Efcore: Unable to join using composite keys. Type cannot be inferred.

[cdarrigo]

I want to be able to join to queries on multiple fields (composite keys).

Joining on a single field works fine:

  var query = outer.Join(
                inner: inner,
                outerKeySelector: tr => tr.TransactionId
                innerKeySelector: cua => cua.Id
                resultSelector: (tr, cua) => new
                {
                    cua.AmountApplied,
                    tr.Amount
                }
            );

but if I try to create a composite join expression:

   var query = outer.Join(
                inner: inner,
                outerKeySelector: tr => new {tr.TransactionId, tr.UserId},
                innerKeySelector: cua =>new {cua.Id, cua.UserId},
                resultSelector: (tr, cua) => new
                {
                    cua.AmountApplied,
                    tr.Amount
                }
            );

(note that all the fields above are int values).

I get an error:

```
the type arguments for method 'Queryable.Join ````

Do I need to build a non-anonymous type and custom comparer to use composite expressions in my join?

EF Core version: 2.1.0
Database Provider: Microsoft.EntityFrameworkCore.SqlServer
Operating system:
IDE: (e.g. Visual Studio 2017 15.4)

[cdarrigo]

I also tried to writing the query using Linq:

 var query2 = from o in outer
                join i in inner
                    on new {o.TransactionId, o.UserId} equals new {i.Id, i.UserId} into details
                from d in details
                select d;

which produces the error:

The type of one of the expressions in the join clause is incorrect.  The inference failed in the call to 'GroupJoin'.

The type arguments cannot be inferred from the query.

[smitpatel]

Instead of new {o.TransactionId, o.UserId} equals new {i.Id, i.UserId}
try new {Id = o.TransactionId, o.UserId} equals new {i.Id, i.UserId}

[cdarrigo]

NICE! I assumed the anonymous type definitions had match.. It never occurred to me that the property names needed to match as well.

Thanks @smitpatel