EF code first one to many navigate from child to parent

Question

I am a little puzzled as to why this code doesn't work. I have a basic one-to-many relationship where I load a parent and include its children. I later I am trying to navigate from the child back to the parent, but the parent is null and I can't figure out why.

Question:

Why can't I query a graph of parent/child objects and navigate backward through them from child to parent? The parent is always null.

Here are the entities.

public class Budget
{
    [Key]
    public int Id { get; set; }

    public ICollection<Expense> Expenses { get; set; }

    public ICollection<Income> Incomes { get; set; }
}

public class Expense
{
    [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

    [Required]
    [StringLength(200)]
    public string ExpenseName { get; set; }

    [Required]
    public decimal Cost { get; set; }

    [StringLength(800)]
    public string Notes { get; set; }

    public DateTime? DueDate { get; set; }

    public Budget Budget { get; set; }
}

public class Income
{
    [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

    [Required]
    public decimal Amount { get; set; } = 0;

    [Required]
    [StringLength(200)]
    public string Source { get; set; }

    [Required]
    public DateTime? PayDate { get; set; } = DateTime.Now;

    public Budget Budget { get; set; }
}

Here is the repository query.

    public async Task<Budget> GetBudget(int id)
    {
        try
        {
            return await context.Budget
            .Include(e => e.Expenses)
            .Include(i => i.Incomes)
            .SingleAsync(b => b.Id == id);
        }
        catch(Exception x)
        {
            x.ToString();
        }

        return null;
    }

I want to be able to navigate back through the relation from expense to budget to get the Budget.Income collection.

Expected result:

foreach(Expense expense in Budget.Expenses)
{
    if (expense.Budget is not null)
    {
        ICollection<Income> paychecks = expense.Budget.Incomes; // Why is Budget always null?
    }
}

I expected that even if I didn't use the ThenInclude(e => e.Budget) that I should still be able to navigate from the child back to the parent {var budget = expense.Budget}. I'm surprised that this isn't working.

I didn't include the Income entity here, but my goal is to traverse expense.Budget.Incomes to get the collection of incomes in code where I only have access to the expense instance.

After removing ThenInclude(e => e.Budget) I no longer get an error, but the expense.Budget property is still null.

UPDATE

I believe that I found the root cause of my problem. When I added the property Budget to the Expense class I started getting an error when deserializing the objects coming from the API. The HttpClient was throwing an error due to a cyclical reference.

Because I'm fetching the root Budget entity and it's related Expenses, and the Expense has a reference to the Budget I got the cyclical reference error.

I added this code the the startup Blazor server startup class to fix it. I think this is my problem.

services.AddControllersWithViews().AddJsonOptions(x =>
    x.JsonSerializerOptions.ReferenceHandler = ReferenceHandler.IgnoreCycles);

If I change to ReferenceHandler.Preserve I get a different error.

'The JSON value could not be converted to System.Collections.Generic.ICollection`1[BlazorApp.Data.Models.Expense]. Path: $.expenses | LineNumber: 0 | BytePositionInLine: 34.'

What I don't know and would like to solve is how to make this work so I can get the Budget -> Expenses and have the Expense.Budget property point to it's parent Budget instance. My real issue is probably more related to json serialization and deserialization.

Answer 1

You have a bug in your query. Income doesn't depend on expenses, it depends only on budget. The same is about expenses.

So your code can be translated to this

 var budget= await context.Incomes
            .Include(e => e.Expenses)
            .Include(i => i.Incomes)
            .SingleAsync(b => b.Id == id);

foreach (Expense expense in budget.Expenses)
{
      var paychecks = budget.Incomes; 
}

This foreach doesn't make any sense in this case since it just repeats "var paychecks = budget.Incomes;" many times. It is not even saved anywhere.

The same can be done in one line

var paychecks= await context.Incomes
               .Where(b => b.BudgetId == id);
               .ToListAsync();

or if budget is downloaded already

var paychecks = budget.Incomes;

You can not use .ThenInclude(e => e.Budget) since your are querying context.Budget already and as you can see the Budget class doesn't have any extra Budget property.

And Expenses are list. List don't have any Budget property too, only items of the list have. If you try .Include(e => e.Expenses.Budget) it will give a syntax error. But since Expense or Income class has a Budget property this query will be valid

return await context.Incomes
                    .Include(i => i.Budget)
          ....

I think you need only this query that merges budget, income and expenses together

var budget= await context.Budgets
        .Include(e => e.Expenses)
         .Include(i => i.Incomes)
        .FirstOrDefaultAsync(b => b.Id == id);

Also remove public ICollection<Expense> Expenses { get; set; } from Income or make it [NotMapped] if you need it for some reasons.

If you use EF Core 5+, EF Core automatically created foreign keys as the shadow properties. But I personally prefer to have them explicitly. I recommend to add them to your classes (or you still can keep the current version)

public class Budget
{
    [Key]
    public int Id { get; set; }
     .............
   
    [InverseProperty("Budget")]
    public  virtual ICollection<Expense> Expenses { get; set; }

    [InverseProperty("Budget")]
    public  virtual ICollection<Income> Incomes { get; set; }
}

public class Expense
{
    [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

    ...........

    public int BudgetId { get; set; }

    [ForeignKey(nameof(BudgetId ))]
    [InverseProperty("Expenses")]
    public virtual Budget Budget { get; set; }
}
public class Income
{
    [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

   ............

    public int BudgetId { get; }

    [ForeignKey(nameof(BudgetId ))]
    [InverseProperty("Expenses")]
    public virtual Budget Budget { get; set; }

}

Answer 2

Thank you all for the help. Your input helped me know what wasn't the problem.

This fixed my problem.

WebAPI : JSON ReferenceHandler.Preserve

My original question doesn't relate to the actual answer. I did not realize what was really going on. The real problem was a serialization problem. I'm using a hosted Blazor app and have Client and Server projects.

The problem has a number of layers to it. I added the Budget property to the Expense class so that I could traverse back to the budget. After adding the Budget property I started getting an error about cyclical references. This error appears in the Client project during the HttpClient call to the Server controller. I fixed this error by adding the following code to the Server Startup class.

services.AddControllersWithViews().AddJsonOptions(options => {
    options.JsonSerializerOptions.ReferenceHandler = ReferenceHandler.Preserve;
    options.JsonSerializerOptions.PropertyNamingPolicy = null; // prevent camel case
});

Days later when I start doing more work I tried to reference the expense.Budget property. Finding that it was always null and started trying to fix it in the entities which is the wrong place.

It turns out that I also needed to pass the JsonSerializerOptions.ReferenceHandler = ReferenceHandler.Preserve to the HttpClient code during the call to the controller.

public async Task<Budget> GetBudget(int id)
{
    Budget budget = null;
    try
    {
        budget = await httpClient.GetFromJsonAsync<Budget>("api/Budget/" + id, new JsonSerializerOptions() { ReferenceHandler = ReferenceHandler.Preserve });
    }
    catch(Exception x)
    {
        x.ToString();
    }
    return budget;
}

This foreach statement now works as expected.

foreach(Expense expense in Budget.Expenses)
{
    if (expense.Budget is not null)
    {
        ICollection<Income> paychecks = expense.Budget.Incomes; // How do I populate budget?
    }
}