Entity framework is an Object/Relational Mapping (O/RM) framework. It is an enhancement to ADO.NET that gives developers an automated mechanism for accessing & storing the data in the database. ORM framework automatically creates classes based on database tables and the opposite is also true; that is, it can also automatically generate necessary SQL to create database tables based on classes.

First we need to install Entity framework and the best way to install that is by using: -NuGet Package Manager.
Code first approach allows us to create our custom classes first and based on those classes entity framework can generate database automatically for us.
 
Step 1
Create New Project - Select Web Application- Select MVC Template - Cick OK.
 
Step 2
Go toTools - NuGet Package Manager - Manage NuGet Packages For Solution - Browse and type Entity Framework click on Install
 
Step 3
Add the following classes in the Model Folder of your project.
    using System.Collections.Generic;  
    using System.ComponentModel.DataAnnotations;  
    using System.ComponentModel.DataAnnotations.Schema;  
    namespace CodeFirstApproach.Models  
    {  
    [Table("tblClass")]  
    public class Class  
    {  
       [Key]  
       public int ClassID { get; set; }  
       public string ClassName { get; set; }  
       public List<Student> Students { get; set; }  
       public List<Course> Courses { get; set; }  
    }  
    }   
       
       
    [Table("tblStudent")]  
    public class Student  
    {  
       [Key]  
       public int StudentID { get; set; }  
       public string StudentName { get; set; }  
       public Class Class { get; set; }  
    }  
       
    [Table("tblCourse")]  
    public class Course  
    {  
       [Key]  
       public int CourseID { get; set; }  
       public string CourseName { get; set; }  
       public List<Class> Classes { get; set; }  
    }  

Step 4
Add another class as a Context Class and add connection string in web.config. In class add DbSet where each DbSet will map to a table in the database. If we have a property DbSet of Students, and the name of that property is Students, the Entity Framework will by default look for a Student table inside the database which in our case is tblStudent as we used [Table("tblStudent")] as Annotation .
    public class StudentContext: DbContext[DBContext maps to a specific database that has a schema that the DBContext understands] {  
        public StudentContext(): base("name=conn_StudentDB") {}  
        public DbSet < Class > Classes {  
            get;  
            set;  
        }  
        public DbSet < Student > Students {  
            get;  
            set;  
        }  
        public DbSet < Course > Courses {  
            get;  
            set;  
        }  
    }  
    protected override void OnModelCreating(DbModelBuilder modelBuilder) {  
        modelBuilder.Entity < Class > ().HasMany(s => s.Courses).WithMany(c => c.Classes).Map(cs => {  
            cs.MapLeftKey("ClassId");  
            cs.MapRightKey("CourseId");  
            cs.ToTable("tblClassCourse");  
        });  
    }  
    }  

Step 4
Add another class as a Context Class and add connection string in web.config.In class add DbSet where each DbSet will map to a table.
 
Note
We could also say that DBSet class represents an entity set that is used to create, read, update, and delete operations.
 
Web.Config
In the database, if we have a property DbSet of Students, and the name of that property is Students, the Entity Framework will by default look for an Student table inside the database which in our case is tblStudent as we used [Table("tblStudent")] as Annotation .
    <connectionStrings>  
       <add name="conn_StudentDB" connectionString="data source=.;initial catalog=StudentDB;integrated security=True;MultipleActiveResultSets=True;" providerName="System.Data.SqlClient" />  
    </connectionStrings>  

Step 5
 
In HomeController Index Action write the following code and Run (Press F5)

    public ActionResult Index()  
    {  
        StudentContext context = new StudentContext();  
         var x = (from z in context.Courses select z).ToList();  
        return View();  
    }  

Observation
 
It will create the database with all the tables.
 
Point to Start
 
Another Approach is Code Based Migration - Drop the database and follow the step below.
 
Step 1
Entity Framework Code First Migration
 
Go To Package Manager Console and type ,
 
PM> Enable-Migrations
 
Note
It will create a Configuration file in the Migration Folder.
 
Step 2
 
PM> Add-Migration -Name CreateDb
 
Note
It will create a Script in the Migration Folder with xxx_CreateDb.cs. It will consist of two methods,  Up() and Down().
    public partial class CreateDb: DbMigration {  
        public override void Up() {  
            //code to execute when you Update the DB.  
        }  
        public override void Down() {  
            //code to execute when you Rollback.   
        }  
    }  

Step3 - PM> Update-Database
We will see our Database getting created once again just like the image above.
 
Currently we have no data. Insert a few records in the following table.
    insert into [dbo].[tblClass] values('Computer Science'),('Electronics And Communication'),('Mechanical')  
    insert into [dbo].[tblStudent]values('Peter',1),('Scott',2),('Daniel',3),('Raymond',1),('Anthony',2),('Greg',3)  
    insert into [dbo].[tblCourse] values('.Net'),('SQL'),('Java'),('VLSI'),('Analog Communication'),('Optical Fiber Communication'),('Thermodynamics'),('Measurement and Instructions'),('Advanced Solid Mechanics')  
    insert into [dbo].[tblClassCourse] values(1,1),(1,2),(1,3),(2,4),(2,5),(2,6),(2,3),(3,7),(3,8),(3,9),(3,3),(1,5),(3,5);  

Updating Table
 
Step 1
 
Now we will add a property in Student class PhoneNo
    [Table("tblStudent")]  
    public class Student {  
        public int StudentID {  
            get;  
            set;  
        }  
        public string StudentName {  
            get;  
            set;  
        }  
        [Required]  
        public string PhoneNo {  
            get;  
            set;  
        }  
        public Class Class {  
            get;  
            set;  
        }  
    }  

Step 2
 
Go to Console and Add-Migration again for the changes in our model which will create _AddedProperty_tblStudent.cs inside the configuration folder.
 
