EF许多对许多的疯狂

我有一个方法可以更新EF中的ReportRecipient对象。基元工作良好；当试图管理与CCD_ 2对象的M2M关系时。

请看一下这个代码：

    public IReportRecipient ModifyRecipientWithGroupAssignments(IEnumerable<Guid> groupIds, IReportRecipient recipient)
    {
        var entity = Context.ReportRecipients
            .Include("RecipientGroups")
            .FirstOrDefault(e => e.ReportRecipientId == recipient.ReportRecipientId)
            .FromIReportRecipient(recipient);
        var toRemove = entity.RecipientGroups
            .Where(e => !groupIds.Contains(e.GroupId))
            .ToList();
        //remove group assignments that no longer apply
        foreach (var group in toRemove)
        {
            if (group != null)
            {
                entity.RecipientGroups.Attach(group);
                entity.RecipientGroups.Remove(group);
            }
        }
        var toAdd = entity.RecipientGroups
            .Where(e => groupIds.Contains(e.GroupId))
            .ToList();
        //add new groups that weren't there before
        foreach (var group in toAdd)
        {
            if (group != null)
            {
                entity.RecipientGroups.Attach(group);
            }
        }
        return entity;
    }

我的问题在var ToAdd...线上。即使我在groupIds中有一个与表示数据库中RecipientGroup对象的Guid匹配的Guid集合，toAdd的计算结果也总是为空集合。我认为Contains()函数适用于这种情况；有人能解释一下我是否做错了什么吗？