I can't really find a satisfying solution for that problem. I have a n-layers application :
UI
Presentation (Domain Model used as DTO, ViewModels are then presented)
Business Layer (Domain Model)
Repository and DAL (Data Model)
My problem is that I need to work with full objects in my business layer. However I can't figure out what is the best way to load them. The question may look stupid - and perhaps it is - but I'm very puzzled by it. I have the following classes (just to illustrate):
public class Library
{
int ID {get;set;}
string Name {get;set;}
Book[] Novels {get;set;}
Book[] TextBooks {get;set;}
}
public class Book
{
int ID {get;set;}
Library[] SalePoints {get;set;}
string Name {get;set;}
string Type {get;set;}
}
And I have two Service Methods LoadLibrary and LoadBook whose aim is to populate these respective models. My problem is that it looks like I will get into an infinite loop. I'll explain myself right now.
I have pre-loaded domain models coming from the Repositories (The repositories convert the Data Models into domain models using valueinjecter (unflatenning), so basically the simple fields (int, string, etc..), are populated in the resulting domain model but the IEnumerable properties are not. That's why I'm finishing to load them in the Service Layer. (There are also two reaons why I can't fully load them in my repositories : 1/ I'm using a generic repository and 2/ The loading logic relay on business rules)
Let us say that I do the following :
public class LibrairiesProcessor
{
public Library LoadLibrairyObject(int ID)
{
Library l = UnitOfWork.LibraryRepo.GetByID(ID); // Only retrieve "simple" properties, here it will get the Name and the ID properties;
l.Novels = UnitOfWork.BookRepo.Get(b=>b.LibrairyID==l.ID && b.Type="Novel");
l.TextBooks= UnitOfWork.BookRepo.Get(b=>b.LibrairyID==l.ID && b.Type="TextBook");
for(int i=0;i<l.Novels.Count();i++)
{
l.Novels[i] = BooksProcessor.LoadBookObject(l.Novels[i].ID);
}
}
}
and
public class BooksProcessor
{
public Book LoadBookObject(int ID)
{
Book b = UnitOfWork.LibraryRepo.GetByID(ID); // Only retrieve "simple" properties, here it will get the Name and the ID properties;
b.SalesPoint = // Get From repo using the Unit of work
for(int i=0;i<b.SalesPoint.Count();i++)
{
b.SalesPoint[i] = LibrairyProcessor.LoadLibraryObject(b.SalesPoint[i].ID);
}
}
}
Isn't there an infinite Loop ? LoadLibraryObject() calling LoadBookObject() and vice versa... And especially when you come to the book whose SalePoint is the librairy who triggered LoadBookObject()... at best the same work is done twice, but I really suspect that it will never end.
So I'm wondering if I'm doing the things right. My goal is to have fully loaded objects to avoid wondering if the object is enough loaded before using it, but I'm not sure of the solution I've came up with. How do you usually achieve that ?
It looks like a trivial issue but seriously I can't figure out how to load my stuff. I feel like I have a dilemma :
1/ Manually Populating all the properties and sub properties by using UnitOfWork.Repository, but I will have a lot of code and a lot of redundant code.
2/ Live with partially loaded domain models with some properties set to null, but I really don't want that. And since I'm not working with EF objects directly since my domain model is complex, I can't use their lazy loading stuff.
3/Call the Service LoadObject Methods from one LoadObjectMethod to an other, to avoid code redundancy, but it looks like I'm introducing a lot of recursivity.
How should I handle that ?
Thanks !
This is probably best suited for CodeReview, but here are a few thoughts that may save you some trouble :
Having a whole separate DAL object model might be overkill. Most modern ORMs will take care of the domain objects/DB mapping for you without polluting your domain model with persistence stuff. See Should I create in BLL project class the same like poco class in DAL project and return it to UI project if I want to display data from database?
Having a generic repository seems like a poor excuse for not being able to load lists of sub-entities. As for "the loading logic relies on business rules", if you're referring to the Novels/TextBooks distinction, there are other, less class-intensive ways of doing this.
