In many applications, you need to filter data that is only relevant to the particular user that is logged in.  For example, a personal information manager application may only want users to view their own tasks and not the tasks of other users.  Here’s a walkthrough of how you can setup this kind of data filtering in Visual Studio LightSwitch.

I’ll first create a Task table which has two fields: one for the task description and another to store the user name of the user who created the task.

Next, I’ll need to write some code so that whenever a Task is created, it will automatically have it’s CreatedBy field set to the current user.  To do this, I can use Write Code drop-down on the table designer to select the Created method.

Here’s the code:

partial void Task_Created()
{
    this.CreatedBy = this.Application.User.Name;
}

Now we’re at the data filtering step.  What I’d really like to do is have all queries for Tasks be filtered according to the current user.  So even if I model a new query for Tasks then it will automatically get this filtering behavior.  That way I only have to write this code once and it will be applied whenever tasks are queried.  LightSwitch provides a built-in query for each table that returns all instances within that table.  The name of this query is TableName_All.  All other queries for that table are based on that All query.  So if I can modify the behavior of that All query, then every other query that queries the same table will also get that behavior.  LightSwitch just so happens to provide a way to modify the default behavior of the All query.  This can be done through the PreprocessQuery method.  This method is also available through the Write Code drop-down. 

The PreprocessQuery method allows developers to modify the query before it is executed.  In my case, I want to add a restriction to it so that only tasks created by the current user are returned.

partial void Tasks_All_PreprocessQuery(ref IQueryable<LightSwitchApplication.Task> query)
{
    query = query.Where(t => t.CreatedBy == this.Application.User.Name);
}

And that’s all I need to do.  Now, whenever any query is made for tasks it will add this restriction.