How to tell Entity Framework that my ID column is auto-incremented (AspNet Core 2.0 + PostgreSQL)?


Code is simple. Tag.cs entity:

public partial class Tag
    public int Id { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }

HomeController.cs class:

public async Task<IActionResult> Index()
   tagRepository.Insert(new Tag 
                   Name = "name", 
                   Description = "description" 
   await UnitOfWork.SaveChangesAsync();  // calls dbContext.SaveChangesAsync()

   return View();

TagRepository.cs class:

    // Context it's a usual DBContext injected via Repository's constructor
    public virtual void Insert(TEntity item)
                => Context.Entry(item).State = EntityState.Added;

Tag table was created by running:

    Name text NOT NULL,
    Description text NULL

When run my application I get an error:

fail: Microsoft.EntityFrameworkCore.Database.Command[20102]
      Failed executing DbCommand (28ms) [Parameters=[@p0='?', @p1='?', @p2='?'], CommandType='Text', CommandTimeout='30']
      INSERT INTO "tag" ("id", "description", "name")
      VALUES (@p0, @p1, @p2);
Npgsql.PostgresException (0x80004005): 23505: duplicate key value violates unique constraint "tag_pkey"
   at Npgsql.NpgsqlConnector.<DoReadMessage>d__157.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at System.Runtime.CompilerServices.ValueTaskAwaiter`1.GetResult()
   at Npgsql.NpgsqlConnector.<ReadMessage>d__156.MoveNext()

As you can see Entity Framework tries to send id=0 value to the DB, but there is already a record with id=0 in my DB and that's why it throws duplicate key error.

I didn't find any answer so far and my question is: how can I get rid of this error and tell Entity Framework that my id column is auto-incremented and there is no need to update it?
asked on Stack Overflow May 6, 2018 by Andrey Kotov • edited May 9, 2018 by Andrey Kotov

3 Answers


You have to use here "ValueGenerationOnAdd()". As the issue you are getting is already reported on GitHub. Please find the below link.

You can find more info regarding Generated Value pattern from following link.

Value generated on add

public classs SampleContext:DBContext{
public DbSet<Tag> Tag { get; set; }

protected override void OnModelCreating(ModelBuilder modelBuilder){
        .Property(p => p.ID)
public class Tag{
  public int Id { get; set; }
  public string Name { get; set; }
  public string Description{get;set;}


Hope this will help

answered on Stack Overflow May 6, 2018 by Trilok Kumar • edited May 6, 2018 by Trilok Kumar

According to the Postgre SQL docs here is an example which shows how to achieve the desired result:

protected override void OnModelCreating(ModelBuilder modelBuilder)
   => modelBuilder.Entity<Blog>().Property(b => b.Id).UseIdentityAlwaysColumn();
answered on Stack Overflow May 11, 2020 by Yaroslav Trofimov

After a lot of wasted time on research I've solved it absolutely unexpected. Actually, the solution to my problem lies on the surface, because the problem is not in EF and not in it's stuff. In my case it was SEQUENCE. I mean, involved table has serial column and related sequence just was changed by some side-effect. In another words - sequence was restarted from 1 and at the same time the table is already having values with 1, 2, 3 ... etc. That's why postgres throws 'duplicate key' error. So, the solution is:

ALTER SEQUENCE "your_table_seq" RESTART WITH your_value;
answered on Stack Overflow Jun 6, 2020 by anatol

User contributions licensed under CC BY-SA 3.0