MSDN Magazine - February 2009 - (Page 11)

ScOTT MiTchell Toolbox The Active Record Pattern, Aspect-Oriented Programming Implement the Active Record Pattern in Your .NET Application The active record pattern is commonly used in data-driven applications to model a database table or view in terms of a class, where a row of the database table is modeled by an instance of the class. In this pattern, the class’s properties map to the corresponding database table’s columns. The class’s instance methods perform operations on a database record, whereas its static methods work with the whole table. For example, if the database contained a table named Employee with columns EmployeeId as a uniqueidentifier, Name as an nvarchar(50), and Salary as a money, to implement the active record pattern you’d create a class named Employee with properties named EmployeeId, Name, and Salary. The Employee class would include instance methods such as Create, Save, and Delete, and might include static methods such as DeleteAll, Find, and FindAll. Figure 1 shows the active record pattern. It offers a simple, intuitive syntax for working with data and is the pattern used by many object-relational mappers (O/RM). The Castle ActiveRecord (version 1.0, RC3) project offers a quick and easy way to implement the active record pattern for Microsoft .NET Framework-based apps. It’s built on top of NHibernate, which is a free, open-source O/RM tool and was reviewed in the October 2006 Toolbox column (see msdn.microsoft.com/magazine/cc163540). One of the drawbacks of NHibernate is that it has a bit of a steep learning curve and requires the developer to create and maintain the XML mapping files that tie together the database tables, columns, and relationships with the classes and properties in the app. The ActiveRecord project API enFigure 1 Active Record Pattern in Action // Add Andrew Fuller as a new employee Employee emp = new Employee(); emp.Name = "Andrew Fuller"; emp.Salary = 50000.00M; emp.Create(); // Give a 10% raise to all employees Employee[] allEmployees = Employee.FindAll(); foreach (Employee current in allEmployees) { current.Salary *= 1.10M; current.Save(); } capsulates much of this complexity by using .NET attributes and makes implementing the active record pattern easy. To get started, download the installation package from the project’s home page. This installs the ActiveRecord and NHibernate assemblies and makes them visible to Visual Studio; add these assemblies to your project. Next, create the classes in your app that will use the active record pattern to model a database table. This is the code for the Employee class referenced in Figure 1: [ActiveRecord] public class Employee : ActiveRecordBase { [PrimaryKey] public Guid EmployeeId { get; set; } [Property] public string Name { get; set; } [Property] public decimal Salary { get; set; } } Note that the Employee class extends the ActiveRecordBase class and uses attributes to flag which properties map to columns in the database table and which column serves as the primary key. The ActiveRecordBase class defines instance methods such as Create and Save and static methods including Find, FindAll, and DeleteAll. You can provide more detail about each member in the Employee class by passing in parameters through the PrimaryKey and Property attributes, including the length of the string columns, whether the column requires unique values, if the column can be assigned a database NULL value, and whether to ignore the column when inserting or updating a class instance. Along with creating the Employee class, you also need to define a few configuration settings, such as the database connection string and a handful of NHibernate-specific options. But there’s no need to write any data-access code. That’s handled for you automatically by the ActiveRecord project and NHibernate. In fact, you don’t even need to have created the database tables at this point, as the ActiveRecord project can auto generate the tables for you based on the classes you’ve created. The attribute syntax of the ActiveRecord project is also used to establish relationships among classes. For example, the HasMany and BelongsTo attributes can be used in a parent and child class, respectively, to indicate a one-to-many relationship. Imagine that the data model was expanded to include a Department table with columns DepartmentId and Name, and a DepartmentId column in the Employee table establishing a one-to-many relationship between departments and employees. This relationship would be modeled in the application by adding a Department class Send your questions and comments to toolsmm@microsoft.com. All prices confirmed on December 29, 2008, and are subject to change. The opinions expressed in this column are solely those of the author and do not necessarily reflect the opinions at Microsoft. February 2009 11 http://msdn.microsoft.com/magazine/cc163540

Table of Contents Feed for the Digital Edition of MSDN Magazine - February 2009

MSDN Magazine - February 2009 Contents Toolbox CLR Inside Out Data Points Cutting Edge Patterns In Practice Best Practices .Net Interop "Oslo" Basics Patterns Silverlight Under The Table Foundations Windows With C++ .NET Matters Going Places { End Bracket }

MSDN Magazine - February 2009

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