如何使用同一模型中的两个下拉菜单实例

我遇到了一个问题，我认为我已经正确设置了，但结果总是为零。

以下是三种型号：

public class FamilyMember
{
    [Key]
    public int id { get; set; }
    [Required]
    [Display(Name = "First Name")]
    public string firstName { get; set; }
    [Required]
    [Display(Name = "Surname")]
    public string surname { get; set; }
    [Required]
    [Display(Name = "Date of Birth")]
    public DateTime dob { get; set; }
    public virtual FamilyRelationship FamilyRelationship { get; set; }
    [Display(Name = "Full Name")]
    public string fullName
    {
        get
        {
            return string.Format("{0} {1}", firstName, surname);
        }
    }
}
public class RelationshipType
{
    [Key]
    public int id { get; set; }
    [Required]
    [Display(Name="Relationship Type")]
    public string relationshipType { get; set; }
    public virtual FamilyRelationship FamilyRelationship { get; set; }
}
public class FamilyRelationship
{
    [Key]
    public int id { get; set; }
    [ForeignKey("FamilyMembers")]
    [Display(Name = "First Family Member")]
    public int familyMemberPrimary { get; set; }
    [ForeignKey("FamilyMembers")]
    [Display(Name = "Second Family Member")]
    public int familyMemberSecondary { get; set; }
    [Display(Name = "Relationship Type")]
    public int relationshipType { get; set; }
    public virtual ICollection<FamilyMember> FamilyMembers { get; set; }
    public virtual ICollection<RelationshipType> RelationshipTypes { get; set; }
}

因此，我已经成功地将数据添加到FamilyMember和RelationshipType，CRUD工作得很好。

问题出现在FamilyRelationship的创建控制器/视图中。下拉菜单工作得很好，在两个相关菜单中显示了家庭成员，关系也显示在relationType下拉菜单上。但是，当我单击create时，所有值都设置为null。

创建控制器：

    // GET: FamilyRelationships/Create
    public ActionResult Create()
    {
        ViewBag.familyMember = new SelectList(db.FamilyMembers, "id", "fullName");
        ViewBag.relationship = new SelectList(db.RelationshipTypes, "id", "relationshipType");
        return View();
    }
    // POST: FamilyRelationships/Create
    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Create([Bind(Include = "id,familyMemberPrimary,familyMemberSecondary,relationshipType")] FamilyRelationship familyRelationship)
    {
        if (ModelState.IsValid)
        {
            db.FamilyRelationships.Add(familyRelationship);
            db.SaveChanges();
            return RedirectToAction("Index");
        }
        return View(familyRelationship);
    }

创建视图：

    @model FamilyTree.Models.FamilyRelationship
    @{
        ViewBag.Title = "Create";
    }
    <h2>Create</h2>

    @using (Html.BeginForm()) 
    {
        @Html.AntiForgeryToken()
        <div class="form-horizontal">
            <h4>FamilyRelationship</h4>
            <hr />
            @Html.ValidationSummary(true, "", new { @class = "text-danger" })
            <div class="form-group">
                @Html.LabelFor(model => model.familyMemberPrimary, htmlAttributes: new { @class = "control-label col-md-2" })
                <div class="col-md-10">
                    @*@Html.EditorFor(model => model.familyMemberPrimary, new { htmlAttributes = new { @class = "form-control" } })
                    @Html.ValidationMessageFor(model => model.familyMemberPrimary, "", new { @class = "text-danger" })*@
                    @Html.DropDownList("familyMember", null, htmlAttributes: new { @class = "form-control" })
                    @Html.ValidationMessageFor(model => model.familyMemberPrimary, "", new { @class = "text-danger" })
                </div>
            </div>
            <div class="form-group">
                @Html.LabelFor(model => model.familyMemberSecondary, htmlAttributes: new { @class = "control-label col-md-2" })
                <div class="col-md-10">
                    @Html.DropDownList("familyMember", null, htmlAttributes: new { @class = "form-control" })
                    @Html.ValidationMessageFor(model => model.familyMemberPrimary, "", new { @class = "text-danger" })
                </div>
            </div>
            <div class="form-group">
                @Html.LabelFor(model => model.relationshipType, htmlAttributes: new { @class = "control-label col-md-2" })
                <div class="col-md-10">
                    @Html.DropDownList("relationship", null, htmlAttributes: new { @class = "form-control" })
                    @Html.ValidationMessageFor(model => model.relationshipType, "", new { @class = "text-danger" })
                </div>
            </div>
            <div class="form-group">
                <div class="col-md-offset-2 col-md-10">
                    <input type="submit" value="Create" class="btn btn-default" />
                </div>
            </div>
        </div>
    }
    <div>
        @Html.ActionLink("Back to List", "Index")
    </div>
    @section Scripts {
        @Scripts.Render("~/bundles/jqueryval")
    }

请让我知道我哪里出了问题，如果可能的话，请提供一个例子来实现这一点。