C# 4 and .NET 4

878 ❘ ChaPTer 31 AdO.net entity frAmewOrk
Now it would be interesting to filter the racers based on a condition. This can be done by using the Where()
method of the ObjectQuery class. Where() is one of the Query Builder methods that create Entity
SQL. This method requires a predicate as a string, and optional parameters of type ObjectParameter.
The predicate shown here specifies that only the racers from Brazil are returned. The it specifies the item
of the result and Country is the column Country. The first parameter of the ObjectParameter constructor
references the @Country parameter of the predicate, but doesn’t list the @ sign:
string country = "Brazil";
ObjectQuery racers = data.Racers.Where("it.Country = @Country",
new ObjectParameter("Country", country));
The magic behind it can be seen immediately by accessing the CommandText property of the query. With
Entity SQL, SELECT VALUE it declares it to access the columns:
SELECT VALUE it
FROM (
[Formula1Entities].[Racers]
) AS it
WHERE
it.Country = @Country
The method ToTraceString() shows the generated SQL statement:
SELECT
[Extent1].[Id] AS [Id],
[Extent1].[Firstname] AS [Firstname],
[Extent1].[Lastname] AS [Lastname],
[Extent1].[Country] AS [Country],
[Extent1].[Starts] AS [Starts],
[Extent1].[Wins] AS [Wins]
FROM [dbo].[Racers] AS [Extent1]
WHERE [Extent1].[Country] = @Country
Of course, you can also specify the complete Entity SQL:
string country = "Brazil";
ObjectQuery racers = data.CreateQuery(
"SELECT VALUE it FROM ([Formula1Entities].[Racers]) AS " +
"it WHERE it.Country = @Country",
new ObjectParameter("Country", country));
The class ObjectQuery offers several Query Builder methods as explained in the following table. Many
of these methods are very similar to the LINQ extension methods that you learned about in Chapter 11,
“Language Integrated Query.” An important difference with the methods here is that instead of parameters
of type delegate or Expression, the parameter type with ObjectQuery is usually of type string.
objeCTquery query builder meThods
Where()
Distinct()
Except()
GroupBy()
Include()
desCriPTion
This method allows you to filter the results based on a
condition.
This method creates a query with unique results.
This method returns the result without the items that meet
the condition with the except filter.
This method creates a new query to group entities based on
a specified criteria.
With relations, you saw earlier that related items are delay
loaded. It is required to invoke the Load() method of the
EntityCollection class to get related entities into the
object context. Instead of using the Load() method, you can
specify a query with the Include() method to eager fetchrelated
entities.
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