"since I'm not working with EF objects directly since my domain model is complex, I can't use their lazy loading stuff" : how exactly is your model complex ? Technically speaking, why couldn't you use lazy loading ?
As a good practice, Aggregates shouldn't refer to one another with navigatable properties but with (lists of) ids. The object that got an Aggregate root from a Repository will call another Repository if it wants to load associated aggregate roots.
UnitOfWork.LibraryRepo looks awkward. The UoW should be injected or directly instantiated in the method, not made available as a singleton. Same is true for the Repository. Things will be more testable and loosely coupled if you pass an ILibraryRepo around to the method instead. Unit of Work and Repository are two separate concepts, they somehow converge in Entity Framework but from the outside, they shouldn't look as tightly tied togteher as that.
I am implementing a form of CQRS that uses a single data store but separate Query and Command models. For the command side of things I am implementing DDD including Repositories, IoC and Dependency Injection. For the Query side I am using the Finder pattern as described here. Basically, a finder is similar to a Repository, but with Find methods only.
So in my application for the read side, in my DAL, I use ADO.net and raw SQL to do my queries. The ADO.Net stuff is all abstracted away into a nice helper class so that my Finder classes simply pass the query to the ADO helper which returns generic data objects which the finder/mapper class turns into read models.
Currently the Finder methods, like my command repositories, are accessed through interfaces that are injected into my controllers, but I am wondering if the interfaces, DI and IoC are overkill for the query side, as everything I have read about the read side of CQRS recommends a "thin data layer".
Why not just access my Finders directly? I understand the arguments for interfaces and DI. ie Separation of Concerns and testability. In the case of SOC, my DAL has already separated out database specific logic by using a mapper class and putting the ADO.net stuff in a helper class. As far as testing is concerned, according to this question unit testing read models is not a necessity.
So in summary, for read models, can I just do this:
public class PersonController : Controller
{
public ActionResult Details(int id)
{
var person = new Person();
person = PersonFinder.GetByID(id);
// TODO: Map person to viewmodel
return this.View(viewmodel);
}
}
Instead of this:
public class PersonController : Controller
{
private IPersonFinder _person;
public PersonController(IPersonFinder person)
{
_person = person;
}
public ActionResult Details(int id)
{
Person person = _person.GetByID(id);
// TODO: Map person to viewmodel
return this.View(viewmodel);
}
}
Are you using both IoC and DI? That's bad ass! Anyways, the second version is the better one because it doesn't depend on a static class. Using statics will open Pandora's box, don't do it, for all the reasons that using static is bad.
You really don't get any benefits for using a static class and once you are already using a DI Container, there's no additional cost. And you are using the Finders directly but you let the DI Container instantiate one instead of you calling a static object.
Update
A thin read layer refers to using a simplified read model instead of the rich domain objects. It is unrelated to DI, it doesn't matter how the query service is built or by whom, it matters to not involve the business objects in queries.
Read/Write separation is completely unrelated to coding techniques like dependency injection. Your read models are serving fewer purposes than your combined read/write models were before. Could you consider ditching all the server-side code and just using your database's native REST API? Could you wire your controller to directly query the database with SQL and return the data as JSON? Do you need a generic repository-like pattern to deal with specific read requests?
I am learning the well-known Onion Architecture from Jeffrey Palermo.
Not specific to this pattern, but I cannot see clearly the separation between repositories and domain services.
I (mis)understand that repository concerns data access and service are more about business layer (reference one or more repositories).
In many examples, a repository seems to have some kind of business logic behind like GetAllProductsByCategoryId or GetAllXXXBySomeCriteriaYYY.
For lists, it seems that service is just a wrapper on repository without any logic.
For hierarchies (parent/children/children), it is almost the same problem : is it the role of repository to load the complete hierarchy ?
The repository is not a gateway to access Database. It is an abstraction that allow you to store and load domain objects from some form of persistence store. (Database, Cache or even plain Collection). It take or return the domain objects instead of its internal field, hence it is an object oriented interface.
