From a0b8146700e0c34b09b41dacc8a81bb05f1e36e1 Mon Sep 17 00:00:00 2001
From: Yaroslav <1.yaroslav.kazeev@gmail.com>
Date: Sun, 14 Sep 2025 01:50:40 +0200
Subject: [PATCH 01/13] DB is seeded

---
 Week4/homework/ex1-aggregation/index.js |  35 +++++
 package-lock.json                       | 185 ++++++++++++++++++++++++
 package.json                            |   8 +
 3 files changed, 228 insertions(+)
 create mode 100644 Week4/homework/ex1-aggregation/index.js
 create mode 100644 package-lock.json
 create mode 100644 package.json

diff --git a/Week4/homework/ex1-aggregation/index.js b/Week4/homework/ex1-aggregation/index.js
new file mode 100644
index 000000000..4ef55d7c0
--- /dev/null
+++ b/Week4/homework/ex1-aggregation/index.js
@@ -0,0 +1,35 @@
+import { MongoClient } from "mongodb";
+import "dotenv/config";
+const clientMongo = new MongoClient(process.env.MONGODB_URL);
+import fs from "fs";
+import csv from "csv-parser";
+
+async function processCSV(fileName) {
+  return new Promise((resolve, reject) => {
+    const data = [];
+    fs.createReadStream(fileName)
+      .pipe(csv())
+      .on("data", (row) => data.push(row))
+      .on("end", () => resolve(data))
+      .on("error", (err) => reject(err));
+  });
+}
+
+async function main() {
+  try {
+    await clientMongo.connect();
+    const db = clientMongo.db("databaseWeek4");
+    const collection = db.collection("population_pyramid");
+    let data = await processCSV(
+      "./Week4/homework/ex1-aggregation/population_pyramid_1950-2022.csv"
+    );
+    await collection.insertMany(data);
+  } catch (error) {
+    console.error("Error connecting to MongoDB:", error);
+  } finally {
+    await clientMongo.close();
+    console.log("Connection to MongoDB closed");
+  }
+}
+
+main();
diff --git a/package-lock.json b/package-lock.json
new file mode 100644
index 000000000..da3a745b9
--- /dev/null
+++ b/package-lock.json
@@ -0,0 +1,185 @@
+{
+  "name": "databases-Cohort53",
+  "lockfileVersion": 3,
+  "requires": true,
+  "packages": {
+    "": {
+      "dependencies": {
+        "csv-parser": "^3.2.0",
+        "dotenv": "^17.2.2",
+        "mongodb": "^6.19.0"
+      }
+    },
+    "node_modules/@mongodb-js/saslprep": {
+      "version": "1.3.0",
+      "resolved": "https://registry.npmjs.org/@mongodb-js/saslprep/-/saslprep-1.3.0.tgz",
+      "integrity": "sha512-zlayKCsIjYb7/IdfqxorK5+xUMyi4vOKcFy10wKJYc63NSdKI8mNME+uJqfatkPmOSMMUiojrL58IePKBm3gvQ==",
+      "license": "MIT",
+      "dependencies": {
+        "sparse-bitfield": "^3.0.3"
+      }
+    },
+    "node_modules/@types/webidl-conversions": {
+      "version": "7.0.3",
+      "resolved": "https://registry.npmjs.org/@types/webidl-conversions/-/webidl-conversions-7.0.3.tgz",
+      "integrity": "sha512-CiJJvcRtIgzadHCYXw7dqEnMNRjhGZlYK05Mj9OyktqV8uVT8fD2BFOB7S1uwBE3Kj2Z+4UyPmFw/Ixgw/LAlA==",
+      "license": "MIT"
+    },
+    "node_modules/@types/whatwg-url": {
