A relationship in Entity framework is that there is a
situation that exists between two database tables in which one table has a
foreign key and the foreign key has reference the primary key(PK)
Of the another
table, A relationship allow relationship database to split and store data in
different tables. for example if we want to store student private information
and student education information then we need two tables.
One table for student private information and another
table for student education information.
In Entity framework there are 3 type of relationship
- One-to-One
Relationships
- One-to-many/Many to One Relationships
- Many-to-Many
Relationships
One-to-One Relationships :
Here we will understand
one to one Entity relationship b/w student and StudentCollageInfo. As One-to-
one relationship happens when primary key of one table become PK and FK of
another table.
In our example StudentID is the PK(primary key) of student table and
StudentID will be Foreign key (FK) in StudentCollageInfo table for the
relationship b/w the tables.
Notice that in SqlServer One To One relationship
technically not possible it always One to Zero or Zero.
Firstly we will understand this with Code first
approach for this we have to declare property.
public class Student
{
public Student() { }
public int
StudentId { get; set; }
public string
StudentName { get; set; }
public virtual StudentCollageInfo _studentCollageInfo { get; set; }
}
public class StudentCollageInfo
{
[Key, ForeignKey("Student")] // This is dataAnoation we are using here
public int
StudentId { get; set; }
public string CollageName
{ get; set; }
public string StudentBranch
{ get; set; }
public virtual Student Student { get; set; }
}
One-to-Zero-or-One entity relationship using Fluent API:
The below example for one-to-zero or one
relationship between Student and StudentAddress using Fluent API.
Public void OnModelCreating(DbModelBuilder modelBuilder)
{
//
Configure StudentId as PK for StudentAddress
modelBuilder.Entity<StudentCollageInfo>()
.HasKey(e => e.StudentId);
// StudentId
as FK for StudentCollageInfo
modelBuilder.Entity<Student>()
.HasOptional(s => s._studentCollageInfo)
// Mark StudentCollageInfo is optional
for Student
.WithRequired(ad =>
ad.Student); // it will Create inverse relationship
}
In above example, we set primary key(PK) to
StudentId for Studentcollageinfo because it doen't follow Code First conventions
for primary(PK). Then in second line, HasOptional method make StudentCollageInfo
property nullable then WithRequired method will create inverse relationship by
making StudentId column as forgion key in StudentcollageInfo table. Thus,
StudentId will be PK and FK in StudentAddress which makes one-to-zero or one
relationship.
Alternatively, we can also configure Studentcollageinfo entity as shown below.
Public void OnModelCreating(DbModelBuilder _builder)
{
//
Configure StudentId as priamry for Studentcollageinfo
_builder.Entity<StudentCollageInfo>()
.HasKey(e => e.StudentId);
//
Configure StudentId as FK for Studentcollageinfo
_builder.Entity<StudentCollageInfo>()
.HasRequired(c => c.Student)
.WithOptional(c => c._studentCollageInfo);
}