It is not recommended to add some methods like GetAllProductsByCategoryId or GetProductByName to the repository, because you will add more and more methods the repository as your use case/ object field count increase. Instead it is better to have a query method on the repository which takes a Specification. You can pass different implementations of the Specification to retrieve the products.
Overall, the goal of repository pattern is to create a storage abstraction that does not require changes when the use cases changes. This article talks about the Repository pattern in domain modelling in great detail. You may be interested.
For the second question: If I see a ProductRepository in the code, I'd expect that it returns me a list of Product. I also expect that each of the Product instance is complete. For example, if Product has a reference to ProductDetail object, I'd expect that Product.getDetail() returns me a ProductDetail instance rather than null. Maybe the implementation of the repository load ProductDetail together with Product, maybe the getDetail() method invoke ProductDetailRepository on the fly. I don't really care as a user of the repository. It is also possible that the Product only returns a ProductDetail id when I call getDetail(). It is perfect fine from the repository's contract point of view. However it complicates my client code and forces me to call ProductDetailRepository myself.
By the way, I've seen many service classes that solely wrap the repository classes in my past. I think it is an anti-pattern. It is better to have the callers of the services to use the repositories directly.
Repository pattern mediates between the domain and data mapping layers using a collection-like interface for accessing domain objects.
So, repositories is to provide interface for CRUD operation on domain entities. Remember that Repositories deals with whole Aggregate.
Aggregates are groups of things that belong together. An Aggregate Root is the thing that holds them all together.
Example Order and OrderLines:
OrderLines have no reason to exist without their parent Order, nor can they belong to any other Order. In this case, Order and OrderLines would probably be an Aggregate, and the Order would be the Aggregate Root
Business logic should be in Domain Entities, not in Repository layer , application logic should be in service layer like your mention, services in here play a role as coordinator between repositoies.
While I'm still struggling with this, I want to post as an answer but also I accept (and want) feedback about this.
In the example GetProductsByCategory(int id)
First, let's think from the initial need. We hit a controller, probably the CategoryController so you have something like:
public CategoryController(ICategoryService service) {
// here we inject our service and keep a private variable.
}
public IHttpActionResult Category(int id) {
CategoryViewModel model = something.GetCategoryViewModel(id);
return View()
}
so far, so good. We need to declare 'something' that creates the view model.
Let's simplify and say:
public IHttpActionResult Category(int id) {
var dependencies = service.GetDependenciesForCategory(id);
CategoryViewModel model = new CategoryViewModel(dependencies);
return View()
}
ok, what are dependencies ? We maybe need the category tree, the products, the page, how many total products, etc.
so if we implemented this in a repository way, this could look like more or less like this :
public IHttpActionResult Category(int id) {
var products = repository.GetCategoryProducts(id);
var category = repository.GetCategory(id); // full details of the category
var childs = repository.GetCategoriesSummary(category.childs);
CategoryViewModel model = new CategoryViewModel(products, category, childs); // awouch!
return View()
}
instead, back to services :
public IHttpActionResult Category(int id) {
var category = service.GetCategory(id);
if (category == null) return NotFound(); //
var model = new CategoryViewModel(category);
return View(model);
}
much better, but what is exactly inside service.GetCategory(id) ?
public CategoryService(ICategoryRespository categoryRepository, IProductRepository productRepository) {
// same dependency injection here
public Category GetCategory(int id) {
var category = categoryRepository.Get(id);
var childs = categoryRepository.Get(category.childs) // int[] of ids
var products = productRepository.GetByCategory(id) // this doesn't look that good...
return category;
}
}
Let's try another approach, the unit of work, I will use Entity framework as the UoW and Repositories, so no need to create those.
public CategoryService(DbContext db) {
// same dependency injection here
public Category GetCategory(int id) {
var category = db.Category.Include(c=> c.Childs).Include(c=> c.Products).Find(id);
return category;
}
}
So here we are using the 'query' syntax instead of the method syntax, but instead of implementing our own complex, we can use our ORM. Also, we have access to ALL repositories, so we can still do our Unit of work inside our service.
