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If the order was removed from the corresponding s col, that would have been considered lection by calling and the order a case of severing the association between the entities; it would not be a case of deletion The result would be an update to to re ect the severance of the Order setting the foreign key value to association Thus, unlike in the case of insertion, LINQ to SQL does not infer command should be generated for removal from a relationship that a command requires a call to avoid collection A accidental deletion This is a safer choice
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Updating Entities After entities are retrieved from the database, you are free to manipulate them as you like They are your objects; use them as you will As you do this, LINQ to SQL tracks changes so that it can persist them into the datais called base when LINQ to SQL starts tracking your entities as soon as they are materialized, even before giving them to you as returned results of a query Indeed, the identity management service discussed in 8, Reading Objects with LINQ to SQL, has already kicked in as well Change tracking costs very little in additional overhead until you actually start making changes statements when you call It allows LINQ to SQL to generate Unlike in the case of deletion, which requires , no additional API needs to be called before The property following code shows a simple example in which the : is changed for a
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9: Modifying Obje cts with LINQ to SQL
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The automatic detection of a change is made possible by LINQ to SQL s ability to keep a copy of the original entity for comparison when is called A new interface was added to make this ef cient by allowing a copy to be made just before an entity is changed through a property setter The code generated by the designer or SqlMetal for entities implements this interface and noti es the LINQ to SQL runtime about an imminent change The runtime makes a copy of the entity before the rst property change Entities that are queried but not modi ed do not need to have their original versions copied for comparison and update statement generation The original values also allow LINQ to SQL to generate a minimal update statement Only the columns that are modi ed are set to new values Unchanged column values are not set The SQL generated for the previous sets only the corresponding column C# code setting , as follows: with parameter
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As in the case of insertions, some column values may be set in the database A common example is a timestamp column The value of the timestamp is set for an updated row in the database Another example is a and trigger-updated column In such cases, using the properties of the mapping attribute, you can tell LINQ to SQL to skip updating the column and instead retrieve the value after successful update
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Automatically Maintained Relationships Relationships pose an interesting challenge They involve two entities Hence, a change in the relationship affects two entities In the database, this is simpli ed through normalized data models and foreign keys In the case and , the Orders of the database relationship between table contains a foreign key recording the relationship, and the Customers table keeps no direct record of the relationship In the corresponding object collection and the model, both the
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Entity Life cycle
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references pointing back to entities need to be in sync If the collection contains an , the corresponding better point back to the same Customer entity Maintaining such a bidirectional relationship would be quite a chore if you had to take care of it every time there was any change Fortunately, LINQ to SQL automates this process through a combination of the gener classes is of ated code and the , and is of type type Consider the following code, which moves an order from one to another Here, we use the terser but semantically identical lambda function syntax to retrieve single customers:
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The same objective can be accomplished just as well through the fol, sequence used in the prelowing assignment instead of the ceding example:
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If you assign a null to a relationship reference, you are severing the relationship This is distinct from deleting the target of the relationship In the entity following code, the order is no longer associated with any is not deleted Likewise, if you remove The order s original target collection, the effect is symmetric: the an order from the relationship is severed, but the order is not deleted In each case, LINQ to to null in the SQL attempts to set the foreign key column is called Such an operation may succeed database when if the foreign key column is nullable; otherwise, it will fail
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