+      "version": "11.0.5",
+      "resolved": "https://registry.npmjs.org/@types/whatwg-url/-/whatwg-url-11.0.5.tgz",
+      "integrity": "sha512-coYR071JRaHa+xoEvvYqvnIHaVqaYrLPbsufM9BF63HkwI5Lgmy2QR8Q5K/lYDYo5AK82wOvSOS0UsLTpTG7uQ==",
+      "license": "MIT",
+      "dependencies": {
+        "@types/webidl-conversions": "*"
+      }
+    },
+    "node_modules/bson": {
+      "version": "6.10.4",
+      "resolved": "https://registry.npmjs.org/bson/-/bson-6.10.4.tgz",
+      "integrity": "sha512-WIsKqkSC0ABoBJuT1LEX+2HEvNmNKKgnTAyd0fL8qzK4SH2i9NXg+t08YtdZp/V9IZ33cxe3iV4yM0qg8lMQng==",
+      "license": "Apache-2.0",
+      "engines": {
+        "node": ">=16.20.1"
+      }
+    },
+    "node_modules/csv-parser": {
+      "version": "3.2.0",
+      "resolved": "https://registry.npmjs.org/csv-parser/-/csv-parser-3.2.0.tgz",
+      "integrity": "sha512-fgKbp+AJbn1h2dcAHKIdKNSSjfp43BZZykXsCjzALjKy80VXQNHPFJ6T9Afwdzoj24aMkq8GwDS7KGcDPpejrA==",
+      "license": "MIT",
+      "bin": {
+        "csv-parser": "bin/csv-parser"
+      },
+      "engines": {
+        "node": ">= 10"
+      }
+    },
+    "node_modules/dotenv": {
+      "version": "17.2.2",
+      "resolved": "https://registry.npmjs.org/dotenv/-/dotenv-17.2.2.tgz",
+      "integrity": "sha512-Sf2LSQP+bOlhKWWyhFsn0UsfdK/kCWRv1iuA2gXAwt3dyNabr6QSj00I2V10pidqz69soatm9ZwZvpQMTIOd5Q==",
+      "license": "BSD-2-Clause",
+      "engines": {
+        "node": ">=12"
+      },
+      "funding": {
+        "url": "https://dotenvx.com"
+      }
+    },
+    "node_modules/memory-pager": {
+      "version": "1.5.0",
+      "resolved": "https://registry.npmjs.org/memory-pager/-/memory-pager-1.5.0.tgz",
+      "integrity": "sha512-ZS4Bp4r/Zoeq6+NLJpP+0Zzm0pR8whtGPf1XExKLJBAczGMnSi3It14OiNCStjQjM6NU1okjQGSxgEZN8eBYKg==",
+      "license": "MIT"
+    },
+    "node_modules/mongodb": {
+      "version": "6.19.0",
+      "resolved": "https://registry.npmjs.org/mongodb/-/mongodb-6.19.0.tgz",
+      "integrity": "sha512-H3GtYujOJdeKIMLKBT9PwlDhGrQfplABNF1G904w6r5ZXKWyv77aB0X9B+rhmaAwjtllHzaEkvi9mkGVZxs2Bw==",
+      "license": "Apache-2.0",
+      "dependencies": {
+        "@mongodb-js/saslprep": "^1.1.9",
+        "bson": "^6.10.4",
+        "mongodb-connection-string-url": "^3.0.0"
+      },
+      "engines": {
+        "node": ">=16.20.1"
+      },
+      "peerDependencies": {
+        "@aws-sdk/credential-providers": "^3.188.0",
+        "@mongodb-js/zstd": "^1.1.0 || ^2.0.0",
+        "gcp-metadata": "^5.2.0",
+        "kerberos": "^2.0.1",
+        "mongodb-client-encryption": ">=6.0.0 <7",
+        "snappy": "^7.3.2",
+        "socks": "^2.7.1"
+      },
+      "peerDependenciesMeta": {
+        "@aws-sdk/credential-providers": {
+          "optional": true
+        },
+        "@mongodb-js/zstd": {
+          "optional": true
+        },
+        "gcp-metadata": {
+          "optional": true
+        },
+        "kerberos": {
+          "optional": true
