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Entity Framework

Entity

Types of Entities

Entity State

An entity in Entity Framework is a class that maps to a database table. This class must be included as a DbSet<TEntity> type property in the DbContext class. EF API maps each entity to a table and each property of an entity to a column in the database.

For example, the following Student, and Grade are domain classes in the school application.

public class Student

{

public int StudentID { get; set; }

public string StudentName { get; set; }

public DateTime? DateOfBirth { get; set; }

public byte[] Photo { get; set; }

public decimal Height { get; set; }

public float Weight { get; set; }

public Grade Grade { get; set; }

}

public class Grade

{

public int GradeId { get; set; }

public string GradeName { get; set; }

public string Section { get; set; }

public ICollection<Student> Students { get; set; }

}

The above classes become entities when they are included as DbSet<TEntity> properties in a context class (the class which derives from DbContext), as shown below.

public class SchoolContext : DbContext

{

public SchoolContext()

{

}

public DbSet<Student> Students { get; set; }

public DbSet<Grade> Grades { get; set; }

}

In the above context class, Students, and Grades properties of type DbSet<TEntity> are called entity sets. The Student, and Grade are entities. EF API will create the Students and Grades tables in the database, as shown below.

⁠

An Entity can include two types of properties: Scalar Properties and Navigation Properties.

Scalar Property

The primitive type properties are called scalar properties. Each scalar property maps to a column in the database table which stores an actual data. For example, StudentID, StudentName, DateOfBirth, Photo, Height, Weight are the scalar properties in the Student entity class.

public class Student

{

// scalar properties

public int StudentID { get; set; }

public string StudentName { get; set; }

public DateTime? DateOfBirth { get; set; }

public byte[] Photo { get; set; }

public decimal Height { get; set; }

public float Weight { get; set; }

//reference navigation properties

public Grade Grade { get; set; }

}

EF API will create a column in the database table for each scalar property, as shown below.

⁠

Navigation Property

The navigation property represents a relationship to another entity.

There are two types of navigation properties: Reference Navigation and Collection Navigation

Reference Navigation Property

If an entity includes a property of another entity type, it is called a Reference Navigation Property. It points to a single entity and represents multiplicity of one (1) in the entity relationships.

EF API will create a ForeignKey column in the table for the navigation properties that points to a PrimaryKey of another table in the database. For example, Grade are reference navigation properties in the following Student entity class.

public class Student

{

// scalar properties

public int StudentID { get; set; }

public string StudentName { get; set; }

public DateTime? DateOfBirth { get; set; }

public byte[] Photo { get; set; }

public decimal Height { get; set; }

public float Weight { get; set; }

//reference navigation property

public Grade Grade { get; set; }

}

In the database, EF API will create a ForeignKey Grade_GradeId in the Students table, as shown below.

⁠

⁠

Collection Navigation Property

If an entity includes a property of generic collection of an entity type, it is called a collection navigation property. It represents multiplicity of many (*).

EF API does not create any column for the collection navigation property in the related table of an entity, but it creates a column in the table of an entity of generic collection. For example, the following Grade entity contains a generic collection navigation property ICollection<Student>. Here, the Student entity is specified as generic type, so EF API will create a column Grade_GradeId in the Students table in the database.

⁠

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