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Last modified on August 15, 2012 17:22
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We've seen how DevForce supports discovery by context, something particularly useful in testing. DevForce also contains built-in support for faking using this facility.
There are a few definitions of faking, but let's use one from Martin Fowler in his post "Mocks Aren't Stubs": "Fake objects actually have working implementations, but usually take some shortcut which makes them not suitable for production (an in memory database is a good example)."
Faking with CompositionContext.Fake
CompositionContext.Fake is a composition context in which "fake" implementations of several important DevForce interfaces are provided to simulate end-to-end query and save operations against a real backing store. Faking is also useful in facilitating model-first development, and providing data and functionality for demo applications.
CompositionContext.Fake components include:
- Id generation with the FakeIdGenerator to allow both temporary ids for new entities and translation to "real" ids when a save is performed.
- An in-memory data store with the EntityServerFakeBackingStore.
- Query functionality with the EdmQueryExecutorFake.
- Save functionality with the EdmSaveExecutorFake.
These are not stubs. They are rich implementation that provide functionality almost identical to that of their "real" counterparts. Faked queries and saves return virtually the same results you would expect from a real data store, but instead use a fake data store, called the EntityServerFakeBackingStore. Once you populate this store, you can issue standard queries and saves from an EntityManager created for the context without any additional code.
Note that other DevForce extensions, such as IEntityLoginManager, are not faked, so either DevForce defaults or your custom implementations will be used for those.
You can create multiple named fake contexts, and custom contexts based on the standard CompositionContext.Fake. Each unique context uses its own backing store. With a custom fake context you can supply additional fake functionality of your own.
Simple Faking
At the simplest, all you need to do is create an EntityManager for the default fake context and issue queries and saves against it. Of course, unless you've pre-populated the fake backing store queries will return no data until you've first saved some entities.
Here we create a new EntityManager and populate the backing store with a few test entities (remember to add a using statement to IdeaBlade.Core.Composition):
| C# | var em = new DomainModelEntityManager(compositionContextName: CompositionContext.Fake.Name); PopulateBackingStore(em); |
| VB | Dim em As New DomainModelEntityManager(compositionContextName:= CompositionContext.Fake.Name) PopulateBackingStore(em) |
| C# | private void PopulateFakeBackingStore(EntityManager em) { var customer = new Customer { CompanyName = "Test Company" }; em.AddEntity(customer); var employee = new Employee { FirstName= "Fred", LastName = "Smith"}; em.AddEntity(employee); for (int i = 0; i < 5; i++) { var salesOrder = new OrderSummary { Employee=employee, Customer = customer}; em.AddEntity(salesOrder); } em.SaveChanges(); } |
| VB | Private Sub PopulateFakeBackingStore(ByVal em As EntityManager) Dim customer = New Customer() With {.CompanyName = "Test Company"} em.AddEntity(customer) Dim employee = New Employee() With {.FirstName = "Fred", .LastName = "Smith"} em.AddEntity(employee) For i As Integer = 0 To 4 Dim salesOrder = New OrderSummary() With {.Employee = employee, .Customer = customer} em.AddEntity(salesOrder) Next em.SaveChanges() End Sub |
Once the backing store is populated with test data we can then use the EntityManager as usual. Here we clear it first, to ensure that the EntityManager is querying from the backing store.
| C# | em.Clear(); // clear cache; no memory of prior entities var query = em.Customers.Include(c => c.OrderSummaries) .Where(c => c.OrderSummaries.Any(os => os.Employee.FirstName == "Fred")); |
| VB | em.Clear() ' clear cache; no memory of prior entities Dim query As em.Customers.Include(Function(c) c.OrderSummaries).Where _ (Function(c) c.OrderSummaries.Any(Function(os) os.Employee.FirstName = "Fred")) |
Working directly with a fake backing store
You can access a fake backing store via a fake CompositionContext, for example:
| C# | var store EntityManager.CompositionContext.GetFakeBackingStore(); |
| VB | Dim store = EntityManager.CompositionContext.GetFakeBackingStore() |
Local vs. remote stores
Different backing stores are used in 2-tier vs. n-tier applications, EntityServerFakeBackingStore.Local and EntityServerFakeBackingStore.Remote. The Local store, which is not available in Silverlight applications, allows you to perform synchronous queries and saves when not using a remote EntityServer. The Remote store is used with Silverlight applications and any others using a remote EntityServer. Generally, you won't be concerned whether the store in use is local or remote unless you're working directly with it, for example to clear or populate it.
Store methods
The entities stored and retrieved by the EntityServerFakeBackingStore are actually held in an EntityCacheState. The EntityServerFakeBackingStore provides methods to allow you to work directly with both it and the underlying EntityCacheState. You can use Save or SaveAsync to save the store to a file or stream, and Restore or RestoreAsync to restore data into the store. Clear or ClearAsync can be used to clear the store (for example to clear the store between tests).
Note that only the asynchronous versions of these methods are available in Silverlight applications.
The EntityCacheState makes it easy to load test data into the backing store, possibly from a test database. The EntityCacheState can be initially loaded from an EntityManager, and allows you to save to and restore from a file or stream. Using these techniques you can write a small helper method or application, a "TestDataMother" if you will, which is responsible for creating and loading test data.
Troubleshooting
If you deploy an application using faking to IIS, you must also modify a web server setting to allow services using the fake composition context name to start successfully. Set allowDoubleEscaping to true:
| XML | <system.webServer> <security> <requestFiltering allowDoubleEscaping="true"/> </security> </system.webServer> |
Additional resources
The BookShelf sample application contains an example of a "TestDataMother".