+        },
+        "mongodb-client-encryption": {
+          "optional": true
+        },
+        "snappy": {
+          "optional": true
+        },
+        "socks": {
+          "optional": true
+        }
+      }
+    },
+    "node_modules/mongodb-connection-string-url": {
+      "version": "3.0.2",
+      "resolved": "https://registry.npmjs.org/mongodb-connection-string-url/-/mongodb-connection-string-url-3.0.2.tgz",
+      "integrity": "sha512-rMO7CGo/9BFwyZABcKAWL8UJwH/Kc2x0g72uhDWzG48URRax5TCIcJ7Rc3RZqffZzO/Gwff/jyKwCU9TN8gehA==",
+      "license": "Apache-2.0",
+      "dependencies": {
+        "@types/whatwg-url": "^11.0.2",
+        "whatwg-url": "^14.1.0 || ^13.0.0"
+      }
+    },
+    "node_modules/punycode": {
+      "version": "2.3.1",
+      "resolved": "https://registry.npmjs.org/punycode/-/punycode-2.3.1.tgz",
+      "integrity": "sha512-vYt7UD1U9Wg6138shLtLOvdAu+8DsC/ilFtEVHcH+wydcSpNE20AfSOduf6MkRFahL5FY7X1oU7nKVZFtfq8Fg==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=6"
+      }
+    },
+    "node_modules/sparse-bitfield": {
+      "version": "3.0.3",
+      "resolved": "https://registry.npmjs.org/sparse-bitfield/-/sparse-bitfield-3.0.3.tgz",
+      "integrity": "sha512-kvzhi7vqKTfkh0PZU+2D2PIllw2ymqJKujUcyPMd9Y75Nv4nPbGJZXNhxsgdQab2BmlDct1YnfQCguEvHr7VsQ==",
+      "license": "MIT",
+      "dependencies": {
+        "memory-pager": "^1.0.2"
+      }
+    },
+    "node_modules/tr46": {
+      "version": "5.1.1",
+      "resolved": "https://registry.npmjs.org/tr46/-/tr46-5.1.1.tgz",
+      "integrity": "sha512-hdF5ZgjTqgAntKkklYw0R03MG2x/bSzTtkxmIRw/sTNV8YXsCJ1tfLAX23lhxhHJlEf3CRCOCGGWw3vI3GaSPw==",
+      "license": "MIT",
+      "dependencies": {
+        "punycode": "^2.3.1"
+      },
+      "engines": {
+        "node": ">=18"
+      }
+    },
+    "node_modules/webidl-conversions": {
+      "version": "7.0.0",
+      "resolved": "https://registry.npmjs.org/webidl-conversions/-/webidl-conversions-7.0.0.tgz",
+      "integrity": "sha512-VwddBukDzu71offAQR975unBIGqfKZpM+8ZX6ySk8nYhVoo5CYaZyzt3YBvYtRtO+aoGlqxPg/B87NGVZ/fu6g==",
+      "license": "BSD-2-Clause",
+      "engines": {
+        "node": ">=12"
+      }
+    },
+    "node_modules/whatwg-url": {
+      "version": "14.2.0",
+      "resolved": "https://registry.npmjs.org/whatwg-url/-/whatwg-url-14.2.0.tgz",
+      "integrity": "sha512-De72GdQZzNTUBBChsXueQUnPKDkg/5A5zp7pFDuQAj5UFoENpiACU0wlCvzpAGnTkj++ihpKwKyYewn/XNUbKw==",
+      "license": "MIT",
+      "dependencies": {
+        "tr46": "^5.1.0",
+        "webidl-conversions": "^7.0.0"
+      },
+      "engines": {
+        "node": ">=18"
+      }
+    }
+  }
+}
diff --git a/package.json b/package.json
new file mode 100644
index 000000000..5822ae68d
--- /dev/null
+++ b/package.json
@@ -0,0 +1,8 @@
+{
+  "dependencies": {
+    "csv-parser": "^3.2.0",
+    "dotenv": "^17.2.2",
+    "mongodb": "^6.19.0"
+  },
+  "type": "module"
+}

