I am trying to return JsonResult using MVC controller standard Json(object) method. My object of type Model1 is built by Fluent NHibernate.
Model1 has property of type Model2. In debug mode I see that the environment creates a proxy descendant class of Model2 called Castle.Proxies.Model2Proxy. This is used internally by Fluent Nhibernate, I believe, to satisfy my mappings. And in run time, actual model1.Model2 is of type Castle.Proxies.Model2Proxy.
The problem is that when my Model1 is serialized, Model2 is being serialized too. And the serializer seems to try to serialize all the properties of this object, including those generated by Castle and not needed by me. I would be OK with it if it did not cause an exception. Namely, somewhere inside this object a circular reference presents and the exception is caused by it. Here is the exception text:
System.InvalidOperationException: A circular reference was detected while serializing an object of type 'System.Reflection.RuntimeModule'
I double checked my domain and found no circular references there, so I am blaming Castle. Am I correct? Is Castle really to blame for that? If so, what are my options? How do I tell serializer to ignore Castle properties? Particularly, how do I tell it to serialize the defined type, not the actual one?
I tend to covering my domain models with ViewModels to fight this issue, which is a recommended approach, but I would really love to know another cure, if it exists.
in general, it's not good practice to serialize your model entities.
this is because you want to have complete control over what you serialize and send to the client.
when you serialize your model entities, you might be serializing the whole object graph associated with them, which you don't necessarily need / want.
(for example- if you want the user to view just a Model1 entity, you might be sending along also a Model2 entity, along with its Model3 collection etc.)
the standard way to deal with it is using some sort of DTOs, which are adapted to display precisely what you want to display. for example:
public class Model1DTO
{
public int Id;
public string Name;
public string Model2Name;
//whatever other properties you need to display
}
Related
I've started to develop ASP.NET MVC application using Entity Framework and I wish to use DDD. It's my first time using DDD in ASP.NET (used until now in PHP), so I'm little confused.
I'm using code-first approach, so I'm creating my entites in the core and then the DbContext in the Infrastructure.
My questions is about data annotations: Is it OK to annonate the entities in the core? with Required, DataType, etc. or I have to create entries with pure C# validation (in the setters and getters) and then create a map objects for the database creation?
For example, I got:
public class Account
{
public string AccountName { get; set; }
}
Can I annonate the AccountName with [Required] or I need to create a map class which will just reflect the exact same properties in the Account class but will have attributes and that will be the class that I'll use in Entity Framework DbContext?
Thanks!
Neither.
Your entities should have barely any public getters or setters. Domain model is a behavioral model, not a data model. Your entities should have private fields and methods that changes the entity state. Those methods should have a business meaning and should protect entities invariants (validate input). For example - we have UserAddress and want to change it. Is it just user.Address = newAddress? NO. What's the meaning of changing that? Maybe your User want to FixMistypedAddress(str). Or maybe your UserMoved(newLocation)? Different business rules may apply tho those scenarios. For example when UserMoved() we want to send him a champagne bottle, but not when he just fixed a typo. You should already see that there is no use of data annotations here because we don't just set properties but we do meaningful operations.
Entities should always be valid. That mean there should be no way to put it in an invalid state. Data annotations only allow you to check whether the object is valid or not. They not guarantee is will be valid all the time.
IMO Data annotations are fine for:
Using Entity Framework in a CRUD application (no DDD applied)
In DTO or ViewModels. In other words - for validating user forms, not entities.
So the first quiestion for you is: Are you doing CRUD or DDD?
I'd say either way is fine.
If you want a more pure approach you would create a buddy class that has the metadata on it however it is also acceptable to put it directly on the domain class.
In the end it comes down to how much you want to remain "pure" and how much extra work you want to put into maintaining buddy classes, not to say that it is a bad thing.
I'm new to the whole ASP world and I'm getting my feet wet by building a C# MVC3/EF4 project. I'm finding it hard to keep from duplicating a bunch of code in my models and view models. Consider an object Foo. I need to do the following things with Foo:
Store records of type Foo in my database.
