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Enabling Lazy Loading in the Child Class
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When using lazy loading, the code in a child class is different from normal. In fact, the child object will implement the same kind of factory and DataPortal_Fetch() as a root object. And you ll need to call MarkAsChild() manually because the data portal won t know to call it automatically on your behalf. The child factory method will look like this: internal ChildType GetChild(ParentType parent) { return DataPortal.Fetch<ChildType>( new SingleCriteria<ChildType, int>(parent.Id)); } The data portal is invoked to retrieve the child object, using properties from the parent object as criteria. In many cases a child object is loaded based on the parent object s unique ID value, which can be used as a foreign key, but the actual criteria values you use will depend on your specific object and data models.
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Notice that a reference to the parent object is provided as a parameter rather than specific parent properties. This provides good decoupling between the parent and child because this way the parent has no idea what data is required by the child s factory method.
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As you ll see in 15, the data portal ultimately creates an instance of the object and invokes the DataPortal_Fetch() method. Normally, a child object wouldn t implement this method at all, but when using lazy loading you do implement this method. The method is responsible for loading the child object with data from the database: private void DataPortal_Fetch(SingleCriteria<ChildType, int> criteria) { MarkAsChild(); // load child with data from database }
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CHAPTER 9 PARE NT-C HILD RELA TION SHIPS
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The important thing to remember in this case is that you must call MarkAsChild() to indicate that this is a child object. You must do this manually because the data portal is being used to load the object here as though it were a root object. The data portal doesn t know to mark the object as a child automatically, so you must do it explicitly. The end result is that the child object is created or retrieved on demand, using lazy loading. At this point you should understand how an editable object can act as a parent or a child and how a parent object manages the references to its child objects. In the next section, I discuss how this works when the parent object is an editable collection.
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Parent Editable Collection
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Editable collections are created by inheriting from BusinessListBase, as discussed in s 4 and 5. By definition, a collection is a parent because it contains the items in the collection. To a large degree, the interactions between parent and child objects discussed already in this chapter also apply when the parent is a collection. For example, the BusinessListBase class implements the IParent interface. And BusinessListBase calls SetParent() as each child object is added to the collection or when the collection is deserialized. The same benefits and features I ve already discussed apply to collections as well as editable objects. However, editable collections provide some different behaviors as well, most notably around how items are deleted from the collection and how child events are cascaded up as ListChanged events from the collection (especially after deserialization of the collection).
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Parent-Child Interaction
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The BusinessListBase class provides the same set of behaviors in Table 9-1, earlier in the chapter. The primary difference in the implementation is that a collection has no intrinsic state of its own. Instead, its state comes from its child objects. In other words, the collection s IsValid and IsDirty properties simply reflect the underlying state of the child objects. For example, a collection is valid only if all the child objects it contains are valid. The one big difference between an editable object and an editable collection is in terms of how ChildChanged events are handled.
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