From b8eff2310e4bb8326b36c3e71cb837f3f33b6c44 Mon Sep 17 00:00:00 2001
From: Yaroslav <1.yaroslav.kazeev@gmail.com>
Date: Sun, 14 Sep 2025 11:36:20 +0200
Subject: [PATCH 02/13] getTotalPopulation works

---
 Week4/homework/ex1-aggregation/index.js | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/Week4/homework/ex1-aggregation/index.js b/Week4/homework/ex1-aggregation/index.js
index 4ef55d7c0..01aec1034 100644
--- a/Week4/homework/ex1-aggregation/index.js
+++ b/Week4/homework/ex1-aggregation/index.js
@@ -3,6 +3,7 @@ import "dotenv/config";
 const clientMongo = new MongoClient(process.env.MONGODB_URL);
 import fs from "fs";
 import csv from "csv-parser";
+let data;
 
 async function processCSV(fileName) {
   return new Promise((resolve, reject) => {
@@ -15,15 +16,37 @@ async function processCSV(fileName) {
   });
 }
 
+async function getTotalPopulationByYear(collection, country) {
+  const result = await collection
+    .aggregate([
+      { $match: { Country: country } },
+      {
+        $group: {
+          _id: { $toInt: "$Year" },
+          countPopulation: {
+            $sum: {
+              $add: [{ $toInt: "$M" }, { $toInt: "$F" }],
+            },
+          },
+        },
+      },
+      { $sort: { _id: 1 } },
+    ])
+    .toArray();
+  return result;
+}
+
 async function main() {
   try {
     await clientMongo.connect();
     const db = clientMongo.db("databaseWeek4");
     const collection = db.collection("population_pyramid");
-    let data = await processCSV(
+    data = await processCSV(
       "./Week4/homework/ex1-aggregation/population_pyramid_1950-2022.csv"
     );
     await collection.insertMany(data);
+    data = await getTotalPopulationByYear(collection, "Netherlands");
+    console.log(JSON.stringify(data, null, 2));
   } catch (error) {
     console.error("Error connecting to MongoDB:", error);
   } finally {

From 618f596da7a466ef38a14e090aa9160f058108f1 Mon Sep 17 00:00:00 2001
From: Yaroslav <1.yaroslav.kazeev@gmail.com>
Date: Sun, 14 Sep 2025 12:22:36 +0200
Subject: [PATCH 03/13] ContinentPopulation completed

---
 Week4/homework/ex1-aggregation/index.js | 50 ++++++++++++++++++++++++-
 1 file changed, 49 insertions(+), 1 deletion(-)

diff --git a/Week4/homework/ex1-aggregation/index.js b/Week4/homework/ex1-aggregation/index.js
index 01aec1034..a4e9dced4 100644
--- a/Week4/homework/ex1-aggregation/index.js
+++ b/Week4/homework/ex1-aggregation/index.js
@@ -36,6 +36,46 @@ async function getTotalPopulationByYear(collection, country) {
   return result;
 }
 
+async function getContinentsPopulation(collection, year, age) {
+  const result = await collection
+    .aggregate([
+      {
+        $match: {
+          Year: year.toString(),
+          Age: age.toString(),
+          // Filter only continents by checking if Country is uppercase
+          $expr: {
+            $eq: ["$Country", { $toUpper: "$Country" }],
+          },
+        },
+      },
+      {
+        $addFields: {
+          M: { $toInt: "$M" },
+          F: { $toInt: "$F" },
+        },
+      },
+      {
+        $addFields: {
+          TotalPopulation: { $add: ["$M", "$F"] },
+        },
+      },
+      {
+        $project: {
+          _id: 1,
+          Country: 1,
+          Year: 1,
+          Age: 1,
+          M: 1,
+          F: 1,
+          TotalPopulation: 1,
+        },
+      },
+    ])
+    .toArray();
+  return result;
+}
+
 async function main() {
   try {
     await clientMongo.connect();
@@ -46,7 +86,15 @@ async function main() {
     );
     await collection.insertMany(data);
     data = await getTotalPopulationByYear(collection, "Netherlands");
-    console.log(JSON.stringify(data, null, 2));
+    console.log(
+      "\nPopulation by Year (Netherlands):\n",
+      JSON.stringify(data, null, 2)
+    );
+    data = await getContinentsPopulation(collection, "2020", "100+");
+    console.log(
+      "\nPopulation by Continent (2020, 100+):\n",
+      JSON.stringify(data, null, 2)
+    );
   } catch (error) {
     console.error("Error connecting to MongoDB:", error);
   } finally {

From 83afd999b209ee685599c60308871ec97b073c2a Mon Sep 17 00:00:00 2001
From: Yaroslav <1.yaroslav.kazeev@gmail.com>
Date: Sun, 14 Sep 2025 12:31:16 +0200
Subject: [PATCH 04/13] cleaned up

---
 Week4/homework/ex1-aggregation/index.js | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/Week4/homework/ex1-aggregation/index.js b/Week4/homework/ex1-aggregation/index.js
index a4e9dced4..5adf824d3 100644
--- a/Week4/homework/ex1-aggregation/index.js
+++ b/Week4/homework/ex1-aggregation/index.js
@@ -51,13 +51,7 @@ async function getContinentsPopulation(collection, year, age) {
       },
       {
         $addFields: {
-          M: { $toInt: "$M" },
-          F: { $toInt: "$F" },
-        },
-      },
-      {
-        $addFields: {
-          TotalPopulation: { $add: ["$M", "$F"] },
+          TotalPopulation: { $add: [{ $toInt: "$M" }, { $toInt: "$F" }] },
         },
       },
       {

