Efficient way of updating list of entities

You can try the follwoing to minimize queries:

using (var ctx = new MyContext())
{
    var entityDict = ctx.Entities
        .Where(e => viewModels.Select(v => v.ID).Contains(e.ID))
        .ToDictionary(e => e.ID); // one DB query

    foreach (var viewModel in viewModels)
    {
        Entity entity;
        if (entityDict.TryGetValue(viewModel.ID, out entity))
            entity.Value = viewModel.Value;
    }

    ctx.SaveChanges(); //single transaction with multiple UPDATE statements
}

Be aware that Contains can be potentially slow if the list of viewModels is very long. But it will only run a single query.

Here are two ways I know of to update an entity in the database without doing a retrieval of the entity first:

//Assuming person is detached from the context
//for both examples
public class Person
{
  public int Id { get; set; }
  public string Name { get; set; }
  public DateTime BornOn { get; set; }   
}

public void UpdatePerson(Person person)
{
  this.Context.Persons.Attach(person)
  DbEntityEntry<Person> entry = Context.Entry(person);
  entry.State = System.Data.EntityState.Modified;
  Context.SaveChanges();
}

Should yield:

Update [schema].[table]
Set Name = @p__linq__0, BornOn = @p__linq__1
Where id = @p__linq__2

Or you can just specify fields if you need to (probably good for tables with a ton of columns, or for security purposes, allows only specific columns to be updated:

public void UpdatePersonNameOnly(Person person)
{
  this.Context.Persons.Attach(person)
  DbEntityEntry<Person> entry = Context.Entry(person);
  entry.Property(e => e.Name).IsModified = true;
  Context.SaveChanges();
}

Should yield:

Update [schema].[table]
Set Name = @p__linq__0
Where id = @p__linq__1

Doesn't the .Attach() go to the database to retrieve the record first and then merges your changes with it ? so you end up with roundtrip anyway

No. We can test this

using System;
using System.Data.Entity;
using System.Linq;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations.Schema;
using System.ComponentModel.DataAnnotations;

public class Program
{
    public static void Main()
    {

        var movie1 = new Movie { Id = 1, Title = "Godzilla" };
        var movie2 = new Movie { Id = 2, Title = "Iron Man" };
        using (var context = new MovieDb())
        {
            /*
            context.Database.Log = (s) => {
                Console.WriteLine(s);
            };
            */

            Console.WriteLine("========= Start Add: movie1 ==============");
            context.Movies.Add(movie1);
            context.SaveChanges();
            Console.WriteLine("========= END Add: movie1 ==============");

            // LET EF CREATE ALL THE SCHEMAS AND STUFF THEN WE CAN TEST

            context.Database.Log = (s) => {
                Console.WriteLine(s);
            };

            Console.WriteLine("========= Start SELECT FIRST movie ==============");
            var movie1a = context.Movies.First();
            Console.WriteLine("========= End SELECT FIRST movie ==============");

            Console.WriteLine("========= Start Attach Movie2 ==============");
            context.Movies.Attach(movie2);
            Console.WriteLine("========= End Attach Movie2 ==============");

            Console.WriteLine("========= Start SELECT Movie2 ==============");
            var movie2a = context.Movies.FirstOrDefault(m => m.Id == 2);
            Console.WriteLine("========= End SELECT Movie2 ==============");
            Console.Write("Movie2a.Id = ");
            Console.WriteLine(movie2a == null ? "null" : movie2a.Id.ToString());
        }
    }

    public class MovieDb : DbContext
    {
        public MovieDb() : base(FiddleHelper.GetConnectionStringSqlServer()) {}
        public DbSet<Movie> Movies { get; set; }
    }

    public class Movie
    {
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.None)]
        public int Id { get; set; }

        public string Title { get; set; }
    }
}

If attach makes any DB calls, we will see them between the Start Attach Movie2 and End Attach Movie2. We also verify that the documentation that states:

Remarks

Attach is used to repopulate a context with an entity that is known to already exist in the database.

SaveChanges will therefore not attempt to insert an attached entity into the database because it is assumed to already be there.

After attaching the movie2, we can attempt to select it from the DB. It should not be there (because EF only assumes it is there).

========= Start Add: movie1 ==============

========= END Add: movie1 ==============

========= Start SELECT FIRST movie ==============

Opened connection at 1/15/2020 5:29:23 PM +00:00

SELECT TOP (1)

[c].[Id] AS [Id],

[c].[Title] AS [Title]

FROM [dbo].[Movies] AS [c]

-- Executing at 1/15/2020 5:29:23 PM +00:00

-- Completed in 23 ms with result: SqlDataReader

Closed connection at 1/15/2020 5:29:23 PM +00:00

========= End SELECT FIRST movie ==============

========= Start Attach Movie2 ==============

========= End Attach Movie2 ==============

========= Start SELECT Movie2 ==============

Opened connection at 1/15/2020 5:29:23 PM +00:00

SELECT TOP (1)

[Extent1].[Id] AS [Id],

[Extent1].[Title] AS [Title]

FROM [dbo].[Movies] AS [Extent1]

WHERE 2 = [Extent1].[Id]

-- Executing at 1/15/2020 5:29:23 PM +00:00

-- Completed in 2 ms with result: SqlDataReader

Closed connection at 1/15/2020 5:29:23 PM +00:00

========= End SELECT Movie2 ==============

Movie2a.Id = null

So no SQL called during the attach, no error message attaching it, and it's not in the database.