Merge pull request learning-zone#4 from srinivasKandukuri/master

Join 3 or more collections

Author: learning-zone

mongodb-code.md

@@ -354,4 +354,142 @@ The operation results in the following document:
 }
 ```
 
+## Join 3 or more collections
+
+Let's say we have 3 hypothetical collections in MongoDB: customers, orders, and orderItems.
+
+Each customer has multiple orders, and each order has multiple order items.
+
+
+### customers
+
+```js
+[
+    {
+        customer_id: 1,
+        name: "Jim Smith",
+        email: "jim.smith@example.com"
+    },
+    {
+        customer_id: 2,
+        name: "Bob Jones",
+        email: "bob.jones@example.com"
+    }
+]
+```
+### orders
+```js
+[
+    {
+        order_id: 1,
+        customer_id: 1
+    },
+    {
+        order_id: 2,
+        customer_id: 1
+    }
+]
+```
+
+### orderItems
+```js
+[
+    {
+        order_item_id: 1,
+        name: "Foo",
+        price: 4.99,
+        order_id: 1
+    },
+    {
+        order_item_id: 2,
+        name: "Bar",
+        price: 17.99,
+        order_id: 1
+    },
+    {
+        order_item_id: 3,
+        name: "baz",
+        price: 24.99,
+        order_id: 2
+    }
+]
+```
+
+### Desired Result
+```js
+[
+    {
+        customer_id: 1,
+        name: "Jim Smith",
+        email: "jim.smith@example.com"
+        orders: [
+            {
+                order_id: 1,
+                items: [
+                    {
+                        name: "Foo",
+                        price: 4.99
+                    },
+                    {
+                        name: "Bar",
+                        price: 17.99
+                    }
+                ]
+            },
+            {
+                order_id: 2,
+                items: [
+                    {
+                        name: "baz",
+                        price: 24.99
+                    }
+                ]
+            }
+        ]
+    },
+    {
+        customer_id: 2,
+        name: "Bob Jones",
+        email: "bob.jones@example.com"
+        orders: []
+    }
+]
+```
+
+
+# Answer
+
+Do nested lookup using lookup with pipeline,
+
+1. ```$lookup``` with orders collection.
+2. ```let```, define variable customer_id that is from main collection, to access this reference variable inside pipeline using ```$$``` like ```$$customer_id```.
+3. ```pipeline``` can add pipeline stages same as we do in root level pipeline
+4. ```$expr``` whenever we match internal fields it requires expression match condition, so ```$$customer_id``` is parent collection field that declared in let and $customer_id is child collection's/current collection's field
+5. ```$lookup``` with orderitems collection
+
+
+```js
+db.customers.aggregate([
+  {
+    $lookup: {
+      from: "orders",
+      let: { customer_id: "$customer_id" },
+      pipeline: [
+        { $match: { $expr: { $eq: ["$$customer_id", "$customer_id"] } } },
+        {
+          $lookup: {
+            from: "orderitems",
+            localField: "order_id",
+            foreignField: "order_id",
+            as: "items"
+          }
+        }
+      ],
+      as: "orders"
+    }
+  }
+])
+```
+
+