Now we need to select which data we want, I probably don't want all the fields of my entities.
The best place I can see this is happening is actually on the ViewModel, each ViewModel may need to map it's own data, so let's change the implementation of the service again.
public CategoryService(DbContext db) {
// same dependency injection here
public Category GetCategory(int id) {
var category = db.Category.Find(id);
return category;
}
}
so where are all the products and inner categories?
let's take a look at the ViewModel, remember this will ONLY map data to values, if you are doing something else here, you are probably giving too much responsibility to your ViewModel.
public CategoryViewModel(Category category) {
Name = category.Name;
Id = category.Id;
Products = category.Products.Select(p=> new CategoryProductViewModel(p));
Childs = category.Childs.Select(c => c.Name); // only childs names.
}
you can imagine the CategoryProductViewModel by yourself right now.
BUT (why is there always a but??)
We are doing 3 db hits, and we are fetching all the category fields because of the Find. Also Lazy Loading must be enable. Not a real solution isn't it ?
To improve this, we can change find with where... but this will delegate the Single or Find to the ViewModel, also it will return an IQueryable<Category>, where we know it should be exactly one.
Remember I said "I'm still struggling?" this is mostly why. To fix this, we should return the exact needed data from the service (also know as the ..... you know it .... yes! the ViewModel).
so let's back to our controller :
public IHttpActionResult Category(int id) {
var model = service.GetProductCategoryViewModel(id);
if (category == null) return NotFound(); //
return View(model);
}
inside the GetProductCategoryViewModel method, we can call private methods that return the different pieces and assemble them as the ViewModel.
this is bad, now my services know about viewmodels... let's fix that.
We create an interface, this interface is the actual contract of what this method will return.
ICategoryWithProductsAndChildsIds // quite verbose, i know.
nice, now we only need to declare our ViewModel as
public class CategoryViewModel : ICategoryWithProductsAndChildsIds
and implement it the way we want.
The interface looks like it has too many things, of course it can be splitted with ICategoryBasic, IProducts, IChilds, or whatever you may want to name those.
So when we implement another viewModel, we can choose to do only IProducts.
We can have our services having methods (private or not) to retrieve those contracts, and glue the pieces in the service layer. (Easy to say than done)
When I get into a fully working code, I might create a blog post or a github repo, but for now, I don't have it yet, so this is all for now.
I believe the Repository should be only for CRUD operations.
public interface IRepository<T>
{
Add(T)
Remove(T)
Get(id)
...
}
So IRepository would have: Add, Remove, Update, Get, GetAll and possibly a version of each of those that takes a list, i.e, AddMany, RemoveMany, etc.
For performing search retrieval operations you should have a second interface such as an IFinder. You can either go with a specification, so IFinder could have a Find(criteria) method that takes criterias. Or you can go with things like IPersonFinder which would define custom functions such as: a FindPersonByName, FindPersonByAge etc.
public interface IMyObjectFinder
{
FindByName(name)
FindByEmail(email)
FindAllSmallerThen(amount)
FindAllThatArePartOf(group)
...
}
The alternative would be:
public interface IFinder<T>
{
Find(criterias)
}
This second approach is more complex. You need to define a strategy for the criterias. Are you going to use a query language of some sort, or a more simple key-value association, etc. The full power of the interface is also harder to understand from simply looking at it. It's also easier to leak implementations with this method, because the criterias could be based around a particular type of persistence system, like if you take a SQL query as criteria for example. On the other hand, it might prevent you from having to continuously come back to the IFinder because you've hit a special use case that requires a more specific query. I say it might, because your criteria strategy will not necessarily cover 100% of the querying use cases you might need.
You could also decide to mix both together, and have an IFinder defining a Find method, and IMyObjectFinders that implement IFinder, but also add custom methods such as FindByName.
