使用实体框架与SQL Server 2012(本地*.mdf数据库)

我正试图在MVC项目中创建一个数据库。代码优先方法。我编写了我的模型和DbContext数据库。我在连接数据库时遇到问题。我怀疑问题出在我的连接字符串中，但我不确定。我想要的数据库将由两个表组成，定义如下：

public class EmployeeRequest
{
    [Key]
    public int EmployeeRequestId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string UserName { get; set; }
    public int ExtNumber { get; set; }
    public bool isProcessed { get; set; }
    public virtual ICollection<ChangeOrder> ChangeOrders { get; set; }
}

和

public class ChangeOrder
{
    [Key]
    public int ChangeOrderId { get; set; }
    public short Operation { get; set; } //0 - Add, 1- delete
    public int TargetExt { get; set; }
    public short Status { get; set; } //0- Pending, 1- Approved, 2- Denied
    public DateTime DtRequested { get; set; }
    public DateTime DtProcessed { get; set; }
    public DateTime DtChangesApplied { get; set; }
    public int EmployeeRequestID { get; set; }
    public virtual EmployeeRequest Request { get; set; }
 }

DbContext类为：

public class RequestsContext : DbContext
{
    public DbSet<EmployeeRequest> EmployeeRequests { get; set; }
    public DbSet<ChangeOrder> ChangeOrders { get; set; }
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
    }
}

连接字符串：

<add name="RequestsContext" 
     providerName="System.Data.SqlClient" 
     connectionString="Data Source=(LocalDb);Database=Requests.mdf;Trusted_Connection=True"/>

我得到的错误表明无法连接到主机。

我也不知道为什么会发生这种事。有什么想法吗？

public class RequestsContext : DbContext
{
    public DbSet<EmployeeRequest> EmployeeRequests { get; set; }
    public DbSet<ChangeOrder> ChangeOrders { get; set; }
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
    }
    public RequestsContext ()
    : base("RequestsContext")
    {}
}