Seeding relationship data in Entity Framework Core 2.1 According to .NET Core docs (https://learn.microsoft.com/en-us/ef/core/modeling/data-seeding), I could seed the data with HasData in ModelBuilder. 
But I need to seed a relational data like: 
public class ModuleComponent
{
public int Id { get; set; }
public int DisplayOrder { get; set; }
public string ModuleName { get; set; }
public int Level { get; set; }
public int? ParentId { get; set; }
public ModuleComponent Parent { get; set; }
public ICollection<ModuleComponent> Children {get; set;}
}

Before I did create Seeder class and calling it from Program.cs
namespace CRSApp.API.Data
{
    public class SeederModuleComponent
    {
private readonly DataContext _context;

public SeederModuleComponent(DataContext context)
{
_context = context;
}

public void Seeding()
{
if (!_context.ModuleComponent.Any())
{
// create Admin etc 
var admin = new ModuleComponent
{
DisplayOrder = 0,
ModuleName = "Admin",
Level = 0
};

var dashboard = new ModuleComponent
{
DisplayOrder = 1,
ModuleName = "Dashboard",
Level = 0
};

var clients = new ModuleComponent
{
DisplayOrder = 2,
ModuleName = "Clients",
Level = 0
};

var timesheet = new ModuleComponent
{
DisplayOrder = 3,
ModuleName = "Time Sheet",
Level = 0
};

var billings = new ModuleComponent
{
DisplayOrder = 3,
ModuleName = "Billing",
Level = 0
};

var reports = new ModuleComponent
{
DisplayOrder = 4,
ModuleName = "Reports",
Level = 0
};

_context.ModuleComponent.Add(admin);
_context.ModuleComponent.Add(dashboard);
_context.ModuleComponent.Add(clients);
_context.ModuleComponent.Add(timesheet);
_context.ModuleComponent.Add(billings);
_context.ModuleComponent.Add(reports);

//ADMIN
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Organisation", Level = 1, DisplayOrder = 0, Parent = admin });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "User Group", Level = 1, DisplayOrder = 1, Parent = admin });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Users", Level = 1, DisplayOrder = 2, Parent = admin });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "All Entitlements", Level = 1, DisplayOrder = 3, Parent = admin });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Staff Leave Approval", Level = 1, DisplayOrder = 4, Parent = admin });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Settings", Level = 1, DisplayOrder = 5, Parent = admin });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Department Productivity", Level = 1, DisplayOrder = 6, Parent = admin });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Staff Productivity Chart", Level = 1, DisplayOrder = 7, Parent = admin });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Job Profitability", Level = 1, DisplayOrder = 8, Parent = admin });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Activity Codes", Level = 1, DisplayOrder = 9, Parent = admin });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Master Password", Level = 1, DisplayOrder = 10, Parent = admin });

//Dashboard
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Summary of Active Clients", Level = 1, DisplayOrder = 0, Parent = dashboard });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Practice Work In Progress", Level = 1, DisplayOrder = 1, Parent = dashboard });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Employee Productivity", Level = 1, DisplayOrder = 2, Parent = dashboard });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Leave", Level = 1, DisplayOrder = 3, Parent = dashboard });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Staff Default Rates", Level = 1, DisplayOrder = 4, Parent = dashboard });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Activity Codes", Level = 1, DisplayOrder = 5, Parent = dashboard });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Change Password", Level = 1, DisplayOrder = 6, Parent = dashboard });

// Clients
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Clients Active Case", Level = 1, DisplayOrder = 0, Parent = clients });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Client Personal", Level = 1, DisplayOrder = 1, Parent = clients });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Client Company", Level = 1, DisplayOrder = 2, Parent = clients });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "My Clients Profile", Level = 1, DisplayOrder = 3, Parent = clients });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "All Clients Profile", Level = 1, DisplayOrder = 4, Parent = clients });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Hourly Rate Assigned", Level = 1, DisplayOrder = 5, Parent = clients });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Edit Hourly Rate Assigned", Level = 1, DisplayOrder = 6, Parent = clients });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Reassigning Clients", Level = 1, DisplayOrder = 7, Parent = clients });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "WIP Pre Billed", Level = 1, DisplayOrder = 8, Parent = clients });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "My Client Billing History", Level = 1, DisplayOrder = 9, Parent = clients });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "All Client Billing History", Level = 1, DisplayOrder = 10, Parent = clients });