The service acts as a supervisor. Say you need to retrieve an item but must also process the item before it is returned to the client, and that processing might require information found in other items. So the service would retrieve all appropriate items using the Repositories and the Finders, it would then send the item to be processed to objects that encapsulates the necessary processing logic, and finally it would return the item requested by the client. Sometime, no processing and no extra retrievals will be required, in such cases, you don't need to have a service. You can have clients directly call into the Repositories and the Finders. This is one difference with the Onion and a Layered architecture, in the Onion, everything that is more outside can access everything more inside, not only the layer before it.
It would be the role of the repository to load the full hierarchy of what is needed to properly construct the item that it returns. So if your repository returns an item that has a List of another type of item, it should already resolve that. Personally though, I like to design my objects so that they don't contain references to other items, because it makes the repository more complex. I prefer to have my objects keep the Id of other items, so that if the client really needs that other item, he can query it again with the proper Repository given the Id. This flattens out all items returned by the Repositories, yet still let's you create hierarchies if you need to.
You could, if you really felt the need to, add a restraining mechanism to your Repository, so that you can specify exactly which field of the item you need. Say you have a Person, and only care for his name, you could do Get(id, name) and the Repository would not bother with getting every field of the Person, only it's name field. Doing this though, adds considerable complexity to the repository. And doing this with hierarchical objects is even more complex, especially if you want to restrict fields inside fields of fields. So I don't really recommend it. The only good reason for this, to me, would be cases where performance is critical, and nothing else can be done to improve the performance.
In Domain Driven Design the repository is responsible for retrieving the whole Aggregate.
Onion and Hexagonal Architectures purpose is to invert the dependency from domain->data access.
Rather than having a UI->api->domain->data-access,
you'll have something like UI->api->domain**<-**data-access
To make your most important asset, the domain logic, is in the center and free of external dependencies.
Generally by splitting the Repository into Interface/Implementation and putting the interface along with the business logic.
Now to services, there's more that one type of services:
Application Services: your controller and view model, which are external concerns for UI and display and are not part of the domain
Domain Services: which provide domain logic. In you're case if the logic you're having in application services starts to do more that it's presentation duties. you should look at extracting to a domain service
Infrastructure Services: which would, as with repositories, have an interface within the domain, and an implementation in the outer layers
#Bart Calixto, you may have a look at CQRS, building your view model is too complex when you're trying to use Repositories which you design for domain logic.
you could just rewrite another repo for the ViewModel, using SQL joins for example, and it doesn't have to be in the domain
is it the role of repository to load the complete hierarchy ?
Short answer: yes, if the repository's outcome is a hierarchy
The role of repository is to load whatever you want, in any shape you need, from the datasource (e.g. database, file system, Lucene index, etc).
Let's suppose a repository (interface) has the GetSomeHierarchyOrListBySomeCriteria operation - the operation, its parameters and its outcome are part of the application core!
Let's focus on the outcome: it doesn't matter it's shape (list, hierarchy, etc), the repository implementation is supposed to do everything necessary to return it.
If one is using a NoSql database than GetSomeHierarchyOrListBySomeCriteria implementation might need only one NoSql-database-query with no other conversions or transformations to get the desired outcome (e.g. hierarchy). For a SQL database on the other hand, the same outcome might imply multiple queries and complex conversions or transformations - but that's an implementation detail, the repository interface is the same.
repositories vs domain services
According to The Onion Architecture : part 1, and I'm pointing here about the official page, not someone's else interpretation:
The first layer around the Domain Model is typically where we would
find interfaces that provide object saving and retrieving behavior,
called repository interfaces. [...] Only the interface is in the application core.
Notice the Domain Services layer above Domain Model one.
Starting with the second official page, The Onion Architecture : part 2, the author forgets about Domain Services layer and is depicting IConferenceRepository as part of the Object Services layer which is right above Domain Model, replacing Domain Services layer! The Object Services layer continues in The Onion Architecture : part 3, so I ask: what Domain Services? :)))
It seems to me that author's intent for Object Services or Domain Services is to consist only of repositories, otherwise he leaves no clue for something else.
I'm developing an application using asp.net mvc, NHibernate and DDD. I have a service layer that are used by controllers of my application. Everything are using Unity to inject dependencies (ISessionFactory in repositories, repositories in services and services in controllers) and works fine.
