c# - 实体框架，获取孤儿记录/没有父母的记录

Question

保存数据库上下文时出现以下异常：无法更改关系，因为一个或多个外键属性不可为空。

如此处所述，这可能是由于缺少级联删除。但是，这不是我的代码，我不知道哪些表可能包含孤立记录。错误消息没有这样说。

有没有办法检索那些孤儿记录。（至少知道他们在哪个表）

然后我将能够确定我需要调整代码的哪一部分。

Answer 1

在实体框架中，当您具有多对多关系时，您尝试从这样的对象中删除parent.Children.Remove(child)只会将子对象与中间连接表分离。因此，您必须像这样找到孩子并将其从 DbContext ChildrenToParent 实体中删除DbContext.ChildrenToParent.Remove(child)。如果您提供一些代码示例和/或数据库图，我想我可以更准确地解释它。

Answer 2

Can you try the following solution? The DeleteOrphans extension method must be called between DetectChanges and SaveChanges methods.

public static class DbContextExtensions
{
    private static readonly ConcurrentDictionary< EntityType, ReadOnlyDictionary< string, NavigationProperty>> s_navPropMappings = new ConcurrentDictionary< EntityType, ReadOnlyDictionary< string, NavigationProperty>>();

    public static void DeleteOrphans( this DbContext source )
    {
        var context = ((IObjectContextAdapter)source).ObjectContext;
        foreach (var entry in context.ObjectStateManager.GetObjectStateEntries(EntityState.Modified))
        {
            var entityType = entry.EntitySet.ElementType as EntityType;
            if (entityType == null)
                continue;

            var navPropMap = s_navPropMappings.GetOrAdd(entityType, CreateNavigationPropertyMap);
            var props = entry.GetModifiedProperties().ToArray();
            foreach (var prop in props)
            {
                NavigationProperty navProp;
                if (!navPropMap.TryGetValue(prop, out navProp))
                    continue;

                var related = entry.RelationshipManager.GetRelatedEnd(navProp.RelationshipType.FullName, navProp.ToEndMember.Name);
                var enumerator = related.GetEnumerator();
                if (enumerator.MoveNext() && enumerator.Current != null)
                    continue;

                entry.Delete();
                break;
            }
        }
    }

    private static ReadOnlyDictionary<string, NavigationProperty> CreateNavigationPropertyMap( EntityType type )
    {
        var result = type.NavigationProperties
            .Where(v => v.FromEndMember.RelationshipMultiplicity == RelationshipMultiplicity.Many)
            .Where(v => v.ToEndMember.RelationshipMultiplicity == RelationshipMultiplicity.One || (v.ToEndMember.RelationshipMultiplicity == RelationshipMultiplicity.ZeroOrOne && v.FromEndMember.GetEntityType() == v.ToEndMember.GetEntityType()))
            .Select(v => new { NavigationProperty = v, DependentProperties = v.GetDependentProperties().Take(2).ToArray() })
            .Where(v => v.DependentProperties.Length == 1)
            .ToDictionary(v => v.DependentProperties[0].Name, v => v.NavigationProperty);

        return new ReadOnlyDictionary<string, NavigationProperty>(result);
    }
}