Help Understanding Enumerable.Join Method

Question

Yesterday I posted this question regarding using lambdas inside of a Join() method to check if 2 conditions exist across 2 entities. I received an answer on the question, which worked perfectly. I thought after reading the MSDN article on the Enumerable.Join() method, I'd understand exactly what was happening, but I don't. Could someone help me understand what's going on in the below code (the Join() method specifically)? Thanks in advance.

if (db.TableA.Where( a => a.UserID == currentUser )
      .Join( db.TableB.Where( b => b.MyField == someValue ),
             o => o.someFieldID,
             i => i.someFieldID,
             (o,i) => o )
      .Any()) 
{
    //...
}

Edit: Specifically, I'm curious about the last 3 parameters, and what's actually going on. How do they result in the signature requirements of Func(TOuter, TKey), Func(TInner, TKey) etc.

Answer 1

Explanation of the Join.

b = object type of first table o = object type of first table i = object type of second table

1. db.TableB.Where( b => b.MyField == someValue ) This is the element type of the second table
2. o => o.someFieldID The key of the first table
3. i => i.someFieldID The key of the second table (which will match the key in the first table)
4. (o,i) => o The object to return, in this case the object type of the first table.

Answer 2

This query is saying join TableA to TableB where TableA.someFieldID == TableB.someFieldID and selecting the results from TableA and seeing if there are any results at all

In terms of SQL think of it like this, even if it's not Linq-to-SQL...if you are familiar with SQL maybe this makes more sense:

Select Count(*)
From TableA a
     Join TableB b
       On a.someFieldID = b.someFieldID

Then checking if Count(*) is > 0