From 53f4153e63bc5e382bd29b793d93bd5c56d7fa96 Mon Sep 17 00:00:00 2001
From: Yaroslav <1.yaroslav.kazeev@gmail.com>
Date: Sun, 14 Sep 2025 22:20:00 +0200
Subject: [PATCH 05/13] ex2 files created

---
 Week4/homework/ex2-transactions/index.js    | 0
 Week4/homework/ex2-transactions/setup.js    | 0
 Week4/homework/ex2-transactions/transfer.js | 0
 3 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 Week4/homework/ex2-transactions/index.js
 create mode 100644 Week4/homework/ex2-transactions/setup.js
 create mode 100644 Week4/homework/ex2-transactions/transfer.js

diff --git a/Week4/homework/ex2-transactions/index.js b/Week4/homework/ex2-transactions/index.js
new file mode 100644
index 000000000..e69de29bb
diff --git a/Week4/homework/ex2-transactions/setup.js b/Week4/homework/ex2-transactions/setup.js
new file mode 100644
index 000000000..e69de29bb
diff --git a/Week4/homework/ex2-transactions/transfer.js b/Week4/homework/ex2-transactions/transfer.js
new file mode 100644
index 000000000..e69de29bb

From f9582940d4fe5f8f7f329e3a635409b18c2fb3c1 Mon Sep 17 00:00:00 2001
From: Yaroslav <1.yaroslav.kazeev@gmail.com>
Date: Mon, 15 Sep 2025 00:32:52 +0200
Subject: [PATCH 06/13] data inserted into collections

---
 Week4/homework/ex2-transactions/data.json | 41 +++++++++++++++++++++++
 Week4/homework/ex2-transactions/index.js  | 14 ++++++++
 Week4/homework/ex2-transactions/setup.js  | 17 ++++++++++
 3 files changed, 72 insertions(+)
 create mode 100644 Week4/homework/ex2-transactions/data.json

diff --git a/Week4/homework/ex2-transactions/data.json b/Week4/homework/ex2-transactions/data.json
new file mode 100644
index 000000000..7c813b96d
--- /dev/null
+++ b/Week4/homework/ex2-transactions/data.json
@@ -0,0 +1,41 @@
+{
+  "accounts": [
+    { "account_number": 101, "balance": 5000 },
+    { "account_number": 102, "balance": 12000 },
+    { "account_number": 103, "balance": 7500 },
+    { "account_number": 104, "balance": 3000 },
+    { "account_number": 105, "balance": 9500 }
+  ],
+  "account_changes": [
+    {
+      "account_number": 101,
+      "amount": 1000,
+      "changed_date": "2025-09-01",
+      "remark": "Salary deposit"
+    },
+    {
+      "account_number": 102,
+      "amount": -500,
+      "changed_date": "2025-09-02",
+      "remark": "Grocery shopping"
+    },
+    {
+      "account_number": 103,
+      "amount": 2000,
+      "changed_date": "2025-09-03",
+      "remark": "Freelance payment"
+    },
+    {
+      "account_number": 104,
+      "amount": -1000,
+      "changed_date": "2025-09-04",
+      "remark": "Rent payment"
+    },
+    {
+      "account_number": 105,
+      "amount": 1500,
+      "changed_date": "2025-09-05",
+      "remark": "Gift received"
+    }
+  ]
+}
diff --git a/Week4/homework/ex2-transactions/index.js b/Week4/homework/ex2-transactions/index.js
index e69de29bb..4f6ce1014 100644
--- a/Week4/homework/ex2-transactions/index.js
+++ b/Week4/homework/ex2-transactions/index.js
@@ -0,0 +1,14 @@
+import { setup } from "./setup.js";
+let clientMongo, account, accountChanges;
+async function main() {
+  try {
+    ({ clientMongo, account, accountChanges } = await setup());
+  } catch (error) {
+    console.error("Error connecting to MongoDB:", error);
+  } finally {
+    await clientMongo.close();
+    console.log("Connection to MongoDB closed");
+  }
+}
+
+main();
diff --git a/Week4/homework/ex2-transactions/setup.js b/Week4/homework/ex2-transactions/setup.js
index e69de29bb..f9288375e 100644
--- a/Week4/homework/ex2-transactions/setup.js
+++ b/Week4/homework/ex2-transactions/setup.js
@@ -0,0 +1,17 @@
+import { MongoClient } from "mongodb";
+import "dotenv/config";
+import fs from "fs";
+
+export async function setup() {
+  const { accounts, account_changes } = JSON.parse(
+    fs.readFileSync("./Week4/homework/ex2-transactions/data.json")
+  );
+  const clientMongo = new MongoClient(process.env.MONGODB_URL);
+  await clientMongo.connect();
+  const db = clientMongo.db("databaseWeek4");
+  const account = await db.createCollection("accounts");
+  await account.insertMany(accounts);
+  const accountChanges = await db.createCollection("account_changes");
+  await accountChanges.insertMany(account_changes);
+  return { clientMongo, account, accountChanges };
+}

