具有 VB.NET 的 Fluent API

Code First 允许你使用 C# 或 VB.NET 类定义模型。 可以选择使用类和属性上的特性或使用 Fluent API 执行其他配置。 本演练演示了如何使用 VB.NET 执行 Fluent API 配置。

本页假定你对 Code First 有基本的了解。 请查看以下演练，详细了解 Code First：

• 对新数据库使用 Code First
• 对现有数据库使用 Code First

先决条件

至少需要安装 Visual Studio 2010 或 Visual Studio 2012 才能完成本演练。

如果使用 Visual Studio 2010，还需要安装 NuGet

创建应用程序

为简单起见，我们将构建一个使用 Code First 执行数据访问的基本控制台应用程序。

• 打开 Visual Studio
• “文件”->“新建”->“项目…”
• 从左侧菜单中选择“Windows”并选择“控制台应用程序”
• 输入“CodeFirstVBSample”作为名称
• 选择“确定”

定义模型

在此步骤中，定义表示概念模型的 VB.NET POCO 实体类型。 这些类不需要派生自任何基类或实现任何接口。

• 向项目添加新类，输入“SchoolModel”作为类名
• 将新类的内容替换为以下代码

Public Class Department
    Public Sub New()
        Me.Courses = New List(Of Course)()
    End Sub

    ' Primary key
    Public Property DepartmentID() As Integer
    Public Property Name() As String
    Public Property Budget() As Decimal
    Public Property StartDate() As Date
    Public Property Administrator() As Integer?
    Public Overridable Property Courses() As ICollection(Of Course)
End Class

Public Class Course
    Public Sub New()
        Me.Instructors = New HashSet(Of Instructor)()
    End Sub

    ' Primary key
    Public Property CourseID() As Integer
    Public Property Title() As String
    Public Property Credits() As Integer

    ' Foreign  key that does not follow the Code First convention.
    ' The fluent API will be used to configure DepartmentID_FK  to be the foreign key for this entity.
    Public Property DepartmentID_FK() As Integer

    ' Navigation properties
    Public Overridable Property Department() As Department
    Public Overridable Property Instructors() As ICollection(Of Instructor)
End Class

Public Class OnlineCourse
    Inherits Course

    Public Property URL() As String
End Class

Partial Public Class OnsiteCourse
    Inherits Course

    Public Sub New()
        Details = New OnsiteCourseDetails()
    End Sub

    Public Property Details() As OnsiteCourseDetails
 End Class

' Complex type
Public Class OnsiteCourseDetails
    Public Property Time() As Date
    Public Property Location() As String
    Public Property Days() As String
End Class

Public Class Person
    ' Primary key
    Public Property PersonID() As Integer
    Public Property LastName() As String
    Public Property FirstName() As String
End Class

Public Class Instructor
    Inherits Person

    Public Sub New()
        Me.Courses = New List(Of Course)()
    End Sub

    Public Property HireDate() As Date

    ' Navigation properties
    Private privateCourses As ICollection(Of Course)
    Public Overridable Property Courses() As ICollection(Of Course)
    Public Overridable Property OfficeAssignment() As OfficeAssignment
End Class

Public Class OfficeAssignment
    ' Primary key that does not follow the Code First convention.
    ' The HasKey method is used later to configure the primary key for the entity.
    Public Property InstructorID() As Integer

    Public Property Location() As String
    Public Property Timestamp() As Byte()

    ' Navigation property
    Public Overridable Property Instructor() As Instructor
End Class

定义派生上下文

我们将开始使用实体框架中的类型，因此需要添加 EntityFramework NuGet 包。

• **“项目”->“管理 NuGet 包…”

注意

如果没有“管理 NuGet 包…”选项，应安装最新版本的 NuGet

• 选择“联机”选项卡
• 选择 EntityFramework 包
• 单击“安装”

现在，可以定义一个派生上下文，该上下文表示与数据库的会话，使我们能够查询和保存数据。 我们定义了一个派生自 System.Data.Entity.DbContext 的上下文，并为模型中的每个类公开一个类型化 DbSet<TEntity>。

• 向项目添加新类，输入“SchoolContext”作为类名
• 将新类的内容替换为以下代码

Imports System.ComponentModel.DataAnnotations
Imports System.ComponentModel.DataAnnotations.Schema
Imports System.Data.Entity
Imports System.Data.Entity.Infrastructure
Imports System.Data.Entity.ModelConfiguration.Conventions

