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I am testing Knockout 2.1.0 and Upshot 1.0.0.2 with Entity Framework 4.3 (Code-First) and am running into the following error:
{"Object graph for type 'System.Collections.Generic.HashSet`1[[KnockoutTest.Models.Person, KnockoutTest, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]]' contains cycles and cannot be serialized if reference tracking is disabled."}
I am using a fairly typical model for testing with some basic parent-child entities.
public class Client
{
public Client()
{
Projects = new HashSet<Project>();
Persons = new HashSet<Person>();
}
[Key]
public int ClientId { get; set; }
[Required]
[Display(Name = "Client Name", Description = "Client's name")]
[StringLength(30)]
public string Name { get; set; }
public ICollection<Project> Projects { get; set; }
public ICollection<Person> Persons { get; set; }
}
public class Project
{
public Project()
{
}
[Key]
public int ProjectId { get; set; }
[StringLength(40)]
public string Name { get; set; }
public int? ClientId { get; set; }
public virtual Client Client { get; set; }
}
public class Person
{
public Person()
{
PhoneNumbers=new HashSet<PhoneNumber>();
}
[Key]
public int PersonId { get; set; }
[Required]
[Display(Name="First Name", Description = "Person's first name")]
[StringLength(15)]
public string FirstName { get; set; }
[Required]
[Display(Name = "First Name", Description = "Person's last name")]
[StringLength(15)]
public string LastName { get; set; }
[ForeignKey("HomeAddress")]
public int? HomeAddressId { get; set; }
public Address HomeAddress { get; set; }
[ForeignKey("OfficeAddress")]
public int? OfficeAddressId { get; set; }
public Address OfficeAddress { get; set; }
public ICollection<PhoneNumber> PhoneNumbers { get; set; }
public int? ClientId { get; set; }
public virtual Client Client { get; set; }
}
public class Address
{
[Key]
public int AddressId { get; set; }
[Required]
[StringLength(60)]
public string StreetAddress { get; set; }
[Required]
[DefaultValue("Laurel")]
[StringLength(20)]
public string City { get; set; }
[Required]
[DefaultValue("MS")]
[StringLength(2)]
public string State { get; set; }
[Required]
[StringLength(10)]
public string ZipCode { get; set; }
}
public class PhoneNumber
{
public PhoneNumber()
{
}
[Key]
public int PhoneNumberId { get; set; }
[Required]
[Display(Name = "Phone Number", Description = "Person's phone number")]
public string Number { get; set; }
[Required]
[Display(Name = "Phone Type", Description = "Type of phone")]
[DefaultValue("Office")]
public string PhoneType { get; set; }
public int? PersonId { get; set; }
public virtual Person Person { get; set; }
}
My context is very generic.
public class KnockoutContext : DbContext
{
public DbSet<Client> Clients { get; set; }
public DbSet<Project> Projects { get; set; }
public DbSet<Person> Persons { get; set; }
public DbSet<Address> Addresses { get; set; }
public DbSet<PhoneNumber> PhoneNumbers { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
}
}
I also have a little bit of sample data--though it should not be relevant.
