Migrating project from dotnet core 2.0 to 3.1 and having some problems on Entity Framework

I'm bulding a project using DDD. So the only way to update/add/delete a child entity is through its parent, this was not a problem when I was using dotnet core 2.0 but now that i'm migrating the project to dotnet core 3.1 I'm getting the following error:

> Database operation expected to affect 1 row(s) but actually affected 0 row(s). Data may have been modified or deleted since entities were loaded. See http://go.microsoft.com/fwlink/?LinkId=527962 for information on understanding and handling optimistic concurrency exceptions.

I have a Client class like that:

public class Client
{
    /// <summary>
    /// DDD Patterns comment Using a private collection field, better for DDD Aggregate's
    /// encapsulation so Receptions cannot be added from "outside the AggregateRoot" directly to
    /// the collection, but only through the method ClientAggrergateRoot.AddReception() which
    /// includes behaviour.
    /// </summary>
    private readonly List<Reception> _receptions;

    public Client(Guid id, string name, string url, string domainEmail)
    {
        Id = id;
        Name = name;
        Url = url;
        DomainEmail = domainEmail;
        _receptions = new List<Reception>();
    }

    protected Client()
    {
        _receptions = new List<Reception>();
    }

    public string Name { get; private set; }

    public string Url { get; private set; }

    public string DomainEmail { get; private set; }

    public bool IsDeleted { get; private set; }

    public DateTime CreatedAt { get; private set; }

    public DateTime ModifiedAt { get; private set; }

    public IReadOnlyCollection<Reception> Receptions => _receptions;        
}

I have my Reception class:

public class Reception
{
    public Reception(Guid id, string name, string address, string noteToGuest, string noteToReceptionst,
        string timeZone, Guid clientId)
    {
        Id = id;
        Name = name;
        Address = address;
        ClientId = clientId;
    }

    private Reception()
    {
    }

    public Guid ClientId { get; private set; }

    public string Name { get; private set; }

    public string Address { get; private set; }        
}

And this is the Client configuration file:

public class ClientEntityTypeConfiguration
    : IEntityTypeConfiguration<Client>
{
    public void Configure(EntityTypeBuilder<Client> builder)
    {
        builder.ToTable("Clients", BeWelcomeContext.CLIENT_SCHEMA);
        builder.HasKey(c => c.Id);            

        var navigation = builder.Metadata.FindNavigation(nameof(Client.Receptions));

        // DDD Patterns comment: Set as field (New since EF 1.1) to acces the Receptions
        // collection property through its fields
        navigation.SetPropertyAccessMode(PropertyAccessMode.Field);
    }
}

And the reception configuration file:

public class ReceptionEntityTypeConfiguration
    : IEntityTypeConfiguration<Reception>
{
    public void Configure(EntityTypeBuilder<Reception> builder)
    {
        builder.ToTable("Receptions", BeWelcomeContext.CLIENT_SCHEMA);
        builder.HasKey(r => r.Id);

        builder.HasOne<Client>()
            .WithMany(r => r.Receptions)
            .IsRequired()
            .HasForeignKey(r => r.ClientId);
    }
}

This have worked perfectly until I the project migration, I don't know what is happening, if I have to change some configuration.

https://learn.microsoft.com/en-us/ef/core/what-is-new/ef-core-3.0/breaking-changes#detectchanges-honors-store-generated-key-values

This is a breaking change in dot net 3.0.

If you are using Guid key and pregenerating them (I was doing same DDD stuff), then Ef treats it now as modified object rather than new object.

Solution is to turn off Key generating for the model.

The above link has detailed explanation.
But to summarise you need to do this change in the DbContext OnModelCreating

modelBuilder
.Entity<Client>()
.Property(e => e.Id)
.ValueGeneratedNever();

In case models are deriving from some class like Entity that has Id property, then a better workaround maybe to use,

 [DatabaseGenerated(DatabaseGeneratedOption.None)]
 public string Id { get; set; }

Setting this on parent will apply it all the classes.

> Database operation expected to affect 1 row(s) but actually affected 0 row(s). Data may have been modified or deleted since entities were loaded. See http://go.microsoft.com/fwlink/?LinkId=527962 for information on understanding and handling optimistic concurrency exceptions.

It seems like your DB prevent to modify the data. Take a look at your logs, copy the SQL query created by EF Core and try to run it in your DBMS, maybe you will get more detailed error. It is usual that there is a trigger that prevents to do the operation.