Public Class SchoolContext
    Inherits DbContext

    Public Property OfficeAssignments() As DbSet(Of OfficeAssignment)
    Public Property Instructors() As DbSet(Of Instructor)
    Public Property Courses() As DbSet(Of Course)
    Public Property Departments() As DbSet(Of Department)

    Protected Overrides Sub OnModelCreating(ByVal modelBuilder As DbModelBuilder)
    End Sub
End Class

使用 Fluent API 进行配置

本部分演示了如何使用 Fluent API 配置类型到表的映射、属性到列的映射，以及模型中表\类型之间的关系。 Fluent API 通过 DbModelBuilder 类型公开，最常见的访问方式是替代 DbContext 上的 OnModelCreating 方法。

• 复制以下代码，并将其添加到 SchoolContext 类上定义的 OnModelCreating 方法中。注释解释了每个映射的作用

' Configure Code First to ignore PluralizingTableName convention
' If you keep this convention then the generated tables
' will have pluralized names.
modelBuilder.Conventions.Remove(Of PluralizingTableNameConvention)()


' Specifying that a Class is a Complex Type

' The model defined in this topic defines a type OnsiteCourseDetails.
' By convention, a type that has no primary key specified
' is treated as a complex type.
' There are some scenarios where Code First will not
' detect a complex type (for example, if you do have a property
' called ID, but you do not mean for it to be a primary key).
' In such cases, you would use the fluent API to
' explicitly specify that a type is a complex type.
modelBuilder.ComplexType(Of OnsiteCourseDetails)()


' Mapping a CLR Entity Type to a Specific Table in the Database.

' All properties of OfficeAssignment will be mapped
' to columns  in a table called t_OfficeAssignment.
modelBuilder.Entity(Of OfficeAssignment)().ToTable("t_OfficeAssignment")


' Mapping the Table-Per-Hierarchy (TPH) Inheritance

' In the TPH mapping scenario, all types in an inheritance hierarchy
' are mapped to a single table.
' A discriminator column is used to identify the type of each row.
' When creating your model with Code First,      
' TPH is the default strategy for the types that
' participate in the inheritance hierarchy.
' By default, the discriminator column is added
' to the table with the name “Discriminator”
' and the CLR type name of each type in the hierarchy
' is used for the discriminator values.
' You can modify the default behavior by using the fluent API.
modelBuilder.Entity(Of Person)().
    Map(Of Person)(Function(t) t.Requires("Type").
        HasValue("Person")).
        Map(Of Instructor)(Function(t) t.Requires("Type").
        HasValue("Instructor"))


' Mapping the Table-Per-Type (TPT) Inheritance

' In the TPT mapping scenario, all types are mapped to individual tables.
' Properties that belong solely to a base type or derived type are stored
' in a table that maps to that type. Tables that map to derived types
' also store a foreign key that joins the derived table with the base table.
modelBuilder.Entity(Of Course)().ToTable("Course")
modelBuilder.Entity(Of OnsiteCourse)().ToTable("OnsiteCourse")
modelBuilder.Entity(Of OnlineCourse)().ToTable("OnlineCourse")


' Configuring a Primary Key

' If your class defines a property whose name is “ID” or “Id”,
' or a class name followed by “ID” or “Id”,
' the Entity Framework treats this property as a primary key by convention.
' If your property name does not follow this pattern, use the HasKey method
' to configure the primary key for the entity.
modelBuilder.Entity(Of OfficeAssignment)().
    HasKey(Function(t) t.InstructorID)


' Specifying the Maximum Length on a Property

' In the following example, the Name property
' should be no longer than 50 characters.
' If you make the value longer than 50 characters,
' you will get a DbEntityValidationException exception.
modelBuilder.Entity(Of Department)().Property(Function(t) t.Name).
    HasMaxLength(60)


' Configuring the Property to be Required

' In the following example, the Name property is required.
' If you do not specify the Name,
' you will get a DbEntityValidationException exception.
' The database column used to store this property will be non-nullable.
modelBuilder.Entity(Of Department)().Property(Function(t) t.Name).
    IsRequired()


' Switching off Identity for Numeric Primary Keys

' The following example sets the DepartmentID property to
' System.ComponentModel.DataAnnotations.DatabaseGeneratedOption.None to indicate that
' the value will not be generated by the database.
modelBuilder.Entity(Of Course)().Property(Function(t) t.CourseID).
    HasDatabaseGeneratedOption(DatabaseGeneratedOption.None)

'Specifying NOT to Map a CLR Property to a Column in the Database
 modelBuilder.Entity(Of Department)().
     Ignore(Function(t) t.Administrator)