From 7392b0cbd36ee7fa325aaf1e6aac6074a3ef5e44 Mon Sep 17 00:00:00 2001
From: Yaroslav <1.yaroslav.kazeev@gmail.com>
Date: Mon, 15 Sep 2025 00:41:07 +0200
Subject: [PATCH 07/13] collections are cleaned up before being populated

---
 Week4/homework/ex2-transactions/setup.js | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Week4/homework/ex2-transactions/setup.js b/Week4/homework/ex2-transactions/setup.js
index f9288375e..8bc888bb6 100644
--- a/Week4/homework/ex2-transactions/setup.js
+++ b/Week4/homework/ex2-transactions/setup.js
@@ -10,8 +10,10 @@ export async function setup() {
   await clientMongo.connect();
   const db = clientMongo.db("databaseWeek4");
   const account = await db.createCollection("accounts");
+  await account.deleteMany({});
   await account.insertMany(accounts);
   const accountChanges = await db.createCollection("account_changes");
+  await accountChanges.deleteMany({});
   await accountChanges.insertMany(account_changes);
   return { clientMongo, account, accountChanges };
 }

From 7948d7d431e234708e7f928eec5f4e0aad0d659f Mon Sep 17 00:00:00 2001
From: Yaroslav <1.yaroslav.kazeev@gmail.com>
Date: Mon, 15 Sep 2025 10:24:44 +0200
Subject: [PATCH 08/13] account_changes embedded into accounts

---
 Week4/homework/ex2-transactions/index.js |  4 ++--
 Week4/homework/ex2-transactions/setup.js | 20 +++++++++++++++-----
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/Week4/homework/ex2-transactions/index.js b/Week4/homework/ex2-transactions/index.js
index 4f6ce1014..0d62da095 100644
--- a/Week4/homework/ex2-transactions/index.js
+++ b/Week4/homework/ex2-transactions/index.js
@@ -1,8 +1,8 @@
 import { setup } from "./setup.js";
-let clientMongo, account, accountChanges;
+let clientMongo, account;
 async function main() {
   try {
-    ({ clientMongo, account, accountChanges } = await setup());
+    ({ clientMongo, account } = await setup());
   } catch (error) {
     console.error("Error connecting to MongoDB:", error);
   } finally {
diff --git a/Week4/homework/ex2-transactions/setup.js b/Week4/homework/ex2-transactions/setup.js
index 8bc888bb6..c9c38d886 100644
--- a/Week4/homework/ex2-transactions/setup.js
+++ b/Week4/homework/ex2-transactions/setup.js
@@ -11,9 +11,19 @@ export async function setup() {
   const db = clientMongo.db("databaseWeek4");
   const account = await db.createCollection("accounts");
   await account.deleteMany({});
-  await account.insertMany(accounts);
-  const accountChanges = await db.createCollection("account_changes");
-  await accountChanges.deleteMany({});
-  await accountChanges.insertMany(account_changes);
-  return { clientMongo, account, accountChanges };
+  // await account.insertMany(accounts);
+  for (const acc of accounts) {
+    acc.account_changes = account_changes
+      .filter((a) => a.account_number === acc.account_number)
+      .map((a) => {
+        delete a.account_number;
+        return a;
+      });
+    console.log(acc);
+    await account.insertOne(acc);
+  }
+  // const accountChanges = await db.createCollection("account_changes");
+  // await accountChanges.deleteMany({});
+  // await accountChanges.insertMany(account_changes);
+  return { clientMongo, account };
 }

