CODEWITHSUNDEEP
entity-frameworkjson.netef-code-first
matt
matt

Reputation: 321

"Error getting value from 'MyEntity' on 'System.Data.Entity.DynamicProxies..."

I insert an Entity with this request and it is successful.

Request

{
    "Title": "Book 1"
}

Response

{
    "id": 1,
    "title": "Book 1",
    "serialId": "00000000-0000-0000-0000-000000000000",
    "borrows": null,
    "votes": null
}

I checked the database and there is a Book row with id = 1. But now when I try to fetch a list it returns an error because the DbSet is empty after the insert.

var books = db.Books; // count = 0

enter image description here

The response for /books is the exception:

"exceptionMessage": "The 'ObjectContent`1' type failed to serialize the response body for content type 'application/json; charset=utf-8'.",

And the inner exception includes Error getting value from 'Borrows' so I thought it might be to do with how the relationship is defined, also there are no Borrow entities in the database yet.

"exceptionMessage": "Error getting value from 'Borrows' on 'System.Data.Entity.DynamicProxies.Book_6E27A1F717202EA02AE923CCC6405EF9A501FE9A54A71841CEB43E942224D88A'.",
        "exceptionType": "Newtonsoft.Json.JsonSerializationException",

The Book entity has a navigation property of List on it with a defined relationship:

modelBuilder.Entity<Book>().HasMany<Borrow>(b => b.Borrows);

Book's entity defined with a list of Borrow entities:

public virtual List<Borrow> Borrows { get; set; }

The migration of Book only has it's Id for the primary key:

CreateTable(
            "dbo.Books",
            c => new
                {
                    Id = c.Int(nullable: false, identity: true),
                    Title = c.String(),
                    SerialId = c.Guid(nullable: false),
                })
            .PrimaryKey(t => t.Id);

Question: What is causing this exception, is it something to do with how I defined the relationship between Book and Borrow? I've seen examples of relationships having to be defined in both directions, do I need to define the relationship to not require a borrow somehow? I've noticed DbSet Count = 0 occur in the past when there have been results so I think it could also be an issue with NewtonSoft. I'm not sure what is going on here.

Full exception: https://pastebin.com/raw/BvUfBnKU

Upvotes: 2

Views: 4120

Answers (1)

matt
matt

Reputation: 321

I got it working by using this answer: https://stackoverflow.com/a/14317598/11419029, returning a List rather than IQueryably.

I changed it to this:

// GET: api/Books
public List<Book> GetBooks()
{
    var books = db.Books.ToList();
    return books;
}

From this:

// GET: api/Books
public IQueryable<Book> GetBooks()
{
    var books = db.Books;
    return books;
}

But I'm still not sure what the problem was, another answer said it was to do with a query being executed while iterating results of another query. Triggering lazy loading. Now I'm concerned it is executing nested queries, querying borrows for every instance of a book.

Upvotes: 2

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