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6-2. Exposing a Link Table as an Entity
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You want to expose a link table as an entity instead of a many-to-many association.
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Let s say your database has a many-to-many relationship between workers and tasks and looks something like the one in the database diagram in Figure 6-3.
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Figure 6-3. A many-to-many relationship between workers and tasks The WorkerTask link table contains nothing more than the foreign keys supporting the many-to-many relationship. When we import these tables into our model, the designer will create two entities with a many-to-many association as shown in Figure 6-4.
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Figure 6-4. A many-to-many association between the Worker and Task entities To convert the association to an entity representing the WorkerTask link table, follow these steps. 1. Delete the many-to-many association created by the designer. To delete the association, right-click the link and select Delete. When prompted to delete the WorkerTask table from the underlying store model, click No (see Figure 6-5). Right-click the design surface and select Add Entity. Name the new entity WorkerTask and uncheck the Create key property box. Right-click the WorkerTask entity and select Add Scalar property. Rename the property WorkerId. Repeat this step, adding TaskId scalar property.
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2. 3.
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Right-click each scalar property and select Properties. Change the type for each property from String to Int32. Mark both properties as entity key properties by right-clicking the property and selecting Entity Key. Select the WorkerTask entity. In the Mapping Details window select WorkerTask in the Add Table or View drop-down control. This maps the entity to the WorkerTask table. Map the WorkerId and TaskId properties to the WorkerId column and TaskId columns, respectively. See Figure 6-6. Right-click the design surface and select Add Association to add a one-tomany association between the Task entity and the WorkerTask entity. Make sure that WorkerTask is on the many side of the association. Be sure to uncheck the Add foreign key properties check box because we ve already created the foreign key properties. Repeat this step to create a one-to-many association between the Worker entity and the WorkerTask entity. Now we need to create a referential constraint between the Task entity and the WorkerTask entity. This will complete the foreign key association between these entities. Right-click the association link and select Properties. In the properties for the association, click the Referential Constraint box. In the dialog box, choose Task as the Principal and WorkerTask as the Dependent. Choose TaskId as the Principal Key and TaskId as the Dependent Key. See Figure 6-7. Repeat this step to build the referential constraint for the association between the Worker entity and the WorkerTask entity.
6. 7.
The final model should look like the one in Figure 6-8.
Figure 6-5. Answer No, don t delete the underlying WorkerTask table from the store model
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Figure 6-6. Mapping the WorkerTask table to the WorkerTask entity in the Mapping Details window. Make sure that the WorkerId column is mapped to the WorkerId property and the TaskId property is mapped to the TaskId column.
Figure 6-7. Building the referential constraint between the Task entity and the WorkerTask entity. TaskId is the key on both sides of the constraint.
Figure 6-8. The completed model with the WorkerTask link table exposed as an entity in two one-to-many associations
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How It Works
When a payload-free link table in a many-to-many relationship is imported into a model, the designer will create entities to represent the related tables and will represent the link table as a many-to-many association. During the application development lifecycle, developers often find the need to add payload to the many-to-many associations that started life payload-free. In this recipe, we show how to surface the many-to-many association as a separate entity so that additional scalar properties (i.e., payload) can be added. Of course, if your many-to-many relationship started life with a payload, the designer will create a model similar to the one shown in Figure 6-8 when it is imported. Many developers choose to assume that all many-to-many relationships will ultimately hold a payload and create a synthetic key for the link table rather than the traditional composite key formed by combining the foreign keys. This new key becomes a payload and now, when imported, the designer will start off with a model like the one shown in Figure 6-8. The downside to our new model is that we do not have a simple way to navigate the many-to-many association. We have two one-to-many associations that require an additional hop through the linking entity. The code in Listing 6-2 demonstrates this additional bit of work on both the insert side and the query side. Listing 6-2. Inserting into and retrieving Task and Worker entities using (var context = new EFRecipesEntities()) { var worker = new Worker { Name = "Jim" }; var task = new Task { Title = "Fold Envelopes" }; var workertask = new WorkerTask { Task = task, Worker = worker }; context.WorkerTasks.AddObject(workertask); task = new Task { Title = "Mail Letters" }; workertask = new WorkerTask { Task = task, Worker = worker }; context.WorkerTasks.AddObject(workertask); worker = new Worker { Name = "Sara" }; task = new Task { Title = "Buy Envelopes" }; workertask = new WorkerTask { Task = task, Worker = worker }; context.WorkerTasks.AddObject(workertask); context.SaveChanges(); } using (var context = new EFRecipesEntities()) { context.ContextOptions.LazyLoadingEnabled = true; Console.WriteLine("Workers and Their Tasks"); Console.WriteLine("======================="); foreach (var worker in context.Workers) { Console.WriteLine("\n{0}'s tasks:", worker.Name); foreach (var wt in worker.WorkerTasks) { Console.WriteLine("\t{0}", wt.Task.Title); } } }
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