How to populate a List View with data from a LINQ query

NOTE: Starting from version 16.1, a simpler solution can be implemented based on the How to: Display a Non-Persistent Object's List View from the Navigation article, using a non-persistent object to represent the query result set.

Scenario

Sometimes it is important to show a readonly ListView with custom data obtained from the database, rather than through standard XPO mechanisms, by means of loading entire persistent objects.

While this custom data may include certain properties of your XPO classes, in a general case, it may come from anywhere, including but not limited to, a raw SQL query, stored procedure, database view or a very complex LINQ query returning only a subset of persistent class properties or even custom data fields. While the first scenarios can be solved by mapping a persistent class to a database view, the second group of scenarios is optimal in implementing a custom form/control (check How to show custom forms and controls in XAF (Example) for more details) or a custom CollectionSource.
Here, we will demonstrate how the latter option can be realized to display data from a custom LINQ query using the LINQ to XPO approach.

In particular, we want to show a list of employees with maximum orders:

Steps to implement

1. Include the XAF.LinqToXpo module project in your solution and make sure it builds successfully.

This module is supposed to generate custom ListView nodes in the Application Model with the _Linq suffix so that this ListView uses data returned by a predefined method (see the XPQueryMethod attribute for the ListView node) using LINQ.

To provide this custom ListView with data, a custom CollectionSourceBase class descendant is implemented (see the XAF.LinqToXpo\LinqCollectionSource.xx file). It is enabled for a custom ListView via the XafApplication.CreateCustomCollectionSource event (see the XAF.LinqToXpo\Module.xx file).

2. Invoke the Module Designer for YourSolutionName.Module project and drag and drop the XafLinqToXpoModule component to the Required Modules list (this component should automatically appear in the Visual Studio Toolbox after executing the previous step).

3. Implement the public static methods that accept DevExpress.Xpo.Session as a parameter and return System.Linq.IQueryable as a result in your persistent classes, where required.

Consider the following signature for more clarity: public static System.Linq.IQueryable MethodName(DevExpress.Xpo.Session session);

Within the method body, use the LINQ to XPO approach to return a required data set based on XPQuery<T>. If you want to return a custom property as part of your data set which does not exist within your XPO data model, then include it into the selected part of the query with a custom name (e.g., Employee_Linq, Orders_Sum_Linq, etc.).

Mark these methods with the XAF.LinqToXpo.QueryProjectionsAttribute to specify that comma-separated names of data properties will be included as a query result.

Refer to the NorthwindDemo.Module\PersistentObjects.xx for an example.

4. Invoke the Model Editor for YourSolutionName.Module project and add custom calculated fields with dummy expressions for each specified custom property which is not part of your default XPO data model. Refer to the NorthwindDemo.Module\Model.DesignedDiffs.xafml file for an example.

IMPORTANT NOTES

1. This example uses the Northwind database for testing. You can download the database creation scripts for testing from here.

2. Since a Linq-based ListView may contain custom data sets and not entire persistent objects in a general case, certain standard Controllers are disabled here (e.g., you cannot open a detail form for this data record or create a new object). Refer to the XAF.LinqToXpo\DisableStandardActionsLinqListViewController.xx file for more details.

3. If you want your default ListView to include custom calculated values, then it is possible to use the built-in Custom Fields feature for that purpose.