What is the annotation @Metadata.allowExtensionstrue of CDS view

What is the annotation @Metadata.allowExtensions:true of CDS view

This annotation allows us metadata extension(缩写为 MDE) viewto define annotations for using comsumption view.

What MDE strives to achieve is the concept of so-called separation of concerns, that is, the data model and the metadata extension view each perform their own duties. The former focuses on defining fields according to the business, while the latter only defines domain specific semantic objects, such as using UI-related annotations in the Fiori Elements framework.

The important role of Metadata Extension View in SAP CDS Views

In SAP, CDS (Core Data Services) is a powerful tool for defining and managing data models. CDS Views are a key concept in this field and are used to describe different aspects of a data model, including its structure, behavior and semantics. Among CDS Views, Metadata Extension View (hereinafter referred to as MDE View) plays an important role in helping to realize the concept of separation of concerns (Separation of Concerns) and improve the maintainability and scalability of the data model.

1. The importance of separation of concerns:

   Separation of concerns is a fundamental principle in software engineering that aims to separate different aspects of concerns to make it easier to manage and maintain the system. In CDS Views, the focus of the data model usually includes business data definition and UI-related annotations. One of the main roles of MDE View is to separate these two concerns, achieving the following benefits:

   • Improved code clarity: Store data model and UI-related concerns separately, making each part of the code clearer and easier to understand. Developers can focus on specific aspects of their work without being distracted by irrelevant code.

   • Enhanced maintainability: When the data model needs to be modified, UI annotations do not have to be touched, and vice versa. This reduces the risk of errors and simplifies the maintenance process, as changes to one part do not unnecessarily affect another.

   • Collaboration is easier: different teams or developers can focus on their areas of concern without disrupting the work of other teams. This separation also helps multiple people collaborate on development projects and improves development efficiency.

2. MDE View roles and examples:

   MDE View plays a key role in achieving separation of concerns. Its main task is to define UI-related annotations, which are commonly used in the Fiori Elements framework. The following is an example of an MDE View demonstrating its role in CDS Views:

   @Metadata.allowExtensions:true
   define view MyBusinessData as select from BusinessEntity {
     key ID,
     Name,
     Description
   } {
     // 数据模型定义
   }
   

   In the above example, MyBusinessDatait is a CDS View that defines the business data model. This view contains the fields of the business entity, such as ID, Name, and Description. Note @Metadata.allowExtensions:truethe annotation, which indicates that the view allows the use of MDE View to extend its annotations.

   Next, we can create an MDE View specifically for defining UI-related annotations, such as Fiori Elements annotations:

   extend view MyBusinessData with MDE_Extensions {
     // 与UI相关的注解定义
     @UI: {
       LineItem: [
         { Value: ID, Label: 'ID' },
         { Value: Name, Label: 'Name' }
       ],
       SelectionFields: [ ID ],
       PresentationVariant: [
         {
           Visualizations: [{ Type: #AS_TABLE }],
           RequestedKey: [ ID ],
           SortOrder: [
             { Value: ID, Descending: false }
           ]
         }
       ]
     }
   }
   

   In this example, extend viewthe statement is used to associate the MDE View with the business data model view MyBusinessData. MDE View contains UI-related annotations, such as table column definitions, selection fields, and rendering variations. These annotations are for UI-level definitions and have nothing to do with the data model itself.

3. Summary of important functions:

   • Maintainability and scalability: MDE View allows the data model and UI annotations to be managed separately, improving the maintainability of the system. When you need to modify the UI aspects, you only need to edit the MDE View without modifying the data model, and vice versa.

   • Code Clarity: Separation of concerns makes code clearer and readable. Developers can more easily understand and modify specific parts without being distracted by other aspects of the code.

   • Collaboration and team division: Different teams or developers can focus on their areas of concern without interfering with each other. This helps improve collaboration efficiency and development speed.

   • UI customization: MDE View makes customization for different UI requirements more flexible. Different MDE Views can be created for different UI components to meet different needs without having to change the underlying data model.

   All in all, the role of Metadata Extension View in SAP CDS Views is to achieve separation of concerns, improve the maintainability and scalability of the data model, while promoting team collaboration and UI customization. This is an important concept in SAP development and helps in building flexible, maintainable and scalable enterprise applications.