转 Configuring Relationships with the Fluent API

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Configuring a Required-to-Optional Relationship (One-to–Zero-or-One)

The following example configures a one-to-zero-or-one relationship. The OfficeAssignment has the InstructorID property that is a primary key and a foreign key, because the name of the property does not follow the convention the HasKey method is used to configure the primary key.

// Configure the primary key for the OfficeAssignment
modelBuilder.Entity<OfficeAssignment>()
    .HasKey(t => t.InstructorID); 
 
// Map one-to-zero or one relationship
modelBuilder.Entity<OfficeAssignment>()
    .HasRequired(t => t.Instructor)
    .WithOptional(t => t.OfficeAssignment);

Configuring a Relationship Where Both Ends Are Required (One-to-One)

In most cases the Entity Framework can infer which type is the dependent and which is the principal in a relationship. However, when both ends of the relationship are required or both sides are optional the Entity Framework cannot identify the dependent and principal. When both ends of the relationship are required, use WithRequiredPrincipal or WithRequiredDependent after the HasRequired method. When both ends of the relationship are optional, use WithOptionalPrincipal or WithOptionalDependent after the HasOptional method.

// Configure the primary key for the OfficeAssignment
modelBuilder.Entity<OfficeAssignment>()
    .HasKey(t => t.InstructorID); 
 
modelBuilder.Entity<Instructor>()
    .HasRequired(t => t.OfficeAssignment)
    .WithRequiredPrincipal(t => t.Instructor);

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<Course>()
    .HasMany(t => t.Instructors)
    .WithMany(t => t.Courses)

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<Course>()
    .HasMany(t => t.Instructors)
    .WithMany(t => t.Courses)
    .Map(m =>
    {
        m.ToTable("CourseInstructor");
        m.MapLeftKey("CourseID");
        m.MapRightKey("InstructorID");
    });

Configuring a Relationship with One Navigation Property

A one-directional (also called unidirectional) relationship is when a navigation property is defined on only one of the relationship ends and not on both. By convention, Code First always interprets a unidirectional relationship as one-to-many. For example, if you want a one-to-one relationship between Instructor and OfficeAssignment, where you have a navigation property on only the Instructor type, you need to use the fluent API to configure this relationship.

// Configure the primary Key for the OfficeAssignment
modelBuilder.Entity<OfficeAssignment>()
    .HasKey(t => t.InstructorID); 
 
modelBuilder.Entity<Instructor>()
    .HasRequired(t => t.OfficeAssignment)
    .WithRequiredPrincipal();

Enabling Cascade Delete

You can configure cascade delete on a relationship by using the WillCascadeOnDelete method. 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.

You can remove these cascade delete conventions by using:

modelBuilder.Conventions.Remove<OneToManyCascadeDeleteConvention>()
modelBuilder.Conventions.Remove<ManyToManyCascadeDeleteConvention>()

The following code configures the relationship to be required and then disables cascade delete.

modelBuilder.Entity<Course>()
    .HasRequired(t => t.Department)
    .WithMany(t => t.Courses)
    .HasForeignKey(d => d.DepartmentID)
    .WillCascadeOnDelete(false);

Configuring a Composite Foreign Key

If the primary key on the Department type consisted of DepartmentID and Name properties, you would configure the primary key for the Department and the foreign key on the Course types as follows:

// Composite primary key
modelBuilder.Entity<Department>()
.HasKey(d => new { d.DepartmentID, d.Name }); 
 
// Composite foreign key
modelBuilder.Entity<Course>()
    .HasRequired(c => c.Department)
    .WithMany(d => d.Courses)
    .HasForeignKey(d => new { d.DepartmentID, d.DepartmentName });

Renaming a Foreign Key That Is Not Defined in the Model

If you choose not to define a foreign key on the CLR type, but want to specify what name it should have in the database, do the following:

modelBuilder.Entity<Course>()
    .HasRequired(c => c.Department)
    .WithMany(t => t.Courses)
    .Map(m => m.MapKey("ChangedDepartmentID"));

Configuring a Foreign Key Name That Does Not Follow the Code First Convention

If the foreign key property on the Course class was called SomeDepartmentID instead of DepartmentID, you would need to do the following to specify that you want SomeDepartmentID to be the foreign key:

modelBuilder.Entity<Course>()
         .HasRequired(c => c.Department)
         .WithMany(d => d.Courses)
         .HasForeignKey(c => c.SomeDepartmentID);

using System.Data.Entity;
using System.Data.Entity.ModelConfiguration.Conventions;
// add a reference to System.ComponentModel.DataAnnotations DLL
using System.ComponentModel.DataAnnotations;
using System.Collections.Generic;
using System; 
 
public class SchoolEntities : DbContext
{
    public DbSet<Course> Courses { get; set; }
    public DbSet<Department> Departments { get; set; }
    public DbSet<Instructor> Instructors { get; set; }
    public DbSet<OfficeAssignment> OfficeAssignments { get; set; } 
 
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        // Configure Code First to ignore PluralizingTableName convention
        // If you keep this convention then the generated tables will have pluralized names.
        modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
    }
} 
 
public class Department
{
    public Department()
    {
        this.Courses = new HashSet<Course>();
    }
    // Primary key
    public int DepartmentID { get; set; }
    public string Name { get; set; }
    public decimal Budget { get; set; }
    public System.DateTime StartDate { get; set; }
    public int? Administrator { get; set; } 
 
    // Navigation property
    public virtual ICollection<Course> Courses { get; private set; }
} 
 
public class Course
{
    public Course()
    {
        this.Instructors = new HashSet<Instructor>();
    }
    // Primary key
    public int CourseID { get; set; } 
 
    public string Title { get; set; }
    public int Credits { get; set; } 
 
    // Foreign key
    public int DepartmentID { get; set; } 
 
    // Navigation properties
    public virtual Department Department { get; set; }
    public virtual ICollection<Instructor> Instructors { get; private set; }
} 
 
public partial class OnlineCourse : Course
{
    public string URL { get; set; }
} 
 
public partial class OnsiteCourse : Course
{
    public OnsiteCourse()
    {
        Details = new Details();
    } 
 
    public Details Details { get; set; }
} 
 
public class Details
{
    public System.DateTime Time { get; set; }
    public string Location { get; set; }
    public string Days { get; set; }
} 
 
public class Instructor
{
    public Instructor()
    {
        this.Courses = new List<Course>();
    } 
 
    // Primary key
    public int InstructorID { get; set; }
    public string LastName { get; set; }
    public string FirstName { get; set; }
    public System.DateTime HireDate { get; set; } 
 
    // Navigation properties
    public virtual ICollection<Course> Courses { get; private set; }
} 
 
public class OfficeAssignment
{
    // Specifying InstructorID as a primary
    [Key()]
    public Int32 InstructorID { get; set; } 
 
    public string Location { get; set; } 
 
    // When the Entity Framework sees Timestamp attribute
    // it configures ConcurrencyCheck and DatabaseGeneratedPattern=Computed.
    [Timestamp]
    public Byte[] Timestamp { get; set; } 
 
    // Navigation property
    public virtual Instructor Instructor { get; set; }
}