EF6 vs Entity Framework Core : Insert an entity without resetting the primary key (identity) to zero

Question

EF6: inserting an entity that already has a value for the primary key works and assigns a new value to the primary key.

EF Core : it tries to insert the value of the primary key and obviously fails with a SqlException:

Cannot insert explicit value for identity column in table 'Asset' when IDENTITY_INSERT is set to OFF.

The workaround I found is to reset the PK to the default value (0 for an int).

Example :

        Asset asset = new Asset
        {
            Id = 3,
            Name = "Toto",
        };      

        using (AppDbContext context = new AppDbContext())
        {
            asset.Id = 0; // needs to be reset
            context.Assets.Add(asset);
            context.SaveChanges();
        }

I'm migrating a solution from EF 6 to EF Core and I'd like to avoid resetting manually all the IDs in case of an insertion. The example above is really simple, sometimes I have a whole graph to insert, in my case that would mean resetting all the PKs and FKs of the graph.

The only automatic solution I can think of is using reflection to parse the whole graph and reset the IDs. But i don't think that's very efficient...

My question: how to disable that behaviour globally?

Answer 1

不幸的是，目前（从 EF Core 2.0.1 开始）这种行为是不可控的。我想它应该是对 EF6 的改进，因为它允许您执行身份插入。

幸运的是 EF Core 建立在基于服务的架构上，它允许您（（虽然不是那么容易）替换几乎所有方面。在这种情况下，负责的服务被调用IValueGenerationManager并由ValueGenerationManager类实现。提供上述行为的行是

private static IEnumerable<IProperty> FindPropagatingProperties(InternalEntityEntry entry)    
    => entry.EntityType.GetProperties().Where(    
        property => property.IsForeignKey()    
                    && property.ClrType.IsDefaultValue(entry[property]));

private static IEnumerable<IProperty> FindGeneratingProperties(InternalEntityEntry entry)    
    => entry.EntityType.GetProperties().Where(    
        property => property.RequiresValueGenerator()    
                    && property.ClrType.IsDefaultValue(entry[property]));

具体&& property.ClrType.IsDefaultValue(entry[property])情况。

如果这些方法是virtual，那就太好了，但它们不是，因此为了删除该检查，您基本上需要复制几乎所有代码。

将以下代码添加到项目中的新代码文件中：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using System.Threading;
using Microsoft.EntityFrameworkCore.ChangeTracking.Internal;
using Microsoft.EntityFrameworkCore.ValueGeneration;
using Microsoft.EntityFrameworkCore.Metadata;
using Microsoft.EntityFrameworkCore.Metadata.Internal;

namespace Microsoft.EntityFrameworkCore
{
    public static class Extensions
    {
        public static DbContextOptionsBuilder UseEF6CompatibleValueGeneration(this DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.ReplaceService<IValueGenerationManager, EF6CompatibleValueGeneratorManager>();
            return optionsBuilder;
        }
    }
}

namespace Microsoft.EntityFrameworkCore.ChangeTracking.Internal
{
    public class EF6CompatibleValueGeneratorManager : ValueGenerationManager
    {
        private readonly IValueGeneratorSelector valueGeneratorSelector;
        private readonly IKeyPropagator keyPropagator;

        public EF6CompatibleValueGeneratorManager(IValueGeneratorSelector valueGeneratorSelector, IKeyPropagator keyPropagator)
            : base(valueGeneratorSelector, keyPropagator)
        {
            this.valueGeneratorSelector = valueGeneratorSelector;
            this.keyPropagator = keyPropagator;
        }

        public override InternalEntityEntry Propagate(InternalEntityEntry entry)
        {
            InternalEntityEntry chosenPrincipal = null;
            foreach (var property in FindPropagatingProperties(entry))
            {
                var principalEntry = keyPropagator.PropagateValue(entry, property);
                if (chosenPrincipal == null)
                    chosenPrincipal = principalEntry;
            }
            return chosenPrincipal;
        }

        public override void Generate(InternalEntityEntry entry)
        {
            var entityEntry = new EntityEntry(entry);
            foreach (var property in FindGeneratingProperties(entry))
            {
                var valueGenerator = GetValueGenerator(entry, property);
                SetGeneratedValue(entry, property, valueGenerator.Next(entityEntry), valueGenerator.GeneratesTemporaryValues);
            }
        }

        public override async Task GenerateAsync(InternalEntityEntry entry, CancellationToken cancellationToken = default(CancellationToken))
        {
            var entityEntry = new EntityEntry(entry);
            foreach (var property in FindGeneratingProperties(entry))
            {
                var valueGenerator = GetValueGenerator(entry, property);
                SetGeneratedValue(entry, property, await valueGenerator.NextAsync(entityEntry, cancellationToken), valueGenerator.GeneratesTemporaryValues);
            }
        }

        static IEnumerable<IProperty> FindPropagatingProperties(InternalEntityEntry entry)
        {
            return entry.EntityType.GetProperties().Where(property => property.IsForeignKey());
        }

        static IEnumerable<IProperty> FindGeneratingProperties(InternalEntityEntry entry)
        {
            return entry.EntityType.GetProperties().Where(property => property.RequiresValueGenerator());
        }

        ValueGenerator GetValueGenerator(InternalEntityEntry entry, IProperty property)
        {
            return valueGeneratorSelector.Select(property, property.IsKey() ? property.DeclaringEntityType : entry.EntityType);
        }

        static void SetGeneratedValue(InternalEntityEntry entry, IProperty property, object generatedValue, bool isTemporary)
        {
            if (generatedValue == null) return;
            entry[property] = generatedValue;
            if (isTemporary)
                entry.MarkAsTemporary(property, true);
        }
    }
}

然后OnConfiguring在您的DbContext派生类中覆盖并添加以下行：

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
    // ...
    optionsBuilder.UseEF6CompatibleValueGeneration();
}

就是这样 - 问题解决了。