Importance of Model The first point to note that without a model, development cannot be done in AX 7. Whenever you are creating a new Element or you are customizing any existing Element, it is must to do this in model. So, first thing we must be aware of, before thinking to do any development in AX, is creation of a Model Creation of Model Previously, till, AX 2012, as we were creating model it was as straightforward as creating a new model . With arrival of AX7, creation of a new Model involves an additional step, which is “choice of Package”. In AX 7 when we create a model, we have the option to choose whether we want to create that model in a separate package or in an existing Package. The choice of Package, will determine the development approach that we'll be using. If we create a model in a separate package that implies we will be using the Extension development approach If we create a model in an existing package, that means we'll be using the OverLayring or customization development approach. Choice/Decision of Development Approach Remember if we choose to create a model in an existing package, we must select a model that contains the elements you wish to customize as a referenced model in the wizard. This way our new model has access to those elements. Taking this approach allows you to customize metadata and source code at a higher layer. For example, you can change the design of a form in the user layer. If we choose to create a model in a new package, this implies we're going to use the extension model approach. Developing this way allows us to access the elements from any models that we selected as referenced models and the wizard. It also allows us to add functionality to existing code. We can create new elements as well as create extensions of referenced elements in our projects Important Points to remember: A particular model may contain multiple Visual Studio projects. A project may only belong to one model only. You cannot add elements belonging from different models in same project. If you need to use elements from multiple different models, a new project will be created for each set of elements that originates from a different model, or you have to have your model configured to reference the models you need. Each element has a property named as IsExtendable, which is either false or true. If this is set to True, means the element can be extended. Otherwise you cannot extend such element. An example of non-extendable elements can be seen in most of Base Enumerations. It’s very hard or nearly impossible to develop any solution which has all Extendible elements. At some point, you have to use customization. So your solution/product such as any ISV solution can be combination of both (Extensions and OverLayring).
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