protected override void Seed(KnockoutContext context)
{
base.Seed(context);
context.Clients.Add(new Client
{
Name = "Muffed Up Manufacturing",
Persons = new List<Person> {
new Person {FirstName = "Jack", LastName = "Johnson",
PhoneNumbers = new List<PhoneNumber>
{
new PhoneNumber {Number="702-481-0283", PhoneType = "Office"},
new PhoneNumber {Number = "605-513-0381", PhoneType = "Home"}
}
},
new Person { FirstName = "Mary", LastName = "Maples",
PhoneNumbers = new List<PhoneNumber>
{
new PhoneNumber {Number="319-208-8181", PhoneType = "Office"},
new PhoneNumber {Number = "357-550-9888", PhoneType = "Home"}
}
},
new Person { FirstName = "Danny", LastName = "Doodley",
PhoneNumbers = new List<PhoneNumber>
{
new PhoneNumber {Number="637-090-5556", PhoneType = "Office"},
new PhoneNumber {Number = "218-876-7656", PhoneType = "Home"}
}
}
},
Projects = new List<Project>
{
new Project {Name ="Muffed Up Assessment Project"},
new Project {Name ="New Product Design"},
new Project {Name ="Razor Thin Margins"},
new Project {Name ="Menial Managerial Support"}
}
}
);
context.Clients.Add(new Client
{
Name = "Dings and Scrapes Carwash",
Persons = new List<Person> { new Person {FirstName = "Fred", LastName = "Friday"},
new Person { FirstName = "Larry", LastName = "Lipstick" },
new Person { FirstName = "Kira", LastName = "Kwikwit" }
},
Projects = new List<Project>
{
new Project {Name ="Wild and Crazy Wax Job"},
new Project {Name ="Pimp Ride Detailing"},
new Project {Name ="Saturday Night Special"},
new Project {Name ="Soapy Suds Extra"}
}
}
);
IEnumerable<DbEntityValidationResult> p = context.GetValidationErrors();
if (p != null)
{
foreach (DbEntityValidationResult item in p)
{
Console.WriteLine(item.ValidationErrors);
}
}
}
}
Basically, whenever I attempt to use an "Include" from Client, Person, Project, etc. I get a similar error to the one listed above.
namespace KnockoutTest.Controllers
{
public class ClientController : DbDataController<KnockoutTest.Models.KnockoutContext>
{
public IQueryable<Client> GetClients()
{
return DbContext.Clients.Include("Persons").OrderBy(o => o.Name);
}
}
public class ProjectController : DbDataController<KnockoutTest.Models.KnockoutContext>
{
public IQueryable<Project> GetProjects()
{
return DbContext.Projects.OrderBy(o => o.Name);
}
}
public class PersonController : DbDataController<KnockoutTest.Models.KnockoutContext>
{
public IQueryable<Person> GetPersons()
{
return DbContext.Persons.Include("Client").OrderBy(o => o.LastName);
}
}
public class AddressController : DbDataController<KnockoutTest.Models.KnockoutContext>
{
public IQueryable<Address> GetAddresses()
{
return DbContext.Addresses.OrderBy(o => o.ZipCode);
}
}
public class PhoneNumberController : DbDataController<KnockoutTest.Models.KnockoutContext>
{
public IQueryable<PhoneNumber> GetPhoneNumbers()
{
return DbContext.PhoneNumbers.OrderBy(o => o.Number);
}
}
}
Can you see any reason why .NET should be complaining about this model?
Regardless, what options do I have to work around it?
Thank you for any assistance!
If you expose navigation property on both sides of the relation you will get a cycle. For example
Clientinstance contains collection of relatedProjectinstances and thoseProjectinstances contains navigation property back to their principalClientinstance = cycle.The workaround is either using navigation property only on one side of the relation, using serialization which can resolve cycles out of the box or exclude navigation property on one side from serialization (try to mark it with
IgnoreDataMemberAttribute).The short answer is that Steve Sanderson's demonstration of Knockout, Upshot, and Entity Framework 4.x Code-First to build a Single Page Application was (while great!!!) maybe a little misleading. These tools do not play nearly as nicely together as they appear at first glance. [Spoiler: I do believe there is a reasonable workaround but it involves stepping outside of the Microsoft arena ever-so-slightly.]
(For Steve's fantastic Single Page Application (SPA) presentation, please visit http://channel9.msdn.com/Events/TechDays/Techdays-2012-the-Netherlands/2159. It is well worth a watch.)
