ASP.NET 内核 5.0 Linq 外连接

Question

• I've got Movements and Equipment ...
  • but not all Movements have Equipment .
• For example, a " Back Squat " requires a Barbell, Plates, etc.. but a "Pushup" doesn't require any equipment.
• So, I've got a bridge table linking the Movements that do have Equipment .
• I need an OUTER JOIN so I can access all of the Movements even if they do not have any Equipment .
• ...I hope that makes sense.
• My code below only results in Movements that have matching Equipment records in the bridge table.

Movements = (
    from mvmt   in applicationDbContext.Movements
    join mvmteq in applicationDbContext.MovementEquips on mvmt  .MvmtId  equals mvmteq.MvmtId
    join equip  in applicationDbContext.Equipment      on mvmteq.EquipId equals equip .EquipId
    orderby mvmt.MvmtId
    select new Movement
    {
        MvmtId = mvmt.MvmtId,
        MvmtDescr = mvmt.MvmtDescr
    })
    .ToList();

Answer 1

        var query =  
        from mvmt in applicationDbContext.Movements
        join mvmteq in applicationDbContext.MovementEquips on mvmt.MvmtId  
        equals mvmteq.MvmtId
        join equip  in applicationDbContext.Equipment on mvmteq.EquipId equals 
        equip.EquipId into g
        from mvmteq in g.DefaultIfEmpty()
        where mvmt.MvmtId != null
        select new
        {     
          MvmtId = mvmt.MvmtId,
          MvmtDescr = mvmt.MvmtDescr
         }).ToList();

Please refer to https://docs.microsoft.com/en-gb/dotnet/csharp/linq/perform-left-outer-joins

and

http://msdn.microsoft.com/en-us/library/bb397895.aspx

Answer 2

Movements = (
            from mvmt in applicationDbContext.Movements
            join mvmteq in applicationDbContext.MovementEquips on mvmt.MvmtId equals mvmteq.MvmtId into join_1
            from mvmteq in join_1.DefaultIfEmpty()
            join equip in applicationDbContext.Equipment on mvmteq.EquipId equals equip.EquipId into join_2
            from equip in join_2.DefaultIfEmpty()
            select new GetMovementRequest
            {
                MvmtId = mvmt.MvmtId,
                MvmtDescr = mvmt.MvmtDescr,
                MvmtMetrics = mvmt.MvmtMetrics,
                MvmtStandard = mvmt.MvmtStandard,
                EquipId = equip.EquipId,
                EquipDescr = equip.EquipDescr
            }
        ).ToList();

Answer 3

• You really don't want an OUTER JOIN query.
• If you want to load all Movement and all Equipment data then all you need is this:

static async Task<( IReadOnlyList<Movement> allMovements, IReadOnlyList<Equipment> allEquipment )> LoadEverythingAsync( this ApplicationDbContext db, CancellationToken cancellationToken )
{
    IReadOnlyList<Movement>  allMovements;
    IReadOnlyList<Equipment> allEquipment;
    {
        List<Movement>  mov = await db.Movements.ToListAsync( cancellationToken ).ConfigureAwait(false);
        List<Equipment> eqp = await db.Equipment.ToListAsync( cancellationToken ).ConfigureAwait(false);

        await db.MovementEquips.LoadAsync( cancellationToken ).ConfigureAwait(false);

        allMovements = mov;
        allEquipment = eqp;
    }

    return ( allMovements, allEquipment );
}

The code above "works" because the Entity Framework DbContext performs magic whenever you load data into the DbContext (from both ToListAsync and LoadAsync ) such that all attached and loaded entities will have their reference and navigation properties' setters invoked.