C# EF代码首先不使用npgsql创建表

我首先在Npgsql中使用EF代码。在我尝试使用contextclassObj保存更改之前，一切都很好

public class EmployeeRepository
{
    private ESAppraisalcontext DBAccessObj = null;
    public EmployeeRepository()
    {
        DBAccessObj = new ESAppraisalcontext();
        DBAccessObj.Employees.Add(new Employee { EmployeeCode = "1", EmployeeName = "shuk" });
        DBAccessObj.SaveChanges();            
    }
}  

此时，

SaveChanges（）

给出了一个异常，即关系\ESTables.Employee\不存在。下面是我的上下文类：

public class ESAppraisalcontext : DbContext
{
    public DbSet<Entity> Entities { get; set; }
    public DbSet<Employee> Employees { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.HasDefaultSchema("ESTables");
            base.OnModelCreating(modelBuilder);
    }
}

public class Employee 
{
    [Key]
    public string EmployeeCode { get; set; }
    public string EmployeeName { get; set; }
    public string EmailId { get; set; }
    public string DomainName { get; set; }
    public int DesignationId { get; set; }
    public int DepartmentId { get; set; }
    public string MgrCode { get; set; }
    public int LocationId { get; set; }
    public DateTime DOJ { get; set; }
    public bool IsActive { get; set; }
    public bool IsEligibleForCycle { get; set; }
    public bool IsNonEligibleApprover { get; set; }

    [ForeignKey("DepartmentId")]
    public Departments department { get; set; }
    [ForeignKey("DesignationId")]
    public Designations designation { get; set; }
    [ForeignKey("LocationId")]
    public Locations Loation { get; set; }
}