In most any Web application, We conceptually need to move and manipulate data in the following way:
AND
In the past, manipulating data to receive it from and transmit it to the database was a real nightmare. If you have to be around in the .NET 1.0/1.1 days, you may recall a development process that included steps like:
This was just to get the data into the system. Going back the other way we had to repeat several of these steps in reverse order. The point is that database coding was very time consuming (and really quite boring). Obviously, a number code generation and other tools came along and simplified things.
The real breakthroughs were with NHibernate, Entity Framework 4 (Code-First approach), and other similar ORM tools which (almost) completely abstracted the database from the developer. These tools not only increased development speed, but also improved overall code quality since their were fewer opportunities to mistakenly introduce bugs.
Now, in many applications, connectivity to and interaction with a database is (almost) an afterthought since most database details are hidden away.
Microsoft has also provided Upshot.js and the WebAPI with the idea that these two tools, when used in conjunction with one another, are going to revolutionize the communication between the server and the browser in the same way that NHibernate and Entity Framework 4 have done between the server and the database.
This would indeed be a very worthy accomplishment--especially as clients are pushing for more interactive Web applications.
One of the main stumbling blocks that prevents developers from moving more of the user interface to the (browser) client is the significant amount of coding required. Some of the steps include:
This really seems quite like "deja vu" because it is quite similar in complexity to the legacy processes for getting data into and out of a database.
Depending on how the Web application is configured, there may be additional fun mapping the data once it returns to the server to actual database objects. (This will be the case more often than not.)
This server->client->server data transmission requires a lot of coding and offers many opportunities for unexpected challenges Don't forget how much fun it is to debug JavaScript! (Ok, it's less painful now than it was a couple of years ago, but it is still not as developer-friendly as debugging C# code in Visual Studio.)
Steve Sanderson's presentation on Single Page Applications offers a far different (and better) solution.
The idea is that WebAPI, Upshot.js, and Knockout would be able to seamlessly deliver data to and receive data from a browser client while providing a highly interactive user experience. Wow! Doesn't that make you just want to reach out and hug someone?
While this idea is not new, it is one of the first serious efforts I think have seen to really do this in .NET.
Once the data is delivered through the WebAPI and reaches the client (via Upshot), then frameworks like Knockout would be able to consume the data and deliver the very high level of interactivity cutting edge Web applications require. (While it may not be immediately clear, what I describing are applications which do not primarily function by loading "pages" but rather by primarily communicating JSON formatted data through AJAX requests.)
Any tool that cuts down on all of this coding is obviously going to be quickly embraced by the developer community.
Upshot.js (the renamed and upgraded version of RIA/JS) was supposed to take care of several of the mundane tasks listed above. It is supposed to be the glue between the WebAPI and Knockout. It is intended to dynamically map objects which are tansmitted in JSON or XML from .NET and also expose the associated meta-data for things like object properties, required fields, field lengths, display names, descriptions, etc. (The meta-data is what allows the mapping and MAY be accessed for use in validation.)
Note: I am still uncertain how to access the upshot meta-data and tie it to a validation framework like jQuery validation or one of the Knockout validation plugins. This is on my todo list to test.
Note: I am uncertain which of these types of meta-data are supported. This is on my todo list to test. As a side note, I also plan to experiment with meta-data outside of System.ComponentModel.DataAnnotations to also see if NHibernate attributes are supported as well as custom attributes.
So with all of this in mind, I set out to use the same set of technologies that Steve used in his demo in a real-world Web application. These included:
The expectation is that all of these technologies would function well together because a) they are the latest Microsoft tools (with the exception of the open source Knockout) and because Steve Sanderson, currently of Microsoft, used them together in major Microsoft presentation demonstrating the development of single page application.
Unfortunately, what I found in practice was that Entity Framework 4.x and Upshot.js view the world in very different ways and their orientations are somewhat more contradictory than complementary.
As mentioned, Entity Framework Code First does a really fantastic job allowing developers to define highly functional object models which it near-magically translates into a functional database.
