Allocation of header table into detail table

[smko]

I want to allocate the discount amount happened at the invoice level into line item evenly. Basically the discount will spread evenly into each item, for example InvoiceID 1 has 2 items, hence 20/2 = 10 discount per item

Header table

InvoiceID	Discount
1	20
2	40
3	50

Detail table

InvoiceID	Line item	Allocated discount
1	A	10
1	B	10
2	A	13.33
2	B	13.33
2	C	13.33
3	A	25
3	B	25

I know how to do it in DAX, but I prefer to do the modeling in Power Query. How do I achieve this?

[Fowmy · Accepted solution]

@smko 

Please check the attached file. I merged the discount table and added a custom column to divided by the invoice ID count.

[smko]

Thanks Fowmy! I'm new to M, could you explain the meaning of (x)=> and the "x" in front of [Discount] and [InvoiceID]?

(x)=> 
    x[Discount]/
    List.Count(
        List.Select( #"Merged Queries"[InvoiceID] , each _  = x[InvoiceID])
    )