Question

How to write this C# join with composite key clause in F#? :

join k in DataContext.Catalogs on
   new { id = o.IDENT, v = o.VZ } equals
   new { id = k.IDENT, v = k.VZ }

This is similiar question to this:
groupby multiple columns in a F# 3.0 query
which is still not answered. But I can’t believe it is not easy possible to write it in FSharp.

Thanks

Answer 1

Use tuples containing the key fields you want:

query {
  for o in DataContext.OTable do
  join k in DataContext.Catalogs on
   ((o.IDENT, o.VZ) = (k.IDENT, k.VZ))
  select (o.IDENT, k.VZ)
}

Note that you can’t create an anonymous type with named fields in F#, like you can in C#. Tuples are probably the closest and most idiomatic translation. See Tomas’s answer here.