Front Matter Pages i-xiii. Introducing the ADO. NET 4. Pages The Entity Data Model. Querying the EDM. Working with Entities. Stored Procedures and the EDM. Relationships and Associations.

T4 Code Generation. It will also provide more flexibility in a variety of other coding scenarios. See the sidebar, Resources , for more information. Many ORMs provide a feature called lazy loading, which will automatically load related data as needed. Entity Framework v1 does have a Load method for reference data which must be called explicitly.

For example, if you were to query BikeRoutes without specifying that the query should also return the related BikeRides; there would be no knowledge of BideRides in memory. Each BikeRoute would appear to have zero related BikeRides. For a given route, you could call thisBikeRoute. Load to force the related rides to be retrieved from the database. This is referred to as explicit deferred loading and allows the developer to have total control over when calls to the database are made.

To developers who are used to lazy loading, having to explicitly call Load to get at the related data is a big drawback of EFv1. EF4 adds the capability to implicitly load data. The new ContextOptions. DeferredLoadingEnabled property of ObjectContext provides this feature.

This property, which is false by default, allows you to instruct an instantiated context to lazy load related data on demand. In an e-commerce application, if you queried a Customer without eager loading all of the Orders for that Customer, setting DeferredLoadingEnabled to true will ensure that the Orders and any other data gets retrieved from the database as needed. The following query only returns a Customer entity but the foreach enumeration requires the orders along with even more related data, Salesperson and Person.

Because DeferredLoadingEnabled is set to true, Entity Framework will execute additional queries to satisfy the requested related data. In the case of this example above, as the enumeration steps through each Customer, a query is executed to load the related Orders. Then for each order, an additional query is executed to retrieve the SalesPerson and Person data for that Order. Be aware that this means one additional trip to the database for each Customer. If there are ten customers, then twenty additional calls to the database will be executed.

If there are customers, then you are causing additional database trips to be made. There are differing opinions on when to use lazy loading. I prefer to consider each scenario and determine what will provide the best performance in my application. Then I can choose between eager loading with the Include method, explicit deferred loading where I have to call the Load method or lazy loading with DeferredLoadingEnabled set to true.

The new lazy loading feature is tied to the instantiated context, which you are currently using. Some other ORMs allow you to define the lazy loading based on the related properties; for example, the Orders property of Customer might be defined to always lazy load. By default, classes that derive from a model inherit from EntityObject, which gives them the functionality they need to leverage identity management EntityKeys , change-tracking and relationship management.

For developers who prefer to create simple classes that are not tightly bound to a framework, this is a hindrance to using Entity Framework. EFv1 provides the ability to implement interfaces that enable classes to continue to benefit from the ObjectContext.

But even those interfaces bind classes too tightly to the Entity Framework. An important addition to EF4 allows developers to create classes that can be simple classes yet still take part in the Entity Framework. How is it possible? The assumptions are that each class and its properties map directly to an entity and its properties. There is no physical map, only the fact that the entity names and the property names match.

For example, here is a simple Recipe class. It neither inherits from any classes nor implements any interfaces. If your model contains a Recipe entity, as in Figure 3 , that matched not only the class name, but also whose properties match the Recipe properties, EF4 will be able to figure out how to align the model entity to the class at runtime. In this case, the model still needs to be accompanied by the other metadata Store schema and mappings that allow EF to figure out how to get back to the database.

While the naming is critical, there is one other very important ingredient to enabling this feature. You must create a separate class that inherits from ObjectContext to wrap these classes so that they can be used in queries and will be managed by the context. Listing 2 shows the context class which I have placed in the same project as the model. Now you can have your cake and eat it too. The classes themselves know nothing about the Entity Framework, yet they can participate fully as entities and benefit from the functionality of the Entity Framework.

One of the bedeviling problems with EFv1 is controlling the state of objects. These are methods of ObjectStateEntry and cannot be used directly against the objects themselves. ObjectStateEntries are difficult to visualize in the debugger. About a year ago I created my own extension method to help with this.

You can use ObjectStateEntry. You should be well aware of the impact that the various choices will have on your entities. The purpose of these methods is to overcome problems with reconnecting entities that have come from other processes Web applications and services, for example , currently quite tricky to do in EFv1.

Figure 4 shows my visualizer to observe the state of a Person entity when it is first attached to the context. This could be an entity that was returned from a client app to a Web service where the client app assigned a temporary ID to a new object.

Now look at the new state in Figure 5. The EntityState is Added and all of the original values are gone. What is not displayed is the fact that the method also created a new temporary EntityKey as any other Added object would have. If you called SaveChanges at this point this new person will get added into the database with an Insert command. You may wonder how that could be considering that the PersonID is still intact. Because the state is Added and the EntityKey does not have the original tracking information, the PersonID is ignored when the Insert command is created and it is replaced with the new value that is generated by the database.

If you change an Unchanged entity to Modified, notice the effect on the entity as shown in Figure 6. Not only is the state now Modified but all of the properties are seen as modified as well.

Saving changes will cause an Update command to be created with every value passed in for updating. Changing its state from Modified to Unchanged is another interesting scenario. Figure 7 shows the entity after the FirstName was edited. Figure 8 shows the impact of changing the state to Unchanged. All of the current values are pushed into the Original values and the state becomes Unchanged.

Be careful not to think that the entity has just been rolled back to its original state because that is the opposite of what happens here. While this entity will be ignored during SaveChanges, it has still retained the latest version of the properties. ChangeState will play a big role in fixing up EntityState when you are working across processes, e. Do not ignore the impact on graphs. It will still take some more exploration to have an understanding of how to work with graphs using the ChangeState method.

SaveChanges also reads the state of relationships in order to properly set foreign keys in the database. Relationships are tracked by the ObjectContext as well as entities. If a relationship changes, EF notes that one relationship was deleted and another was added. Oh yeah, and it doesn't support enums. Enums for goodness sakes.

I am not sure what you mean by no support for POCO. EF objects are serializable where as L2S objects are not serializable, important difference if you intend to save the objects out of proc for web applications. Its very very helpful and easy to understand.

I hope its not too much to ask, uhm, can you also add a tutorial on how to use this EDM with Web services or WCF or there's no need to go this far. Am planning to use this new tech with my new intranet web app around 18k concurrent users with around 8 Million data.

Thanks Much and looking forward on your reply. Many Thanks I really want a line between L2S and EF and yes you drawn it. I couldnt find an article which made me interesting about EF.

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