Entity

What is an Entity in Entity Framework?

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

Here we are taking the example of Student and Grade domain classes in the school application.

    public class Student
    {
        public int StuID { get; set; }
        public string StuName { 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 GrdId { get; set; }
        public string GrdName { get; set; }
        public string Section { get; set; }

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

The above class becomes the entity when they are included as DbSetproperties in a context class (Class which is 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 are the type of DbSet<TEntity> which is called as the set of entity. Student and Grades are entity. Entity Framework API will create the Students and Grades tables in a database.

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

Scalar Property: The type of primitive property is called scalar properties. Each scalar property maps to a column in the database table which stores the real data. For example, StudentID, StudentName, DateofBirth, height, and 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 decimal height { get; set; }
        public float weight { get; set; }
     //reference navigation properties
        public Grade Grade { get; set; }
    }

Entity Framework API will create a column in the database table for each scalar property.

Navigation Property

Navigation Property represents the relationship with another entity. We have two types of Navigation property: Reference Navigation, Collection Navigation.

Reference Navigation Property

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

Entity Framework API will create a ForeignKey column in the table for the navigation properties that point to a PrimaryKey of another table in the database. For example, Grade is 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 decimal Height { get; set; }
        public float Weight { get; set; }

        //reference navigation property
        public Grade Grade { get; set; }
    }

In the database, Entity Framework API will create a ForeignKey Grade_GradeID in the Student Table.

Collection Navigation Property

If an entity includes a property of the generic collection of an entity type, then it is called a collection navigation property.

Entity Framework API does not create any column for the navigation property in the related table of an entity. Still, it creates a column in the table of the generic collection entity. For example, a Grade entity contains a generic collection navigation property ICollection<Student>. Here the Student entity is specified as a generic type so that Entity Framework API will create a column GradeID in the Student table in the database.

Next TopicEntity Framework Core

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