//Timesheet
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Create New Timesheet", Level = 1, DisplayOrder = 0, Parent = timesheet });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "My Timesheets", Level = 1, DisplayOrder = 1, Parent = timesheet });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "All Employee Timesheets", Level = 1, DisplayOrder = 2, Parent = timesheet });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Approve/Reject Timesheets", Level = 1, DisplayOrder = 3, Parent = timesheet });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Create Expense Sheet", Level = 1, DisplayOrder = 4, Parent = timesheet });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "My Expense Sheets", Level = 1, DisplayOrder = 5, Parent = timesheet });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "All Employee Expense Sheets", Level = 1, DisplayOrder = 6, Parent = timesheet });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Approve/Reject Expense Sheet", Level = 1, DisplayOrder = 7, Parent = timesheet });

// Billings
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Create New Bill", Level = 1, DisplayOrder = 0, Parent = billings });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "My Billing", Level = 1, DisplayOrder = 1, Parent = billings });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "All Billing History", Level = 1, DisplayOrder = 2, Parent = billings });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Approve/Reject Bill", Level = 1, DisplayOrder = 3, Parent = billings });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "My Potentials", Level = 1, DisplayOrder = 4, Parent = billings });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Create New Potential", Level = 1, DisplayOrder = 5, Parent = billings });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "All Potentials", Level = 1, DisplayOrder = 6, Parent = billings });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "My Batch Billing", Level = 1, DisplayOrder = 7, Parent = billings });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "All Batch Billing", Level = 1, DisplayOrder = 8, Parent = billings });

//Reports
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Timesheet Analysis Report", Level = 1, DisplayOrder = 0, Parent = reports });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "WIP Client Report", Level = 1, DisplayOrder = 1, Parent = reports });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "WIP Practice Report", Level = 1, DisplayOrder = 2, Parent = reports });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "My Report", Level = 1, DisplayOrder = 3, Parent = reports });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "All Employee Report", Level = 1, DisplayOrder = 4, Parent = reports });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "My Client Analysis Report", Level = 1, DisplayOrder = 5, Parent = reports });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "All Client Analysis Report", Level = 1, DisplayOrder = 6, Parent = reports });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Create Summary Remuneration Report", Level = 1, DisplayOrder = 7, Parent = reports });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "All Summary Remun. Report", Level = 1, DisplayOrder = 8, Parent = reports });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Create Detailed Remun. Report", Level = 1, DisplayOrder = 9, Parent = reports });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "All Detailed Remun. Report", Level = 1, DisplayOrder = 10, Parent = reports });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Create Summary Expense Report", Level = 1, DisplayOrder = 11, Parent = reports });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "All Summary Expense Report", Level = 1, DisplayOrder = 12, Parent = reports });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "Create Detailed Expense Report", Level = 1, DisplayOrder = 13, Parent = reports });
_context.ModuleComponent.Add(new ModuleComponent { ModuleName = "All Detailed Expense Report", Level = 1, DisplayOrder = 14, Parent = reports });

_context.SaveChanges();
}
}
    }
}

How to do with EF Core 2.1 ? As we might not know the parentId of object.
Thank you 

For HasData(), you will need to provider ModuleComponent[] and explicit set Id and ParentId.       
Here is a demo code:      
    public class SeederData
{
    public static ModuleComponent[] SeederModuleArray()
    {
int index = 1;
var result = new List<ModuleComponent>();
var admin = new ModuleComponent
{
Id = index,
DisplayOrder = 0,
ModuleName = "Admin",
Level = 0
};
result.Add(admin);
var dashboard = new ModuleComponent
{
Id = ++index,
DisplayOrder = 1,
ModuleName = "Dashboard",
Level = 0
};
result.Add(dashboard);
var Organisation = new ModuleComponent
{
Id = ++index,
ModuleName = "Organisation",
Level = 1,
DisplayOrder = 0,
ParentId = admin.Id
};
result.Add(Organisation);
var Leave = new ModuleComponent
{
Id = ++index,
ModuleName = "Leave",
Level = 1,
DisplayOrder = 3,
ParentId = dashboard.Id
};
result.Add(Leave);
return result.ToArray();
    }
}

Use ModuleComponent[] in DbContext 
builder.Entity<ModuleComponent>().HasData(
SeederData.SeederModuleArray()
);