PM> Add-Migration -Name AddedProperty_tblStudent
    public partial class AddedProperty_tblStudent: DbMigration {  
        public override void Up() {  
            AddColumn("dbo.tblStudent", "PhoneNo", c => c.String(nullable: false));  
            //Setting default value for any property  
            //AddColumn("dbo.tblStudent", "PhoneNo", c => c.String(nullable:true,defaultValue:"NULL"));      [Do not use Required Annotation then]  
            //AddColumn("dbo.tblStudent", "JoinDate", c => c.DateTime(nullable: false, defaultValueSql:"GetDate()"));  
        }  
        public override void Down() {  
            DropColumn("dbo.tblStudent", "PhoneNo");  
        }  
    }  

Step 3
 
PM> Update-Database. Check the following changes in DB for tblStudent.
 
RollBack - Update-Database -TargetMigration:"CreateDb"

ORMs like Entity Framework have always been shrouded in a bit of controversy from SQL purists who aren't in control of the types of queries that these tools are building behind the scenes. While this is a perfectly valid stance to take, these same tools can often assist in getting things done much faster than tinkering with an endless chain of JOINs, sub-queries, etc.

But what if you want to know exactly what is going on behind the scenes? What SQL is being generated by Entity Framework? And is it efficient enough to suit your needs, or do you need to handle writing some yourself?

This post will cover a quick tip that will allow you to see the SQL behind the scenes and judge for yourself using Entity Framework Core.

How Did This Work Prior to Entity Framework Core?

Previously, you could use Reflection to create an ObjectQuery object and then call the ToTraceString()method to actually store the query results as seen below:

// Build a query using Entity Framework
var query = _context.Widgets.Where(w => w.IsReal && w.Id == 42); 
// Get the generated SQL
var sql = ((System.Data.Objects.ObjectQuery)query).ToTraceString(); 

And that's really it. The result of the ToTraceString() call will return a string variable containing the entire SQL query being executed.

Options for Entity Framework Core

The previous approach no longer works within the Entity Framework Core (EF7) world, so we have to resort to one of three options, which may vary depending on your needs:

• Using Built-in or Custom Logging. Logging the executing query using your logger of choice or the built-in Logger in .NET Core as mentioned in this tutorial.
• Using a Profiler. Using an SQL Profiler like MiniProfiler to monitor the executing query.
• Using Crazy Reflection Code. You can implement some custom reflection code similar to the older approach to perform the same basic concept.

Since both of the first two options are fairly well documented, we will be focusing on the crazy reflection approach.

Getting Behind the Scenes in Entity Framework Core

Using the following snippets of code, which rely on Reflection to resolve information about the compiler, parser, database, and fields being targeted, we can use those things to reconstruct what is actually going on behind the scenes.

NOTE: Since this relies on Reflection, it may be subject to breaking in the future, especially with any API changes.

public class IQueryableExtensions 
{
    private static readonly FieldInfo QueryCompilerField =
    typeof(EntityQueryProvider).GetTypeInfo().DeclaredFields.First
    (x => x.Name == "_queryCompiler");

    private static readonly PropertyInfo NodeTypeProviderField =
    QueryCompilerTypeInfo.DeclaredProperties.Single(x => x.Name == "NodeTypeProvider");

    private static readonly MethodInfo CreateQueryParserMethod =
    QueryCompilerTypeInfo.DeclaredMethods.First(x => x.Name == "CreateQueryParser");

    private static readonly FieldInfo DataBaseField =
    QueryCompilerTypeInfo.DeclaredFields.Single(x => x.Name == "_database");

    private static readonly FieldInfo QueryCompilationContextFactoryField =
    typeof(Database).GetTypeInfo().DeclaredFields.Single
    (x => x.Name == "_queryCompilationContextFactory");

    public static string ToSql<TEntity>
    (this IQueryable<TEntity> query) where TEntity : class
    {
        if (!(query is EntityQueryable<TEntity>)
        && !(query is InternalDbSet<TEntity>))
        {
            throw new ArgumentException("Invalid query");
        }

        var queryCompiler = (IQueryCompiler)QueryCompilerField.GetValue(query.Provider);
        var nodeTypeProvider =
        (INodeTypeProvider)NodeTypeProviderField.GetValue(queryCompiler);
        var parser = (IQueryParser)CreateQueryParserMethod.Invoke
        (queryCompiler, new object[] { nodeTypeProvider });
        var queryModel = parser.GetParsedQuery(query.Expression);
        var database = DataBaseField.GetValue(queryCompiler);
        var queryCompilationContextFactory =
        (IQueryCompilationContextFactory)QueryCompilationContextFactoryField.GetValue(database);
        var queryCompilationContext = queryCompilationContextFactory.Create(false);
        var modelVisitor =
        (RelationalQueryModelVisitor)queryCompilationContext.CreateQueryModelVisitor();
        modelVisitor.CreateQueryExecutor<TEntity>(queryModel);
        var sql = modelVisitor.Queries.First().ToString();

        return sql;
    }
}

And as far as actual usage goes, you would simply call the ToSql() method to return your SQL query string:

// Build a query using Entity Framework
var query = _context.Widgets.Where(w => w.IsReal && w.Id == 42); 
// Get the generated SQL
var sql = query.ToSql();