x => x.ModuleID == moduleId).ToArray();
}

Answer 2

I would expect that model to error (ModelID and ID are incompatible types). If that were correct, your code should work. Or simpler:
public List<ModuleMenu> GetModuleMenus()
{
return _dbContext.ModuleMenus.ToList();
}