Linq - SelectMany Confusion

Question

From what I understand from the documentation of SelectMany, one could use it to produce a (flattened) sequence of a 1-many relationship.

I have following classes

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Answer 1

Here is another option using SelectMany

var customerOrders = customers.SelectMany(
  c => orders.Where(o => o.CustomerId == c.Id)
    .Select(p => new {CustomerId = c.Id, OrderDescription = p.Description}));

If you use the Entity Framework or LINQ to Sql and you have an association (relationship) between the entities, then you can do so:

var customerOrders = customers.SelectMany(
  c => c.orders
   .Select(p => new {CustomerId = c.Id, OrderDescription = p.Description}));