Monthly Archives: May 2015

How to see the real-time state of your view-model

When testing and debugging your web-application, its convenient to see the real-time state of the view-model. This is very easy when you’re using a data-binding framework such as knockout. You can simply bind the JSON representation of the view-model to some visible DOM element, like this:

<pre data-bind="text: JSON.stringify(ko.toJS(MyViewModel), null, 4)"></pre>

Sometimes, the objects in your view-model might have circular dependencies, like this:

//The ViewModel
var CurrencyViewModel = function () {
    var self         = this;
    this.DataContext = new CurrencyDataContext(this);
    this.Name        = ko.observable();
    ...
}
//The pseudo-model behind the ViewModel
var CurrencyDataContext = function CurrencyDataContext(ViewModel) {
    var self = this
    this.Viewmodel = ViewModel
    ...
}

In that case you’ll get the following error:
0x800a13aa - JavaScript runtime error: Circular reference in value argument not supported
You can fix this by overriding the toJSON() method, like this:

var CurrencyDataContext = function CurrencyDataContext(ViewModel) {
    var self = this
    this.Viewmodel = ViewModel
    ...
    //Needed to avoid circular reference when a viewModel is serialised into JSON
    CurrencyDataContext.prototype.toJSON = function ()
    {
        var copy = ko.toJS(self);
        delete copy.Viewmodel
        return copy;
    }

Entity Framework: How to delete your old database and start fresh with a new one

In a previous post I explained how to recover after deleting a Entity Framework database. In this post we’ll see the proper way to recreate the database in Entity Framework:

Careful

before you start, make sure you’ve got source-control or back-ups of the code in the Migrations folder. When you delete the Migrations folder, you’re deleting code that:

  1. seeds the database with the initial set of data
  2. and up/downgrades the database to the various versions

In Visual Studio:

  1. Go to Server Explorer, right-click on the data collection that represents your context and choose delete.
  2. Go to Solution Explorer and delete the .mdf file. You might have to click on the “Show All Files” icon before you see it.
  3. Go to the Solution Explorer and delete the Migrations folder.

At this point, you have a solution that will create a new database when its run. If you need to seed the database or expect your models to change, then you’ll want to do the following:

  1. Tell EF to create a fresh database bases on the current models by going to the Package Manager Console and running the following commands: 
    Enable-Migrations
Add-Migration Initial
Update-Database
  2. Update the code in Migrations\Configuration.cs to seed the database with the data you need.