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Tuesday, March 3, 2015

Simplifying declaration of calculated properties in code for DataView mode with Entity Framework

Starting with v14.2.4 (and even 14.1.9) you can use the DevExpress.ExpressApp.DC.CalculatedAttribute to specify an expression used to calculate a property value of an Entity Framework class in Data View mode:
public class Payment { public Int32 ID { get; protected set; } public Decimal Rate { get; set; } public Decimal Hours { { get; set; } [NotMapped, DevExpress.ExpressApp.DC.Calculated("Rate * Hours")] public Decimal CalculatedAmount { get { return Rate * Hours; } }
   // Other data properties and logic...
This attribute functions exactly like the DevExpress.Xpo.PersistentAlias attribute applied to a regular business class property. Of course, properties, referenced in the specified expression (using our cross-platform object-oriented criteria language) should be persistent to be able to run an SQL query at the database level. 

To remind you of the DataView mode, this is what you will see in the SQL Server Profiler if you set DataAccessMode = DataView for the Payment_ListView node containing only the Amount column above:

    [Limit1].[ID] AS [ID], 
    [Limit1].[C1] AS [C1]
    FROM ( SELECT TOP (2147483647) 
        [Extent1].[ID] AS [ID], 
        [Extent1].[Rate] * [Extent1].[Hours] AS [C1]
        FROM [dbo].[Payments] AS [Extent1]
    )  AS [Limit1]

Note that even though our entity may contain a way more other persistent properties, the query included only the two from the expression above, which increases performance in certain scenarios.
The advantage of this attribute solution is that it is easier to keep your data model logic in one place/class instead of spreading it across multiple layers, e.g., Application Model, where the DataView settings are specified:

<ListView Id="Payment_ListView" DataAccessMode="DataView">
       <ColumnInfo PropertyName="CalculatedAmount" IsNewNode="True" />

If you are not sure of which ListView data access mode is best for your particular screen, these guidelines in our docs will help you decide.

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