实体框架4.1+多对多关系改变了跟踪

Question

如何检测ICollection <>属性(多对多关系)的更改？

public class Company
{
    ...

    public virtual ICollection<Employee> Employees { get; set; }
}

using (DataContext context = new DataContext(Properties.Settings.Default.ConnectionString))
{
    Company company = context.Companies.First();
    company.Employees.Add(context.Employees.First());

    context.SaveChanges();
}

public class DataContext : DbContext
{
    public override int SaveChanges()
    {
        return base.SaveChanges();

        // Company's entity state is "Unchanged" in this.ChangeTracker
    }
}

Answer 1

以下是如何找到所有已更改的多对多关系.我已经将代码实现为扩展方法:

public static class IaExtensions
{
    public static IEnumerable<Tuple<object, object>> GetAddedRelationships(
        this DbContext context)
    {
        return GetRelationships(context, EntityState.Added, (e, i) => e.CurrentValues[i]);
    }

    public static IEnumerable<Tuple<object, object>> GetDeletedRelationships(
        this DbContext context)
    {
        return GetRelationships(context, EntityState.Deleted, (e, i) => e.OriginalValues[i]);
    }

    private static IEnumerable<Tuple<object, object>> GetRelationships(
        this DbContext context,
        EntityState relationshipState,
        Func<ObjectStateEntry, int, object> getValue)
    {
        context.ChangeTracker.DetectChanges();
        var objectContext = ((IObjectContextAdapter)context).ObjectContext;

        return objectContext
            .ObjectStateManager
            .GetObjectStateEntries(relationshipState)
            .Where(e => e.IsRelationship)
            .Select(
                e => Tuple.Create(
                    objectContext.GetObjectByKey((EntityKey)getValue(e, 0)),
                    objectContext.GetObjectByKey((EntityKey)getValue(e, 1))));
    }
}

一些解释.多对多关系在EF中表示为独立关联或IA.这是因为关系的外键不会暴露在对象模型中的任何位置.在数据库中,FK位于连接表中,并且此连接表对象模型是隐藏的.

使用"关系条目"在EF中跟踪IAs.它们类似于从DbContext.Entry获取的DbEntityEntry对象,除了它们表示两个实体之间的关系而不是实体本身.关系条目未在DbContext API中公开,因此您需要下拉到ObjectContext以访问它们.

创建两个实体之间的新关系时,将创建新的关系条目,例如,通过向Company.Employees集合添加Employee.此关系处于已添加状态.

同样,当删除两个实体之间的关系时,关系条目将进入Deleted状态.

这意味着要查找已更改的多对多关系(或实际上任何已更改的IA),我们需要查找添加和删除的关系条目.这就是GetAddedRelationships和GetDeletedRelationships所做的事情.

一旦我们有关系条目,我们需要理解它们.为此,您需要了解一些内幕知识.已添加(或未更改)关系条目的CurrentValues属性包含两个值,这两个值是关系任一端的实体的EntityKey对象.同样,但令人烦恼地略有不同,Deleted关系条目的OriginalValues属性包含已删除关系任一端的实体的EntityKey对象.

(而且,是的,这太可怕了.请不要怪我 - 这是在我的时间之前.)

CurrentValues/OriginalValues差异是我们将委托传递给GetRelationships私有方法的原因.

一旦我们有了EntityKey对象,我们就可以使用GetObjectByKey来获取实际的实体实例.我们将这些作为元组返回,并且您拥有它.

这是一些实体,一个上下文和一个初始化器,我曾经测试过这个.(注意测试并不广泛.)

public class Company
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Employee> Employees { get; set; }

    public override string ToString()
    {
        return "Company " + Name;
    }
}

public class Employee
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Company> Companies { get; set; }

    public override string ToString()
    {
        return "Employee " + Name;
    }
}

public class DataContext : DbContext
{
    static DataContext()
    {
        Database.SetInitializer(new DataContextInitializer());
    }

    public DbSet<Company> Companies { get; set; }
    public DbSet<Employee> Employees { get; set; }

    public override int SaveChanges()
    {
        foreach (var relationship in this.GetAddedRelationships())
        {
            Console.WriteLine(
                "Relationship added between {0} and {1}",
                relationship.Item1,
                relationship.Item2);
        }

        foreach (var relationship in this.GetDeletedRelationships())
        {
            Console.WriteLine(
                "Relationship removed between {0} and {1}",
                relationship.Item1,
                relationship.Item2);
        }

        return base.SaveChanges();
    }

}

public class DataContextInitializer : DropCreateDatabaseAlways<DataContext>
{
    protected override void Seed(DataContext context)
    {
        var newMonics = new Company { Name = "NewMonics", Employees = new List<Employee>() };
        var microsoft = new Company { Name = "Microsoft", Employees = new List<Employee>() };

        var jim = new Employee { Name = "Jim" };
        var arthur = new Employee { Name = "Arthur" };
        var rowan = new Employee { Name = "Rowan" };

        newMonics.Employees.Add(jim);
        newMonics.Employees.Add(arthur);
        microsoft.Employees.Add(arthur);
        microsoft.Employees.Add(rowan);

        context.Companies.Add(newMonics);
        context.Companies.Add(microsoft);
    }
}

以下是使用它的示例:

using (var context = new DataContext())
{
    var microsoft = context.Companies.Single(c => c.Name == "Microsoft");
    microsoft.Employees.Add(context.Employees.Single(e => e.Name == "Jim"));

    var newMonics = context.Companies.Single(c => c.Name == "NewMonics");
    newMonics.Employees.Remove(context.Employees.Single(e => e.Name == "Arthur"));

    context.SaveChanges();
}