8: Reading Obje cts with LINQ to SQL in C#

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8: Reading Obje cts with LINQ to SQL
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The deferred loading capability requires LINQ to SQL to intercept the access to a relationship property The interception provides a way to check if the target is already loaded so that it can be populated on rst access Two and , are used to provide the special types, is used where the target is a collecinterception capability is used where the target is a singleton For example, tion, and is of type , and is of type The designer and the command-line tool SqlMetal both use the correct types on each end of a relationship based on the cardinality of the relationship As a result, the generated entities are automatically equipped to provide deferred loading for relationships Furthermore, deferred loading is also available for a nonrelationship property of a class For example, if the Products table contains a large image of the product, you can choose to defer-load it by using a property of type , where is the type used for mapping to the image In the designer, you can set the Delay Loading property to true for such a instead of The class member to change the type to image would then be defer-loaded much like the Category entity referproperty in the class The difenced by the ference between the two is that the image is not an entity it is just a part of entity is a different entity with its own identity the
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Eager Loading Whereas deferred loading is handy when only an occasional relationship is navigated, in other cases all or most of the related entities are needed Consider a variant of the previously shown logic in which the s for all s are needed, regardless of the freight In such cases, deferred loading can be too chatty and inef cient due to the number of queries and resulting round-trips to the database Another option called eager loading (also called immediate loading) is to autoavailable for just such a case You can instruct the for all retrieved s, as shown in the matically bring in
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following example The on the can be used for to specify a relationship that should be eager-loaded entities in the example The log shows that, unlike in the previous loop does not result in additional queries All example, the inner s are preloaded with the Order entities the related
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The following query for loading all the related and the s is different from the queries used for multistep deferred queried clause with columns from the Orders and loading For simplicity, the Order Details tables is trimmed and elided LINQ to SQL takes the results of hierarchy the left outer join and constructs the
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8: Reading Obje cts with LINQ to SQL
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Multiple relationships may be set up for eager loading using The only restriction is that the options cannot form a cycle For is speci ed for eager loading, example, after cannot be speci ed, because it would form a cycle among This the classes through the relationships avoids the problem of unending traversal of cycles One consequence is that certain relationships cannot be eager-loaded For example, consider an Employee entity pointing to a collection of Employee entities in a manageris reports relationship Because the target entity type in , the relationship cannot be eager-loaded However, deferred loading may be used in such cases because the relationship is loaded for a given instance without any danger of endless cycles
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The ability to load related objects is a crucial way to bridge the gap between object models and the underlying relational storage Deferred and eager-loading options ensure that the bridging is done as ef ciently as needed in the particular scenario Yet an additional problem with relationships remains Sometimes the sizes of related collections can be too large to be loaded in either eager or deferred fashion Consider a database that stores orders over many years for customers If the application is interested in showing only a small subset of the orders, perhaps for the last three months, it would be wasteful to load all the orders, whether eagerly or in a deferred fashion Hence, LINQ to SQL provides an additional loading option to lter the collections in a consistent fashion
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The load options for eager loading and ltering of relationships must be speci ed before any queries are performed Specifying the options after the instance has been used for queries creates a risk of inconsistent results across queries Hence, it results in an exception option may be speci ed with or without the eager The loading option It is equally applicable to deferred loading as well The
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