Problem Insterting Multiple data on 2 tables - asp.net-mvc

Question

i am having problem on inserting multiple data in two tables.
Here is my code
Context Class
var userId = Convert.ToUInt32(user.Single(logon => logon.Type == CustomClaimTypes.UserId).Value);
/*Create access table for insert*/
var modules = p.Select(collection => new Accountcollection
{
AccountId = userId,
Amount = collection.Amount,
CashSource = collection.CashSource,
CollectionDate = collection.CollectionDate,
CreatedDatetime = DateTime.Now,
UpdatedDatetime = DateTime.Now,
}).ToList();
_context.Accountcollection.AddRange(modules);
var calendar_event = p.Select(collection => new Accountcalendarevents
{
AccountId = userId,
Subject = collection.CashSource,
Description = collection.CashSource,
Start = collection.CollectionDate,
End = collection.CollectionDate,
ThemeColor = "blue",
Isfullday = true,
Status = "1",
CreatedBy = userId,
CreatedDatetime = DateTime.Now,
UpdatedBy = userId,
UpdatedDatetime = DateTime.Now
}).ToList();
_context.Accountcalendarevents.AddRange(calendar_event);
_context.SaveChanges();
}
This is my collection model
public partial class Accountcollection
{
[Key]
public long Id { get; set; }
public long AccountId { get; set; }
public double? Amount { get; set; }
public string CashSource { get; set; }
public DateTime CollectionDate { get; set; }
public DateTime? CreatedDatetime { get; set; }
public DateTime? UpdatedDatetime { get; set; }
//public Accountcalendarevents Accountcalendarevents { get; set; }
public virtual Accountmaster Account { get; set; }
}
And this is my Caldendar Manager Events Model
public class Accountcalendarevents
{
[Key]
public long Id { get; set; }
public long AccountId { get; set; }
public string Subject { get; set; }
public string Description { get; set; }
public DateTime Start { get; set; }
public DateTime End { get; set; }
public string ThemeColor { get; set; }
public bool Isfullday { get; set; }
public string Status { get; set; }
public long CreatedBy { get; set; }
public DateTime CreatedDatetime { get; set; }
public long UpdatedBy { get; set; }
public DateTime UpdatedDatetime { get; set; }
}
The problem is when I insert only 1 data in works fine but if I try to insert 2 or more, i am getting this kind of exception
Exception i got
But, when I commented
var calendar_event = p.Select(collection => new Accountcalendarevents
{
//AccountId = userId, <-- when I comment this line
Subject = collection.CashSource,
Description = collection.CashSource,
Start = collection.CollectionDate,
End = collection.CollectionDate,
ThemeColor = "blue",
Isfullday = true,
Status = "1",
CreatedBy = userId,
CreatedDatetime = DateTime.Now,
UpdatedBy = userId,
UpdatedDatetime = DateTime.Now
}).ToList();
it works properly on multiple data insertion.
Hope you helped me with this. Thank you!