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Filtering data based on current user in LightSwitch apps

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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.

Comments

  • Anonymous
    September 07, 2010
    Hi Matt great post, however, is there some additional post i'm missing?, i am not seeing wher you created the code for 'tasks' in the drop down code generator....Also would love some more posts on c#

  • Anonymous
    September 07, 2010
    @jeremy: The list of methods in the drop-down are the methods available that the developer can implement if they choose.  Selecting Task_Created from the drop-down will automatically generate the method stub and navigate you to the code file for the Task table.  From there, you just write your code.

  • Anonymous
    September 07, 2010
    Hi Matt , thanks for your speedy response, however, i am not seeing the 'task' method listed under General methods, i am using lightswitch with Visual web dev 2010 , could that be an issue?, not sure why i'm not seeing the methos listed..

  • Anonymous
    September 07, 2010
    The comment has been removed

  • Anonymous
    September 07, 2010
    Thanks Matt, some times we write manuals/Instructions  for Pro's forgetting that newbees will also read,my understanding is that Lightswitch is aimed at the 'non-programmers market.just wanted to ask some questions others reaading may be afraid to ask and do not want to appear dumb :<) all the very best!.

  • Anonymous
    September 08, 2010
    This blog post (and the blog) is invaluable. I also appreciate the design of the extensibility points in LightSwitch. It is brilliant.