Allow users to lookup records of an individual Foo (pass instances of Foo to a view).
Allow users to create new instances of Foo (pass instances of Foo to a form).
Let's say I also have a type Bar. A Bar contains a list of Foos. There are two requirements here:
Users can view a list of Bars.
When the user clicks on a specific Bar, it shows all of its Foos.
So, a sketch of my basic objects look like this:
class Foo
{
string FooName;
int Id;
}
class Bar
{
List<Foo> FooList;
int Id;
string Baz;
}
But when I start thinking about the different views, it begins to get messy:
The views shouldn't have any write access to any of the data members.
There's one view that takes a list of Bars but doesn't care about Bar.FooList. Let's say I also want to be good about resource management and close the DbContext as soon as possible (i.e. after the object is in memory but before I render the view). If I just pass it a list of Bars and the designer tries to access the FooList by mistake, we'll get a runtime error. Yuck!
Ok fine, I just create a distinct ViewModel for each view that has read only datamembers, no problem.
But both the database model and the form models will need to have DataAnnotations attached which say which fields are required, max length of the strings, etc. If I create separate form models and database models then I end up having to duplicate all these annotations. Yuck!
So, that's my architectural dilemma: I want to have succinct view models which restrict the views only to reading the data they are supposed to access. I want to avoid repeating data annotations all over the place. And I want to be able to aggressively free my DB resources as soon as possible. What's the best way to achieve my goals?
My advice is do NOT use data annotations in your EF entity classes. Instead, try out the fluent API. It keeps persistence concerns out of the model classes themselves. You can even define the modelbuilder stuff in an entirely different library.
As for having duplicate properties, properties are cheap:
public string MyProp { get; set; }
Not a lot of code, and you may begin to see, the viewmodels and entities need not always be exact duplicates of each other. For example, what if you want to apply [HiddenInput] to a viewmodel, to get it to render as <input type="hidden" />? Would you apply that to the entity? Why? It belongs in the viewmodel (the namespace is even System.Web.Mvc, not System.ComponentModel.DataAnnotations).
As far as duplicating error messages during validation (if you validate the MVC and EF layers separately), you can use resx resources.
As far as freeing db resources, let EF manage that. I find it's best to keep a single DbContext instance per HttpContext. You can open it up using a factory, an OnActionExecuting action filter, Application_BeginRequest, with an IoC container, or whatever. Then dispose context during OnResultExecued, Application_EndRequest, etc. Keeps things simple.
Context:
Code First, Entity Framework 4.3.1;
User ---- Topic, 1 to Many relation;
User with public virtual ICollection<Topic> CreatedTopics Navigation Property(Lazy Loading);
Topic with public virtual User Creator Navigation Property;
DataServiceController : DbDataController<DefaultDbContext>, Web API beta, ASP.NET MVC 4 Beta , Single Page Application;
System.Json for Json serialization;
Web API Action:
public IQueryable<Topic> GetTopics()
{
// return DbContext.Topics; // OK
return DbContext.Topics.Include("Creator"); //With Exception
}
Result: "an unhandled microsoft .net framework exception occurred in w3wp.exe"
The Problem here seems to be: I should not Add Navigation Property in both Entities(Cause Circular Reference?), and if I delete the CreatedTopics Navigation Property in User Class, It will be OK again.
So, In a similar Context like listed above, Here are my questions:
How to deal with Navigation Properties in the situation of 1 to Many relation;
Further more, how about a Many to Many relation, do i have to divide it into two 1 to Many relations;
What is the Best Practices and Precautions of using Navigation Properties?
I Have read many related posts, but still not clear enough :(,
Thanks for any help!
