Advanced Mapping Concepts

The generated objects have concepts attached to them like data type, nullable type, enumerated type, and error-checking. Depending on the settings of an object’s fields in the model, the generated object will have certain properties associated with each field. Of course a field’s data type is used to create a corresponding .NET type for the generated object. However there are more advanced features as well. A field marked as nullable, is generated with a nullable .NET type. You can look at all non-string fields and know if a null value is permissible since the .NET type either allows it or not. A string is a reference type, so you cannot determine this attribute just by its data type.

Strong error-checking is implemented by not allowing client code to assign values that will break the database, as much as possible. If a null value is passed to a non-nullable field, an error is raised. All objects have meta-data descriptors like length, nullable, type, etc. You can use these attributes to limit UI data entry to valid data. There are also common business object functions that allow you to set or retrieve data in a common way, based on interfaces and field enumerations. The common base functionality of the framework allows for the construction of application to be written that are not domain specific. In other words, you can write applications that can take any generated API based on any domain model and plug it in for user manipulation.

Enumerated types are static types that map to unchanging (or seldom-changing) data. A user type table is a good example of this. You may have a number of user types but they almost never change. Instead of assigning “magic numbers” to these values, simply let the generator handle it for you. From there you can assign an enumeration in your code and not a number. You code is much more readable and developers can tell immediately what the value means.

Interfaces and base classes

All generated objects implement one or more interface or abstract base classes. This allows for the creation of very versatile applications. All generated mapped objects implement numerous interfaces, some based on whether the object is a read-only or persistable or other such permissible actions. You can build applications that take in base objects and interact with the user based on the type of object passed in. At run-time you can get and set an object’s fields without know exactly what type of object it is. The audit fields are accessible by interface as are search objects and primary keys.

The business objects come in two flavors, read-only and persistable. You can define objects as read-only, so they can be selected but not saved. This comes in handy for type tables and also base object tables. The latter is quite interesting in that you can define a table that has no meaning by itself but is the base class of a concrete object. An example of this is an abstract Person that is the base for Customer and Employee. There may be no such thing as a generic Person in your business rules only Customers and Employees. In this case, make the parent table of each the Person entity and make the Person entity read-only. In code, developers can only create Customers and Employees but not Persons.
Notice in the above diagram that the interface model is quite robust. Customer inherits not only from Person but also ICustomer which in turn inherits from IPerson. All business objects are auditable. Persistable objects are derived from the less functional, read-only, objects and interfaces. This model exposes many possibilities when creating user interfaces and business logic.

Inheritance

The generator is not hampered by a one-to-one concrete mapping of database objects to classes. The model supports object inheritance. You can define a base type and inherit any number of objects from it. The back-end database synchronization is handled under the covers. There is no special programming around this concept. The only requirement is that a derived class has the same primary key, cardinality, name and data type. When you select an inherited object, you see all the fields like a truly inherited object. There is no way to tell that some fields are derived from a base table in the database. All the complex joins and field mappings are handled under the covers and you never have to worry about which field comes from which table.

In the example above, the associated class diagram would follow. Notice that Person entity is derived from IBusinessObject. This is an interface that defines no save functionality. The child classes of Customer and Employee are derived from Person but also implement the interface IPersistableBusinessObject. This interface exposes a Persist method.

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Last edited Sep 11, 2009 at 11:20 AM by codetools, version 4

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