'Mapping a CLR Property to a Specific Column in the Database
 modelBuilder.Entity(Of Department)().Property(Function(t) t.Budget).
     HasColumnName("DepartmentBudget")

'Configuring the Data Type of a Database Column
 modelBuilder.Entity(Of Department)().Property(Function(t) t.Name).
     HasColumnType("varchar")

'Configuring Properties on a Complex Type
modelBuilder.Entity(Of OnsiteCourse)().Property(Function(t) t.Details.Days).
    HasColumnName("Days")
modelBuilder.Entity(Of OnsiteCourse)().Property(Function(t) t.Details.Location).
    HasColumnName("Location")
modelBuilder.Entity(Of OnsiteCourse)().Property(Function(t) t.Details.Time).
    HasColumnName("Time")


' Map one-to-zero or one relationship

' The OfficeAssignment has the InstructorID
' property that is a primary key and a foreign key.
modelBuilder.Entity(Of OfficeAssignment)().
    HasRequired(Function(t) t.Instructor).
    WithOptional(Function(t) t.OfficeAssignment)


' Configuring a Many-to-Many Relationship

' The following code configures a many-to-many relationship
' between the Course  and Instructor types.
' In the following example, the default Code First conventions
' are used  to create a join table.
' As a result the CourseInstructor table is created with
' Course_CourseID  and Instructor_InstructorID columns.
modelBuilder.Entity(Of Course)().
    HasMany(Function(t) t.Instructors).
    WithMany(Function(t) t.Courses)


' Configuring a Many-to-Many Relationship and specifying the names
' of the columns in the join table

' If you want to specify the join table name
' and the names of the columns in the table
' you need to do additional configuration by using the Map method.
' The following code generates the CourseInstructor
' table with CourseID and InstructorID columns.
modelBuilder.Entity(Of Course)().
    HasMany(Function(t) t.Instructors).
    WithMany(Function(t) t.Courses).
    Map(Sub(m)
            m.ToTable("CourseInstructor")
            m.MapLeftKey("CourseID")
            m.MapRightKey("InstructorID")
        End Sub)


' Configuring a foreign key name that does not follow the Code First convention

' The foreign key property on the Course class is called DepartmentID_FK
' since that does not follow Code First conventions you need to explicitly specify
' that you want DepartmentID_FK to be the foreign key.
modelBuilder.Entity(Of Course)().
    HasRequired(Function(t) t.Department).
    WithMany(Function(t) t.Courses).
    HasForeignKey(Function(t) t.DepartmentID_FK)


' Enabling Cascade Delete

' By default, if a foreign key on the dependent entity is not nullable,
' then Code First sets cascade delete on the relationship.
' If a foreign key on the dependent entity is nullable,
' Code First does not set cascade delete on the relationship,
' and when the principal is deleted the foreign key will be set to null.
' The following code configures cascade delete on the relationship.

' You can also remove the cascade delete conventions by using:
' modelBuilder.Conventions.Remove<OneToManyCascadeDeleteConvention>()
' and modelBuilder.Conventions.Remove<ManyToManyCascadeDeleteConvention>().
modelBuilder.Entity(Of Course)().
    HasRequired(Function(t) t.Department).
    WithMany(Function(t) t.Courses).
    HasForeignKey(Function(d) d.DepartmentID_FK).
    WillCascadeOnDelete(False)

使用模型

让我们使用 SchoolContext 执行一些数据访问，以了解模型的运行情况。

• 打开定义了 Main 函数的 Module1.vb 文件
• 复制并粘贴以下 Module1 定义

Imports System.Data.Entity

Module Module1

    Sub Main()

        Using context As New SchoolContext()

            ' Create and save a new Department.
            Console.Write("Enter a name for a new Department: ")
            Dim name = Console.ReadLine()

            Dim department = New Department With { .Name = name, .StartDate = DateTime.Now }
            context.Departments.Add(department)
            context.SaveChanges()

            ' Display all Departments from the database ordered by name
            Dim departments =
                From d In context.Departments
                Order By d.Name
                Select d

            Console.WriteLine("All Departments in the database:")
            For Each department In departments
                Console.WriteLine(department.Name)
            Next

        End Using

        Console.WriteLine("Press any key to exit...")
        Console.ReadKey()

    End Sub

End Module

现在，可运行应用程序并对其进行测试。

Enter a name for a new Department: Computing
All Departments in the database:
Computing
Press any key to exit...