Dean
This is not a problem of code first or EF - it is a problem of serialization. Simply the serializer used to convert your object graph to some representation passed in a Web API message is not able to work with circular references by default. Depending on the message format you want to use Web API uses different serializers by default - here is more about default serializers used by Web API and about the way how to change it. The following text suppose that you are using DataContractJsonSerializer or DataContractSerializer (should be default for XML serialization) but the same is possible for JSON.NET (should be default for JSON serialization - JSON serialization can be switched to DataContractJsonSerializer but the default serializer is better).
So what you can do? You can tell the serializer that it should track those circular references by marking your classes with DataContract(IsReference = true) and each passed property with DataMember attribute (check the linked article for description how to achieve it with JSON.NET). This will allow serializer correctly recognizing cycles and the serialization will in theory succeed. In theory because this also demands to not using lazy loading. Otherwise you can serialize much more data than you expected (in some catastrophic scenarios it can lead to serializing whole content of your database).
When you serialize entity graph with lazy loading enabled you serailze a Topic and its Creator but serialization will also visit CreatedTopics property => all related topics are lazy loaded and processed by serialization and serialization continues to visit Creator of all newly loaded topics! This process continues until there is no other object to lazy load. Because of this you should never use lazy loading when serializing entities.
Other option is to exclude back reference from serialization. You just need to serialize Creator. You don't need to serialize CreatedTopics so you can mark the property with IgnoreDataMember attribute (JsonIgnore for JSON.NET). The problem is that if you also have Web API action for returning User with all his CreateTopics this will not work because of the attribute.
The last option is not using entities. This option is usually used in web services where you create special DTO objects satisfying requirements for specific operation and you handle conversion between entities and DTOs inside the operation (possible with help of some tool like AutoMapper).
There is no difference between handling one-to-one, one-to-many, or many-to-many relations. If you have navigation properties on both sides you must always deal with this problem.
I'm building an application using MVC 3 and Entity Framework 4.
I've created my Entity Data Model and generated a database from it.
Now I know the validation attributes such as [Required] or [StringLength(5)] can be used on the model properties to provide validation both clientside and serverside.
I would like to know if these attributes can also be generated dynamically instead of having to add them to the model explicitly? I saw that in EF 4.1 RC you can make use of the Fluent API to further configure your model in the OnModelCreating method by using the DbModelBuilder class.
As shown here
I'm working with a framework however that still uses ObjectContext instead of DbContext so I would like to know if the above solution can be used in combination with ObjectContext?
As a final note, since I've been trying to figure out how to generate and use data annotations it seems using view models would increase the complexity of validation. From what I read here it seems that just passing the models directly to the view would remove the need to add annotations to the models as well as the view models. However that means that you can no longer use strongly typed views when you do joins on the models and pass those to the view directly?
No it can't. Fluent API is different approach to describe mapping. You can use fluent API or EDMX (Entity Data Model). Not both. Fluent API also works only with DbContext API. If you want to have annotations generated you can try to modify T4 template generating your classes.
I have come across a disturbing issue when using poco classes that are extending base classes.
For example, let say you have a Person poco class that has a strongly typed Car property. You also have a Spouse poco that also uses the Car Property.
Now you want to display "Person Car" and "Spouses Car" in the view using the Display("Name = xxx") attribute. You cant!!! Becareful of this issue if you are not using flat View Models
Let's start with this basic scenario:
I have a bunch of Tables that are essentially rarely changed Enums (e.g. GeoLocations, Category, etc.) I want to load these into my EF ObjectContext so that I can assign them to entities that reference them as FK. These objects are also used to populate all sorts of dropdown controls. Pretty standard scenarios so far.
Since a new controller is created for each page request in MVC, a new entity context is created and these "enum" objects are loaded repeatedly. I thought about using a static context object across all instances of controllers (or repository object).
But will this require too much locking and therefore actually worsen perf?
Alternatively, I'm thinking of using a static context only for read-only tables. But since entities that reference them must be in the same context anyway, this isn't any different from the above.
I also don't want to get into the business of attaching/detaching these enum objects. Since I believe once I attach a static enum object to an entity, I can't attach it again to another entity??
