Added $out and some info on $unwind

Author: dfreniche

docs/40-using-arrays/2-search-inside-objects-in-arrays.mdx

@@ -51,4 +51,14 @@ db.books.aggregate([
     {$match: {"attributes.key": "msrp", "attributes.value": 9.99} },
    {$project: {title: 1, attributes: 1}}
 ]);
-```
+```
+
+Here we're getting a copy of each book for each object inside the `$attributes` array. This "flattens" the array and returns many copies of the same documents, one for each different attribute that we have which will be different for each of these generated docs.
+
+To understand `$unwind`, run this aggregation
+
+db.books.aggregate([
+  { $match: {_id: "0002158698"} },
+  { $unwind : "$genres" },
+  { $project: {vectorizedSynopsis: 0, attributes: 0}}
+]);

docs/90-exporting-data/saving-to-collection.mdx

@@ -1,7 +1,37 @@
 # Saving to a collection
 
-You can export the result of an aggregation pipeline to a different DB / Collection
+You can export the result of an aggregation pipeline to a different DB / Collection. To do that just add a last aggregation stage to your pipeline:
 
 ```js
 { $out: { db: "<output-db>", coll: "<output-collection>" } }
-```
+```
+
+You can omit the `db` attribute, in which case the new collection is created in the current DB.
+
+```js
+{ $out: "<output-collection>" }
+```
+
+💻 Create a copy of the books with exactly 100 pages and output as a new collection named `OneHundredPagesBooks.
+
+<details>
+<summary>Answer</summary>
+<div>
+
+```js
+let oneHundredPagesBooks = {$match: {pages: 100}};
+let saveToNewCollection = {$out: "OneHundredPagesBooks"}
+
+db.books.aggregate([
+    oneHundredPagesBooks,
+    saveToNewCollection
+]);
+```
+
+After running this, we should see a new collection with:
+
+```js
+show collections
+```
+</div>
+</details>