Update the field names in the example and challenge queries

- Includes suggested change in #16

Author: bohyunjung

docs/40-CRUD/1-WHERE.mdx

@@ -115,7 +115,7 @@ Now, translate the following into a MongoDB query.
   <summary>Answer</summary>
   <div>
     ```js
-      db.books.find({ genres: "Science", pages: {$gt: 300} });
+    db.books.find({ "genre.name": "Science", pages: {$gt: 300} });
     ```
   </div>
 </details>

docs/40-CRUD/2-SELECT.mdx

@@ -55,7 +55,7 @@ Here:
 ## **Example 3: Using projection along with a query**
 
 ```js
-db.books.find({ genres: "Science" }, { title: 1, totalInventory: 1, _id: 0 });
+db.books.find({ "genre.name": "Science" }, { title: 1, totalInventory: 1, _id: 0 });
 ```
 
 **Equivalent SQL query:**
@@ -90,7 +90,7 @@ Here:
   <summary>Answer</summary>
   <div>
     ```js
-      db.books.find({genres: "History"}, {_id: 0, authors: 0});
+    db.books.find({ "genre.name": "History" }, { _id: 0, authors: 0 });
     ```
   </div>
 </details>

docs/40-CRUD/3-ORDER-LIMIT.mdx

@@ -37,7 +37,7 @@ This fetches the **5 books with the highest stock**.
 
 ```js
 db.books
-  .find({ genres: "Fiction" }, { title: 1, pages: 1 })
+  .find({ "genre.name": "Fiction" }, { title: 1, pages: 1 })
   .sort({ pages: -1 })
   .limit(10);
 ```

docs/50-aggregation/4-group.mdx

@@ -137,10 +137,10 @@ GROUP BY year;
     db.reviews.aggregate([
       {
         $group: {
-          _id: "$bookId",
+          _id: "$_id.bookId",
           avgRating: { $avg: "$rating" }
         }
-      },
+      }
     ]);
     ```
 

docs/50-aggregation/5-lookup.mdx

@@ -99,12 +99,12 @@ The $lookup operation creates an array within each book document. Using $unwind
     db.books.aggregate([
       {
         $lookup:
-          {
-            from: "reviews",
-            localField: "_id",
-            foreignField: "bookId",
-            as: "reviews"
-          }
+        {
+          from: "reviews",
+          localField: "_id",
+          foreignField: "_id.bookId",
+          as: "reviews"
+        }
       }
     ]);
     ```

docs/50-aggregation/7-merge.mdx

@@ -13,10 +13,10 @@ The `$merge` stage enables you to store aggregation results into a different col
 
 ### **Key features:**
 
-✔️ Inserts new documents if they don’t exist
-✔️ Updates existing documents based on `_id` or a specified field
-✔️ Can replace, merge, or discard duplicate records
-✔️ Useful for **ETL workflows, reporting tables, and maintaining summary data**
+- ✔️ Inserts new documents if they don’t exist
+- ✔️ Updates existing documents based on `_id` or a specified field
+- ✔️ Can replace, merge, or discard duplicate records
+- ✔️ Useful for **ETL workflows, reporting tables, and maintaining summary data**
 
 ---
 
@@ -51,8 +51,8 @@ The `$merge` stage enables you to store aggregation results into a different col
 
 ```js
 db.books.aggregate([
-  { $unwind: "$genres" },
-  { $group: { _id: "$genres", totalBooks: { $sum: 1 } } },
+  { $unwind: "$genre" },
+  { $group: { _id: "$genre.genreId", totalBooks: { $sum: 1 } } },
   {
     $merge: {
       into: "genre_summary",
@@ -99,18 +99,18 @@ ON DUPLICATE KEY UPDATE totalBooks = VALUES(totalBooks);
   <Tabs groupId="aggregations">
     <TabItem value="books" label="through 'books' collection">
     ```js
-      db.books.aggregate([
-        { $unwind: "$authors" },
-        { $group: { _id: "$authors.name", totalBooks: { $sum: 1 } } },
-        {
-          $merge: {
-            into: "author_stats",
-            on: "_id",
-            whenMatched: "merge",
-            whenNotMatched: "insert",
-          },
+    db.books.aggregate([
+      { $unwind: "$authors" },
+      { $group: { _id: "$authors.name", totalBooks: { $sum: 1 } } },
+      {
+        $merge: {
+          into: "author_stats",
+          on: "_id",
+          whenMatched: "merge",
+          whenNotMatched: "insert",
         },
-      ]);
+      },
+    ]);
     ```
     </TabItem>