title: linq query sum products across multiple collections
body: <p>I have two datasets that look like this:</p>
  
  <pre><code>+------------------------------------+
  | Products                           |
  +------------------------------------+
  | Id | Name             | Price      |
  +------------------------------------+
  |  1 | apples           | 1.00       |
  |  2 | oranges          | 2.00       |
  |  3 | pomengrate       | 3.00       |
  +------------------------------------+
  
  +-------------------------------+
  | Sales                         |
  +-------------------------------+
  | CustId | ProductId | Quantity |
  +-------------------------------+
  |      1 |         1 |        5 |
  |      1 |         2 |        4 |
  |      1 |         3 |        2 |
  |      2 |         1 |        8 |
  |      2 |         3 |        7 |
  +-------------------------------+     
  </code></pre>
  
  <p>I want to get the amount that each customer is spending, essentially ending up with a result like this:</p>
  
  <pre><code>+----------------+
  | CustId | Total |
  +----------------+
  |      1 | 19.00 |
  |      2 | 29.00 |
  +----------------+
  </code></pre>
  
  <p>I can do a sum across a single table but the Sum method in LINQ takes a lambda with only one argument: the reference to the table that the values being summed are in.  These values are in different tables.  How do I sum these together?</p>