But, it's very common I need a method in service to get only object in my repository, like this (in service class):
public class ProductService {
private readonly IUnitOfWork _uow;
private readonly IProductRepository _productRepository;
public ProductService(IUnitOfWork unitOfWork, IProductRepository productRepository) {
this._uow = unitOfWork;
this._productRepository = productRepository;
}
/* this method should be exists in DDD ??? It's very common */
public Domain.Product Get(long key) {
return _productRepository.Get(key);
}
/* other common method... is correct by DDD ? */
public bool Delete(long key) {
usign (var tx = _uow.BeginTransaction()) {
try
{
_productRepository.Delete(key);
tx.Commit();
return true;
} catch {
tx.RollBack();
return false;
}
}
}
/* ... others methods ... */
}
This code is correct by DDD ? For each Service class I have a Repository, and for each service class need I do a method "Get" for an entity ?
Thanks guys
Cheers
Your ProductService doesn't look like it followed Domain-Driven Design principles. If I understand it correctly, it is a part of Application layer between Presentation and Domain. If so, the methods on ProductService should have business meaning with regard to products.
Let's talk about deleting products. Is it as simple as executing delete on the database (NHibernate, or whatever?) I think it is not. What about orders which reference the to-be-deleted product? And so on and so forth. Btw, Udi Dahan wrote a great article on deleting entities.
Bottom line is, if your application is so simple that services do really replicate your repositories and contain only CRUD operations, you probably shouldn't do DDD, throw away your repositories and let services operate on entities (which would be simple data containers in that case).
On the other hand, if there is a complicated behavior (like the one with handling 'deleted' products), there is a point in going DDD path and I strongly advocate doing so.
PS. Despite which approach (DDD or not) you will eventually take I would encourage you to use some Aspect Oriented Programming to handle transaction and exception related stuff. You would end up with way to many methods such as DeleteProduct with same TX and exception handling code.
That looks correct from my perspective. I really didn't like repeating service and repository method names over and over in my asp.net MVC project, so I went for a generic repository approach/pattern. This means that I really only need one or two Get() methods in my repository to retrieve my objects. This is possible for me because I am using Entity Framework and I just have my repository's get() method return a IQueryable. Then I can just do the following:
Product product = from p in _productRepository.Get() where p.Id == Id select p;
You can probably replicate this in NHibernate with linq -> NHibernate.
Edit: This works for DDD because this still allows me to interchange my DAL/repositories as long as the data library I am using (Nhibernate, EF, etc..) supports IQueryable.
I am not sure how to do a generic repository without IQueryable, but you might be able to use delegates/lambda functions to incorporate it.
Edit2: And just in case I didn't answer your question correctly, if you are asking if you are supposed to call your repository's Get() method from the service then yes, that is the correct DDD design as well. The reason is that the service layer is supposed to handle all your business logic, so it decides exactly how and what data to retrieve (for example, do you want it in alphabetical order, unordered, etc...). It also means that it can perform validation after loading if needed or validation before deleting and/or saving.
This means that the service layer doesn't care exactly how that data is stored and retrieved, it only decides what data is stored and retrieved. It then calls on the repository to handle the request correctly and retrieve/store the data in the way the service layer tells it to. Thus you have correct separation of concerns.
I need to make a web application and I want to use MVC. However, my Model can't be one of the standard Models -- the data is not stored in a database but instead in an external application accessible only via a API. Since this is the first MVC application I've implemented I'm relying on examples to understand how to go about it. I can't find any examples of a non-DB based Model. An example of a custom Model would be fine too. Can anyone point me to such a beast? Maybe MVC is just to new and none exist.
It seems like I might be able to get away with the DataSet Model, however I've not seen any examples of how to use this object. I expect an example of DataSet could help me also. (Maybe it is the same thing?)
Please note: I've seen countless examples of custom bindings. This is NOT what I want. I need an example of a custom Model which is not tied to a specific database/table.
