Pomelo.entityframeworkcore.mysql: Adding entity to DB with predefined "Id" column results in generating wrong Id in DB

Can you share the MySQL schema of Category table? It's not entirely clear what behavior you need, is it that the id is user supplied, or generated automatically in DB with auto_increment? If the former, then you don't need ValueGeneratedOnAdd Method.

Can you share the MySQL schema of Category table? It's not entirely clear what behavior you need, is it that the id is user supplied, or generated automatically in DB with auto_increment? If the former, then you don't need ValueGeneratedOnAdd Method.

Well for example when I use MS SQL, if I send a POST request with data like this:

{
    "Id":12137123,
    "Name":"Some Category"
}

this random Id will be omitted when creating entry in database and correct will be set in ID column. The point of all that is if malicious user tries to put a wrong id, my db will be broken

MySQL will use any ID value you supply. Only if you don't supply a value MySQL will generate an auto_increment value for you. When it is generating an ID, it will use the highest value already in the table (or 0 otherwise) as the seed.

Take a look at 3.6.9 Using AUTO_INCREMENT in the MySQL docs:

The AUTO_INCREMENT attribute can be used to generate a unique identity for new rows:
[...]
No value was specified for the AUTO_INCREMENT column, so MySQL assigned sequence numbers automatically. You can also explicitly assign 0 to the column to generate sequence numbers
[...]
If the column is declared NOT NULL, it is also possible to assign NULL to the column to generate sequence numbers.
[...]
When you insert any other value into an AUTO_INCREMENT column, the column is set to that value and the sequence is reset so that the next automatically generated value follows sequentially from the largest column value.

The following code demonstrates that:

```c#
using System.Diagnostics;
using System.Linq;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Logging;

namespace IssueConsoleTemplate
{
public class IceCream
{
public int IceCreamId { get; set; }
public string Name { get; set; }
}

public class Context : DbContext
{
    public DbSet<IceCream> IceCreams { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder
            .UseMySql(
                "server=127.0.0.1;port=3306;user=root;password=;database=Issue1092",
                b => b.ServerVersion("8.0.20-mysql"))
            .UseLoggerFactory(
                LoggerFactory.Create(b => b
                        .AddConsole()
                        .AddFilter(level => level >= LogLevel.Information)))
            .EnableSensitiveDataLogging()
            .EnableDetailedErrors();
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<IceCream>(
            entity =>
            {
                // Because the property contains "Id", the following lines are added automatically,
                // so no need to specify them explicitly (does not do any harm however):
                // entity.Property(e => e.IceCreamId)
                //    .ValueGeneratedOnAdd();
            });
    }
}

internal class Program
{
    private static void Main()
    {
        using var context = new Context();

        context.Database.EnsureDeleted();
        context.Database.EnsureCreated();

        context.IceCreams.AddRange(
            new IceCream
            {
                IceCreamId = 42, // explicitly set ID
                Name = "Vanilla",
            },
            new IceCream
            {
                // <-- let MySQL use it's auto_increment behavior
                Name = "Chocolate",
            });

        context.SaveChanges();

        var iceCreams = context.IceCreams
            .OrderBy(i => i.IceCreamId)
            .ToList();

        Debug.Assert(iceCreams.Count == 2);
        Debug.Assert(iceCreams[0].IceCreamId == 42);
        Debug.Assert(iceCreams[0].Name == "Vanilla");
        Debug.Assert(iceCreams[1].IceCreamId == 43);
        Debug.Assert(iceCreams[1].Name == "Chocolate");
    }
}

}

It generates the following SQL:

```sql
info: Microsoft.EntityFrameworkCore.Infrastructure[10403]
      Entity Framework Core 3.1.3 initialized 'Context' using provider 'Pomelo.EntityFrameworkCore.MySql' with options: ServerVersion 8.0.20 MySql SensitiveDataLoggingEnabled DetailedErrorsEnabled

info: Microsoft.EntityFrameworkCore.Database.Command[20101]
      Executed DbCommand (12ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
      CREATE DATABASE `Issue1092`;

info: Microsoft.EntityFrameworkCore.Database.Command[20101]
      Executed DbCommand (56ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
      CREATE TABLE `IceCreams` (
          `IceCreamId` int NOT NULL AUTO_INCREMENT,
          `Name` longtext CHARACTER SET utf8mb4 NULL,
          CONSTRAINT `PK_IceCreams` PRIMARY KEY (`IceCreamId`)
      );

info: Microsoft.EntityFrameworkCore.Database.Command[20101]
      Executed DbCommand (8ms) [Parameters=[@p0='42', @p1='Vanilla' (Size = 4000)], CommandType='Text', CommandTimeout='30']
      INSERT INTO `IceCreams` (`IceCreamId`, `Name`)
      VALUES (@p0, @p1);

info: Microsoft.EntityFrameworkCore.Database.Command[20101]
      Executed DbCommand (4ms) [Parameters=[@p0='Chocolate' (Size = 4000)], CommandType='Text', CommandTimeout='30']
      INSERT INTO `IceCreams` (`Name`)
      VALUES (@p0);
      SELECT `IceCreamId`
      FROM `IceCreams`
      WHERE ROW_COUNT() = 1 AND `IceCreamId` = LAST_INSERT_ID();

info: Microsoft.EntityFrameworkCore.Database.Command[20101]
      Executed DbCommand (1ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
      SELECT `i`.`IceCreamId`, `i`.`Name`
      FROM `IceCreams` AS `i`
      ORDER BY `i`.`IceCreamId`

So the behavior you experience is expected and is in accordance with the documentation.

Possible solution:

To solve your issue, just reset your Category.Id to 0 before calling SaveChanges().

Since you are using an explicit repository pattern, a simple solution would be to just override the Add() method:

```c#
public abstract class Repository : IRepository where TEntity : class
{
public virtual void Add(TEntity entity) // <-- made virtual
{
Context.Set().Add(entity);
}
}

public class CategoryRepository : Repository, ICategoryRepository
{
public override void Add(Category entity)
{
entity.Id = 0;
base.Add(entity);
}
}
```

Thanks for the answer and help