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Entity Relationships:

Here, you will learn how entity framework manages the relationships between entities.

Entity framework supports three types of relationships, same as database: 1) One to One 2) One to Many, and 3) Many to Many.

We have created an Entity Data Model for the SchoolDB database in the Create Entity Data Modelsection. The following figure shows the visual designer for that EDM with all the entities and relationships among them.

Let‘s see how each relation (association) is being managed by entity framework.

One-to-One Relationship:

As you can see in the above figure, Student and StudentAddress have a One-to-One relationship (zero or one). A student can have only one or zero address. Entity framework adds Student navigation property into StudentAddress entity and StudentAddress navigation entity into Student entity. Also, StudentAddress entity has StudentId property as PrimaryKey which makes it a One-to-One relationship.

The following code snippet shows Student and StudentAddress entity classes.

public partial class Student
{
    public Student()
    {
        this.Courses = new HashSet<Course>();
    }
    
    public int StudentID { get; set; }
    public string StudentName { get; set; }
    public Nullable<int> StandardId { get; set; }
    public byte[] RowVersion { get; set; }
    
    public virtual Standard Standard { get; set; }
    public virtual StudentAddress StudentAddress { get; set; }
    public virtual ICollection<Course> Courses { get; set; }
}
    
public partial class StudentAddress
{
    public int StudentID { get; set; }
    public string Address1 { get; set; }
    public string Address2 { get; set; }
    public string City { get; set; }
    public string State { get; set; }
    
    public virtual Student Student { get; set; }
}

As you can see in the above code, Student entity class includes StudentAddress navigation property and StudentAddress includes Student navigation property with foreign key property StudentId. This way EF handles one-to-one relationship between entities.

One-to-Many Relationship:

The Standard and Teacher entities have a One-to-Many relationship marked by multiplicity where 1 is for One and * is for many. This means that Standard can have many Teachers whereas Teacher can associate with only one Standard.

To represent this, The Standard entity has the collection navigation property Teachers (please notice that it‘s plural), which indicates that one Standard can have a collection of Teachers (many teachers). And Teacher entity has a Standard navigation property (Not a Collection) which indicates that Teacher is associated with one Standard. Also, it contains StandardId foreign key (StandardId is a PK in Standard entity). This makes it One-to-Many relationship.

The following code snippet shows Standard and Teacher entity class created by EDM.

public partial class Standard
{
    public Standard()
    {
        this.Students = new HashSet<Student>();
        this.Teachers = new HashSet<Teacher>();
    }
    
    public int StandardId { get; set; }
    public string StandardName { get; set; }
    public string Description { get; set; }
    
    public virtual ICollection<Student> Students { get; set; }
    public virtual ICollection<Teacher> Teachers { get; set; }
}

public partial class Teacher
{
    public Teacher()
    {
        this.Courses = new HashSet<Course>();
    }
    
    public int TeacherId { get; set; }
    public string TeacherName { get; set; }
    public Nullable<int> StandardId { get; set; }
    public Nullable<int> TeacherType { get; set; }
    
    public virtual ICollection<Course> Courses { get; set; }
        
    public virtual Standard Standard { get; set; }
}

As you can see in the above code snippet, Standard entity class has Teachers property of type ICollection, so that it can contain multiple Teacher objects. (It initializes Teachers property with HashSet<Teacher> in the constructor, so that you can add Teacher objects into collection without worrying about initializing it.)

Also, Teacher entity class includes Standard property with StandardId for foreign key property. Entity framework includes this foreign key property because we checked Include foreign key columns in the model in the EDM wizard while creating EDM in the Create Entity Data Model section.

Many-to-Many Relationship:

Student and Course have Many-to-Many relationships marked by * multiplicity. It means one Student can enrol for many Courses and also, one Course can be be taught to many Students.

The database design includes StudentCourse joining table which includes the primary key of both the tables (Student and Course table). Entity Framework represents many-to-many relationships by not having entityset for the joining table in CSDL, instead it manages this through mapping.

As you can see in the above figure, Student entity includes Courses property and Course entity includes Students property to represent many-to-many relationship between them.

The following code snippet shows Student and Course entity classes.

public partial class Student
{
    public Student()
    {
        this.Courses = new HashSet<Course>();
    }
    
    public int StudentID { get; set; }
    public string StudentName { get; set; }
    public Nullable<int> StandardId { get; set; }
    public byte[] RowVersion { get; set; }
    
    public virtual Standard Standard { get; set; }
    public virtual StudentAddress StudentAddress { get; set; }
    public virtual ICollection<Course> Courses { get; set; }
}
    
public partial class Course
{
    public Course()
    {
        this.Students = new HashSet<Student>();
    }
    
    public int CourseId { get; set; }
    public string CourseName { get; set; }
    public System.Data.Entity.Spatial.DbGeography Location { get; set; }
    public Nullable<int> TeacherId { get; set; }
    
    public virtual Teacher Teacher { get; set; }
    public virtual ICollection<Student> Students { get; set; }
}

Note: Entity framework supports many-to-many relationship only when the joining table (StudentCourse in this case) does NOT include any columns other than PKs of both the tables. If the join tables contain additional columns, such as DateCreated, then the EDM creates entity for middle table as well and you will have to manage CRUD operation for many-to-many entities manually.

Open EDM in XML view. You can see that SSDL has StudentCourse entityset, but CSDL doesn‘t have StudentCourse entityset. Instead, it‘s being mapped in the navigation property of the Student and Course entity. In MSL (C-S Mapping), it has mapping between Student and Course put into the StudentCourse table in <AssociationSetMapping/>

Thus, Many-to-Many relationship is being managed by C-S mapping in EDM. So when you add a Student in a Course or a Course in a Student entity and when you save it, it will then insert PK of the added student and course in StudentCourse table. So this mapping not only enables a convenient association directly between the two entities, but also manages querying, inserts, and updates across this joint.

Entity Graph:

When an entity has a relationship with other entities, then the full object hierarchy is called an ‘entity graph‘. For example the following is a Student entity graph, that includes hierarchy of Student entity with Standard, StudentAddress & Course entities.

Entity Relationships

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原文地址：http://www.cnblogs.com/purplefox2008/p/5646466.html