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An entity has identity de ned by one or more key members LINQ to SQL ensures that duplicate entity instances are not created for a given key value Hence, entities typically are updatable Arbitrary projections produce results of nonentity types and are not updatable
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Entities are connected to each other through various relationships An Order entity is associated with a Customer entity; an Employee entity is associated with other Employees through a manager-employee relationship LINQ to SQL provides an easy way to represent such relationships and to effectively use them in queries and updates
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The relationship between objects is usually a property that references a In the case of a collection of related related object, such as objects, the relationship is represented as a collection property containing usually is a colreferences to related objects For example, objects lection of references to In relational databases, relationships usually are represented through keys The relationships are often maintained using foreign key constraints For example, in the Northwind database, the Orders table has a foreign key referencing the column in the Customers table LINQ to SQL bridges the database and object concepts of relationships attribute A property referencing a related entity or using the entities is attributed as follows:
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The attribute provides information about the members in the containing class and the related class that map to the key values de ning the relationship in the database In the previous example, names the class members mapped to the primary key of the Customers refers to the key in the table and the foreign key in the Orders table refers to the key member in the other, containing class, and
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8: Reading Obje cts with LINQ to SQL
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related class The attribute indicates that this relationship is enforced as a foreign key relationship in the database This relationship mapping can be used for querying as follows, with a few sample results shown:
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In this query, we can easily dot through the relationship in and expressions refers to a property of the related entity Customer that can be referenced through the range variable of type Likewise, accomplishes similar navigation This is the real power of object relational mapping an explicit join between the Customers and Orders tables is no longer needed The association mapping combined with the power of LINQ makes it easy to write queries simply by using the power of the dot in the object model Any joins that are required are handled by LINQ to SQL under the covers The developer can just keep using dot notation to access the object and its properties This is true whether they are mapped to a column in the same table, such as OrderOrderID, or whether they are mapped to a different table, such as OrderCustomerCountry, which is mapped to the Customers table Collection properties can be used similarly as follows, with the output shown at the end:
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The only additional thing to keep in mind is that a collection valued property must be used with operators that work on collections For example,
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the previous query uses the operator to see if the collection has any orders A collection property cannot be used like the property of an entity instance from the collection and type You cannot directly index an navigate to its property There is a difference between the members of versus Fortunately, IntelliSense is very helpful in this case as well The completion list shows the available set of members, as shown in Figure 82
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The completion list for a collection property
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Collection-valued properties may be used in the result of a query as well SQL is designed for the relational domain; hence, the results in SQL are rectangular If you want Customers and Orders, the result is tabular, with the customer information repeated for each order belonging to the customer LINQ is designed for the object domain Therefore, hierarchies of objects are naturally available in the result displayed using the ObjectDumper:
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