UPDATE
I found a good example from MS located here:
http://msdn.microsoft.com/en-us/library/dd405231.aspx
While this is the "answer" to my question, I don't really like it because it ties me to MS's view of the world. #Aaronaught, #jeroenh, and #tvanfosson give much better answers from a meta perspective of moving my understanding (and yours?) forward with respect to using MVC.
I'm giving the check to #Aaronaught because he actually has example code (which I asked for.) Thanks all and feel free to add even better answers if you have one.
In most cases it shouldn't matter what the backing source is for the actual application data; the model should be exactly the same. In fact, one of the main reasons for using something like a repository is so that you can easily change the underlying storage.
For example, I have an MVC app that uses a lot of web services - rarely does it have access to a local database, except for simple things like authentication and user profiles. A typical model class might look like this:
[DataContract(Namespace = "http://services.acme.com")]
public class Customer
{
[DataMember(Name = "CustomerID")]
public Guid ID { get; set; }
[DataMember(Name = "CustomerName")]
public string Name { get; set; }
}
Then I will have a repository interface that looks like this:
public interface ICustomerRepository
{
Customer GetCustomerByID(Guid id);
IList<Customer> List();
}
The "API" is all encapsulated within the concrete repository:
public class AcmeWSCustomerRepository : ICustomerRepository, IDisposable
{
private Acme.Services.CrmServiceSoapClient client;
public AcmeWSCustomerRepository()
: this(new Acme.Services.CrmServiceSoapClient())
public AcmeWSCustomerRepository(Acme.Services.CrmServiceSoapClient client)
{
if (client == null)
throw new ArgumentNullException("client");
this.client = client;
}
public void Dispose()
{
client.SafeClose(); // Extension method to close WCF proxies
}
public Customer GetCustomerByID(Guid id)
{
return client.GetCustomerByID(id);
}
public IList<Customer> List()
{
return client.GetAllCustomers();
}
}
Then I'll also probably have a local testing repository with just a few customers that reads from something like an XML file:
public class LocalCustomerRepository : ICustomerRepository, IDisposable
{
private XDocument doc;
public LocalCustomerRepository(string fileName)
{
doc = XDocument.Load(fileName);
}
public void Dispose()
{
}
public Customer GetCustomerByID(Guid id)
{
return
(from c in doc.Descendants("Customer")
select new Customer(c.Element("ID").Value, c.Element("Name").Value))
.FirstOrDefault();
}
// etc.
}
The point I'm trying to make here is, well, this isn't tied to any particular database. One possible source in this case is a WCF service; another is a file on disk. Neither one necessarily has a compatible "model". In this case I've assumed that the WCF service exposes a model that I can map to directly with DataContract attributes, but the Linq-to-XML version is pure API; there is no model, it's all custom mapping.
A really good domain model should actually be completely independent of the true data source. I'm always a bit skeptical when people tell me that a Linq to SQL or Entity Framework model is good enough to use throughout the entire application/site. Very often these simply don't match the "human" model and simply creating a bunch of ViewModel classes isn't necessarily the answer.
In a sense, it's actually better if you're not handed an existing relational model. It forces you to really think about the best domain model for your application, and not necessarily the easiest one to map to some database. So if you don't already have a model from a database - build one! Just use POCO classes and decorate with attributes if necessary, then create repositories or services that map this domain model to/from the API.
I think what you are looking for is really a non-DB service layer. Models, typically, are relatively simple containers for data, though they may also contain business logic. It really sounds like what you have is a service to communicate with and need a layer to mediate between the service and your application, producing the appropriate model classes from the data returned by the service.
This tutorial may be helpful, but you'd need to replace the repository with your class that interacts with the service (instead of the DB).
There is no fixed prescription of what a "Model" in MVC should be, just that it should contain the data that needs to be shown on screen, and probably also manipulated.
In a well-designed MVC application, data access is abstracted away somehow anyway, typically using some form of the Repository pattern: you define an abstraction layer (say, an IRepository interface) that defines the contract needed to get and persist data. The actual implementation will usually call a database, but in your case should call your 'service API'.
Here is an example of an MVC application that calls out to a WCF service.