One of the great features of Entity Framework 4.x Code First is the ability to navigate from a parent object to a child AND navigate from a child object back to its parent. These two-way associations are a cornerstone of EF. They save a tremendous amount of time and greatly simplify development. Moreover, Microsoft has repeatedly touted this functionality as great reason to use Entity Framework.
In Scott Guthrie's, blog post (http://weblogs.asp.net/scottgu/archive/2010/07/16/code-first-development-with-entity-framework-4.aspx) where he initially introduced and explained EF 4 Code First approach, he demonstrates the concept of two-way navigation with the following two classes:
As you can see, Dinner contains an association with RSVPs AND RSVPs contains an association to Dinner. There are countless other examples of this on the Internet occurring in many variations.
Because these two way associations are such a core feature of Entity Framework, a reasonable person might expect that Microsoft would support this functionality in the library (Upshot.js) it uses to bring data from a .NET server application to the client. If the functionality was not supported, that is likely something they would want to share as it would significantly key architectural decisions and would most like not work with any properly designed EF 4 Code First implementation.
In my test code (listed in the original question above), I naturally assumed normal EF Code-First functionality (like two-way binding/navigation) was supported because that is what the presentation appeared to show.
However, I immediately started receiving nasty little run-time errors like:
I tried numerous different approaches to try and fix the problem. Based on my ideas and my reading, here are a few of the failed solutions I attempted.
Just in case the issue was the result of the system becoming confused about there being a foreign key, I explicitly specified a [ForeignKey] attribute on the child entity. Everything compiles but .NET returns the "Object graph for type... contains cycles and cannot be serialized..."
Some of my reading indicated that adding an attribute like [DataContract(IsReference = true)] in WCF might keep .NET from getting confused about cyclical references. That's when I get this beauty.
This error is very important because it basically tells us that we NOT going to be able to use Upshot AND Entity Framework Code-First together in their normal configuration. Why? Entity Framework is designed to utilize two-way binding. However, when two-way binding is implemented, Upshot says that it cannot handle cyclical references. When cyclical references are managed, Upshot basically says that it cannot handle references between parent and child objects because JSON doesn't support it.
When I watched Steve's demo, I recalled that he DID have a relationship between Customers and Deliveries. I decided to go back and take a much closer look at his object model.
What we find is that in Steve's demo is that his relationship only goes one way, and it binds the child to the parent rather than the parent to the children.
In this demo, it kind of works. However, in many real-world applications, this approach makes data access impractical. Take, for example, the demo scenario I included in my original question. We have:
Most developers, I think would not want to start their query from addresses or phone numbers. They would expect to be able to select a list of clients or Projects or Persons and then navigate to a list of its descendants.
I am not 100% certain that is impossible to use entities which have two-way binding enabled, but I am unaware of any configuration that is likely to yield success using only the Microsoft tools.
Fortunately, I do think there is a solution (which takes care of the cyclic dependency issue) which I plan to test in the next few days. That solution is... JSON.Net.
JSON.Net supports cyclic dependencies and maintains references to child objects. If it works as expected, it will take care of two of the errors I got in my testing.
Once I have tested, I will report results here.
I think Steve's demo was brilliant, and I loved his demo. I believe Knockout is amazing. I am very thankful for where Microsoft seems to be going. If there are noteworthy limitations in the tool, I think Microsoft maybe should have been more forthcoming with them.
I don't mean to be overly critical of Microsoft (and definitely am not critical of Steve at all) because I think they have done a wonderful job. I love what upshot promises and am looking forward to seeing where it goes.
I would really love to see someone take upshot and re-factor it (and the WebAPI) so that it can integrate fully with Entity Framework without the use of a third-party tool.
I don't know if a similar tool exists for NHiberbnate, but I would love to maybe even see someone extend upshot to integrate with (or develop a similar library for) NHibernate. By extend, I am primarily talking about consuming the meta-data from NHibernate.
When I test JSON.Net, I also plan to test NHibernate as well.