DOCSP-40066: Support SelectMany inside Project/Select (#373)

(cherry picked from commit 8c2bdc5)

Author: mayaraman19

source/fundamentals/linq.txt

@@ -320,6 +320,96 @@ following documents:
    { "date" : ISODate("2012-12-05T00:00:00Z"), "grade" : "A", "score" : 13 }
    { "date" : ISODate("2012-05-17T00:00:00Z"), "grade" : "A", "score" : 11 }
 
+Nested Statements
++++++++++++++++++
+
+You can chain or nest ``Select`` and ``SelectMany`` statements to unwind nested
+arrays. Consider a collection that contains documents with a **new** schema. These
+documents contain a ``restaurants`` field, which holds an array of documents 
+represented by the ``Restaurant`` class. The documents within the array each have
+a ``grades`` field that holds an array of documents represented by
+the ``Grade`` class. The following code is an example of a single document in
+this collection:
+
+.. code-block:: json
+
+   { 
+      "_id": { "$oid": ... },
+      "restaurants": [
+         {
+            "_id": { ... } ,
+            "address": { ... },
+            "name": "Tov Kosher Kitchen",
+            "grades": [
+               { 
+                  "date" : ISODate("2014-11-24T00:00:00Z"), 
+                  "grade" : "Z", 
+                  "score" : 20.0 
+               }, 
+               { 
+                  "date" : ISODate("2013-01-17T00:00:00Z"), 
+                  "grade" : "A", 
+                  "score" : 13.0 
+               }
+            ]
+            ...
+         },
+         {
+            "_id": { ... } ,
+            "address": { ... },
+            "name": "Harriet's Kitchen",
+            "grades": [ 
+               { 
+                  "date" : ISODate("2014-04-19T00:00:00Z"), 
+                  "grade" : "B", 
+                  "score" : 12.0 
+               }
+             ],
+            ...
+         },
+         ...
+      ]
+   }
+
+You can nest ``SelectMany`` statements within ``SelectMany`` or ``Select``
+statements. The following example nests a ``SelectMany`` statement within a
+``Select`` statement to retrieve an array from each document in the collection.
+Each array holds all grade objects from all restaurants in each document.
+
+.. io-code-block::
+   :copyable: true
+
+   .. input:: /includes/fundamentals/code-examples/linq.cs
+      :language: csharp
+      :start-after: start-nested-SelectMany
+      :end-before: end-nested-SelectMany
+
+   .. output::
+      :visible: false
+      :language: json
+
+      // output for first document in collection
+      [
+         { "date" : ISODate("2014-11-24T00:00:00Z"), 
+            "grade" : "Z", 
+            "score" : 20.0 
+         }, 
+         { "date" : ISODate("2013-01-17T00:00:00Z"), 
+            "grade" : "A", 
+            "score" : 13.0 
+         }, 
+         { 
+            "date" : ISODate("2014-04-19T00:00:00Z"), 
+            "grade" : "B", 
+            "score" : 12.0 
+         },
+         ...
+      ], 
+      // output for second document in collection
+      [
+         ...
+      ]
+
 $group
 ~~~~~~
 

source/includes/fundamentals/code-examples/linq.cs

@@ -81,6 +81,11 @@ public class Ingredient
 
 // end-ingredient-model
 
+// start-nested-SelectMany
+var query = queryableCollection
+    .Select(r => r.Restaurants.SelectMany(r => r.Grades));
+// end-nested-SelectMany
+
 // start-bitAnd-example
 
 var query = queryableCollection