如何使用Entity Framework 6按行号进行内部联接

Question

Two tables:

Table A has following data:

Type1 A
Type1 B
Type1 C
Type1 D

Table B has following data:

Type2 X
Type2 Y
Type2 U
Type2 V

I want to get the following output:

new { A, X }
new { B, Y }
new { C, U }
new { D, V }

Is this possible using Entity Framework? The problem I am facing now is there're 8 different entity tables, each represent a type. The app has an index page, we want to show 6 items of each table on the index page. We can do this one by one of course, but that will hit the DB 8 times, which may not be very efficient. What's the right way to solve such a problem using EF 6?

Answer 1

Try this one:

var AX = db.tableA 
.Join( 
tableB, 
foo1 => foo1.A, 
foo2 => foo2.X, 
(foo1, foo2) => new { foo1, foo2}
.ToList()



var BY = db.tableA 
    .Join( 
    tableB, 
    foo1 => foo1.B, 
    foo2 => foo2.Y, 
    (foo1, foo2) => new { foo1, foo2}
    .ToList()

  var CU = db.tableA 
    .Join( 
    tableB, 
    foo1 => foo1.C, 
    foo2 => foo2.U, 
    (foo1, foo2) => new { foo1, foo2}
    .ToList()

  var DV = db.tableA 
    .Join( 
    tableB, 
    foo1 => foo1.D, 
    foo2 => foo2.V, 
    (foo1, foo2) => new { foo1, foo2}
    .ToList()

Answer 2

If you use EF , probably all your tables have int PK with identity. If you also have never deleted separated rows, located between others, from these tables, you can use Id columns for join condition. You can simple extend this example to desired number of tables with same manner.

var tableA = from itemA in ctx.tableA
             let minId = ctx.tableA.Min(x => x.Id)
             select new { itemA, index = itemA.Id - minId };

var tableB = from itemB in ctx.tableB 
             let minId = ctx.tableB.Min(x => x.Id)
             select new { itemB, index = itemB.Id - minId };

var answer = (from itemAA in tableA
              join itemBB in tableB on itemAA.index equals itemBB.index
              select new { itemAA.itemA, itemBB.itemB}).ToList().SelectMany(x => {
                  var result = new List<Tuple<Type1, Type2>>();
                  var props1 = x.itemA.GetType().GetProperties().ToList();
                  var props2 = x.itemB.GetType().GetProperties().ToList();
                  for(var i = 0; i < Math.Min(props1.Count, props2.Count); i++)                              
                       result.Add(new Tuple<Type1, Type2>((Type1)props1[i].GetValue(x.itemA), (Type2)props2[i].GetValue(x.itemB)));
                   return result;                                  
              }).ToList();