实体框架保存了太多子实体

我正在一个项目，该项目包含多个要保存到数据库中的对象。我使用单个上下文和一系列存储库类来访问它们。

当我尝试保存一个实体时，似乎已经保存了与之关联的所有虚拟实体，即使该实体已经存在于数据库中。

这些是我的课程：

public class Requirement
{
    public int ID { get; set; }
    public DateTime DateDue { get; set; }
    public DateTime DateCompleted { get; set; }

    public virtual Standard Standard { get; set; }
    public virtual Project Project { get; set; }
}

public class Standard
{
    public int ID { get; set; }
    public int AgencyID { get; set; }
    public string Name { get; set; }

    public virtual Agency Agency { get; set; }
}


public class Project
{
    public int ID { get; set; }
    public bool Active { get; set; }

    public virtual Agency Agency { get; set; }
    public virtual Department Department { get; set; }
}

这是我创建一些数据的方法：

public class RequirementRepository
{
    public static string CreateMockData()
    {
        StandardRepository stdRep = new StandardRepository();
        ProjectRepository projRep = new ProjectRepository();
        RequirementRepository reqRep = new RequirementRepository();
        Project project = projRep.Find(1);
        StringBuilder sb = new StringBuilder()
        foreach (Standard s in stdRep.FindByAgencyID(project.Agency.ID))
        {
            Requirement r = new Requirement();
            r.Project = project;
            r.Standard = s;
            r.DateCompleted = (DateTime)SqlDateTime.MaxValue;
            r.DateDue = DateTime.Now.AddDays(90);

            r = reqRep.Save(r);

            sb.AppendLine(String.Format("Saved Requirement ID {0} with Project ID {1}<br>", r.ID, r.Project.ID));
        }

        return sb.ToString();
    }
}

这是关联的存储库代码：

public class ProjectRepository
{
    public Project Find(int id)
    {
        using (var db = new MyContext())
        {
            return db.Projects
                .Include(p => p.Agency)
                .Include(p => p.Department)
                .First(p => p.ID.Equals(id));
        }
    }
}


public class StandardRepository
{
    public List<Standard> FindByAgencyID(int agencyID)
    {
        using (var db = new MyContext())
        {
            return db.Standards.Where(r => r.AgencyID == agencyID).ToList();
        }
    }
}



public class RequirementRepository
{
    public Requirement Save(Requirement requirement)
    {
        using (var db = new MyContext())
        {
            Requirement retVal = requirement;
            if (requirement.ID.Equals(0))
            {
                retVal = db.Requirements.Add(requirement);
            }
            else
            {
                db.Entry(requirement).State = EntityState.Modified;
            }

            db.SaveChanges();
            return retVal;
        }
    }
}

当我运行此方法时，我希望它可以将许多新的Requirements插入到数据库中，项目ID为1，标准ID为所采用的任何标准。相反，它为添加的每个需求创建一个全新的项目和全新的标准，然后将这些ID分配给需求。