LINQ for many-to-many in EF Core 2 - entity-framework

Question

I have searched on google and stackoverflow for a while and yes I have found many articles on the subject but I still can't seem to know what I am doing wrong.
Problem: I do have a "link" table (many-to-many) between three tables:
- check
- car
- gate
My 'link' class looks like follows:
public class CheckCarGate
{
public int CheckId { get; set; }
public Check Check{ get; set; }
public int CarId { get; set; }
public Car Car{ get; set; }
public int GateId { get; set; }
public Gate Gate { get; set; }
}
DbContext:
public virtual DbSet<Car> Cars{ get; set; }
public virtual DbSet<Gate> Gates { get; set; }
public virtual DbSet<Check> Checks{ get; set; }
// The key:
protected override void OnModelCreating(ModelBuilder builder)
{
builder.Entity<CheckCarGate>().HasKey(p => new { p.CheckId, p.CarId, p.GateId });
}
I did follow this documentation https://learn.microsoft.com/en-us/ef/core/modeling/relationships#many-to-many when creating models. CheckCarGate is a navigation property in the entity models. E.g.
public class Gate
{
public int GateId { get; set; }
public string Descr { get; set; }
public ICollection<CheckCarGate> CheckCarGates { get; set; }
}
Code first created this table in the database. Now I'm trying to select all Checks with all Gates for one particular CarId.
I tried something like this:
var masterlist = _context.Checks.Where(p => p.CheckCarGate.Any(x => x.CarId == 12));
or:
var masterlist = _context.Checks
.Include(p => p.CheckCarGate)
.ThenInclude(p=>p.Gate)
.Include(p=>p.CheckCarGate)
.ThenInclude(p=>p.Car);
//Edit: I'm sorry for not giving enough information! Bear with me...
Could someone point me to the right direction?
Many thanks in advance!
N.

Answer 1

I'm not sure there is enough information here to answer your question; it appears like each check has its own checkcargate, which doesn't really make sense.
Assuming you had a table of checks and CheckCarGate table seperatatly, you can use where and select to make this query.
var out = CheckCarGateTable.Where(t => t.CarID == targetCarID)
.Select(c => checks.getbyid(c.CheckID))
or, based on your class definition
var out = CheckCarGateTable.Where(t => t.CarID == targetCarID)
.Select(c => c.Check))
First we get all CheckCarGates that refer to our target car, then we transform these into checks using the select call; out should be of type IEnumerable< Check >
and process the list like:
foreach (Check c in out)
{
proc(c);
}
This is the best I can do with the information given.

Answer 2

Your OnModelCreating(...) should have more in it defining the relationship
protected override void OnModelCreating(ModelBuilder builder)
{
builder.Entity<CheckCarGate>().HasKey(p => new { p.CheckId, p.CarId, p.GateId });
builder.Entity<CheckCarGate>().HasOne(c => c.car)
.WithMany(c => c.check)
.HasForeignKey(k => k.carId)
...
}
I'm not sure how you change your query after that, the documentation link you provided didn't have example usage.

Answer 3

I gave up looking for a LINQ solution. (2 days wasted).
Instead I decided to use raw sql and store the data in a new object.
Credits to: https://stackoverflow.com/users/355714/pius and his solution: https://stackoverflow.com/a/46013305/848642