Please help, I'm quite new to EF + MVC, so am wondering what is the best approach.
Personally, I never have any static Context stuff, etc. For me, when i call the database (CRUD) I use that context for that single transaction/unit of work.
So in this case, what you're suggesting is that you wish to retrieve some data from the databse .. and this data is .. more or less .. read only and doesn't change / static.
Lookup data is a great example of this.
So your Categories never change. Your GeoLocations never change, also.
I would not worry about this concept on the database/persistence level, but on the application level. So, just forget that this data is static/readonly etc.. and just get it. Then, when you're in your application (ie. ASP.NET web MVC controller method or in the global.asax code) THEN you should cache this ... on the UI layer.
If you're doing a nice n-tiered MVC app, which contains
UI layer
Services / Business Logic Layer
Persistence / Database data layer
Then I would cache this in the Middle Tier .. which is called by the UI Layer (ie. the MVC Controller Action .. eg. public void Index())
I think it's important to know how to seperate your concerns .. and the database stuff is should just be that -> CRUD'ish stuff and some unique stored procs when required. Don't worry about caching data, etc. Keep this layer as light as possible and as simple as possible.
Then, your middle Tier (if it exists) or your top tier should worry about what to do with this data -> in this case, cache it because it's very static.
I've implemented something similar using Linq2SQL by retrieving these 'lookup tables' as lists on app startup and storing them in ASP's caching mechanism. By using the ASP cache, I don't have to worry about threading/locking etc. Not sure why you'd need to attach them to a context, something like that could easily be retrieved if necessary via the table PK id.
I believe this is as much a question of what to cache as how. When your are dealing with EF, you can quickly run into problems when you try to persist EF objects across different contexts and attempt to detach/attach those objects. If you are using your own POCO objects with custom t4 templates then this isn't an issue, but if you are using vanilla EF then you will want to create POCO objects for your cache.
For most simple lookup items (i.e numeric primary key and string text description), you can use Dictionary. If you have multiple fields you need to pass and back with the UI then you can build a more complete object model. Since these will be POCO objects they can then be persisted pretty much anywhere and any way you like. I recommend using caching logic outside of your MVC application such that you can easily mock the caching activity for testing. If you have multiple lists you need to cache, you can put them all in one container class that looks something like this:
public class MyCacheContainer
{
public Dictionary<int, string> GeoLocations { get; set; }
public List<Category> Categories { get; set; }
}
The next question is do you really need these objects in your entity model at all. Chances are all you really need are the primary keys (i.e. you create a dropdown list using the keys and values from the dictionary and just post the ID). Therefore you could potentially handle all of the lookups to the textual description in the construction of your view models. That could look something like this:
MyEntityObject item = Context.MyEntityObjects.FirstOrDefault(i => i.Id == id);
MyCacheContainer cache = CacheFactory.GetCache();
MyViewModel model = new MyViewModel { Item = item, GeoLocationDescription = GeoLocations[item.GeoLocationId] };
If you absolutely must have those objects in your context (i.e. if there are referential entities that tie 2 or more other tables together), you can pass that cache container into your data access layer so it can do the proper lookups.
As for assigning "valid" entities, in .Net 4 you can just set the foreign key properties and don't have to actually attach an object (technically you can do this in 3.5, but it requires magic strings to set the keys). If you are using 3.5, you might just try something like this:
myItem.Category = Context.Categories.FirstOrDefault(c => c.id == id);
While this isn't the most elegant solution and does require an extra roundtrip to the DB to get a category you don't really need, it works. Doing a single record lookup based on a primary key should not really be that big of a hit especially if the table is small like the type of lookup data you are talking about.
If you are stuck with 3.5 and don't want to make that extra round trip and you want to go the magic string route, just make sure you use some type of static resource and/or code generator for your magic strings so you don't fat finger them. There are many examples here that show how do assign a new EntityKey to a reference without going to the DB so I won't go into that on this question.