From b527da9d69eadb6361537593312cce0053b41119 Mon Sep 17 00:00:00 2001
From: Yaroslav <1.yaroslav.kazeev@gmail.com>
Date: Mon, 15 Sep 2025 10:26:01 +0200
Subject: [PATCH 09/13] cleaned up

---
 Week4/homework/ex2-transactions/setup.js | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/Week4/homework/ex2-transactions/setup.js b/Week4/homework/ex2-transactions/setup.js
index c9c38d886..e35972546 100644
--- a/Week4/homework/ex2-transactions/setup.js
+++ b/Week4/homework/ex2-transactions/setup.js
@@ -11,7 +11,6 @@ export async function setup() {
   const db = clientMongo.db("databaseWeek4");
   const account = await db.createCollection("accounts");
   await account.deleteMany({});
-  // await account.insertMany(accounts);
   for (const acc of accounts) {
     acc.account_changes = account_changes
       .filter((a) => a.account_number === acc.account_number)
@@ -19,11 +18,7 @@ export async function setup() {
         delete a.account_number;
         return a;
       });
-    console.log(acc);
     await account.insertOne(acc);
   }
-  // const accountChanges = await db.createCollection("account_changes");
-  // await accountChanges.deleteMany({});
-  // await accountChanges.insertMany(account_changes);
   return { clientMongo, account };
 }

From d83c05ee38f5a6d8d238e1c34c0c52716e35e15e Mon Sep 17 00:00:00 2001
From: Yaroslav <1.yaroslav.kazeev@gmail.com>
Date: Mon, 15 Sep 2025 11:23:02 +0200
Subject: [PATCH 10/13] transaction in progress

---
 Week4/homework/ex2-transactions/index.js      |  9 +++
 .../homework/ex2-transactions/transaction.js  | 48 +++++++++++++++
 Week4/homework/ex2-transactions/transfer.js   | 60 +++++++++++++++++++
 3 files changed, 117 insertions(+)
 create mode 100644 Week4/homework/ex2-transactions/transaction.js

diff --git a/Week4/homework/ex2-transactions/index.js b/Week4/homework/ex2-transactions/index.js
index 0d62da095..a74409be9 100644
--- a/Week4/homework/ex2-transactions/index.js
+++ b/Week4/homework/ex2-transactions/index.js
@@ -1,8 +1,17 @@
 import { setup } from "./setup.js";
+import { transfer } from "./transfer.js";
 let clientMongo, account;
 async function main() {
   try {
     ({ clientMongo, account } = await setup());
+    const transactionDetails = {
+      donator_account_number: 101,
+      receiver_account_number: 102,
+      amount: 1000,
+      changed_date: new Date().toISOString().slice(0, 10), // 'YYYY-MM-DD'
+      remark: `Transfer from 101 to 102`,
+    };
+    await transfer(clientMongo, account, transactionDetails);
   } catch (error) {
     console.error("Error connecting to MongoDB:", error);
   } finally {
diff --git a/Week4/homework/ex2-transactions/transaction.js b/Week4/homework/ex2-transactions/transaction.js
new file mode 100644
index 000000000..93f4e8f91
--- /dev/null
+++ b/Week4/homework/ex2-transactions/transaction.js
@@ -0,0 +1,48 @@
+import { Client } from "pg";
+const config = {
+  host: "localhost",
+  user: "hyfuser",
+  password: "hyfpassword",
+  database: "transactions_week3",
+  port: 5432,
+};
+const client = new Client(config);
+
+async function seedDatabase(client) {
+  try {
+    await client.connect();
+    console.log("Connected to PostgreSQL database!");
+
+    const donator_account_number = 101;
+    const receiver_account_number = 102;
+    const amount = 1000;
+    const changed_date = new Date().toISOString().slice(0, 10); // 'YYYY-MM-DD'
+    const remark = `Transfer from 101 to 102`;
+
+    await client.query("BEGIN");
+    await client.query(
+      "UPDATE ACCOUNT SET balance = balance - $1 WHERE account_number = $2",
+      [amount, donator_account_number]
+    );
+    await client.query(
+      "UPDATE ACCOUNT SET balance = balance + $1 WHERE account_number = $2",
+      [amount, receiver_account_number]
+    );
+    await client.query(
+      "INSERT INTO account_changes(account_number, amount, changed_date, remark) VALUES($1, $2, $3, $4)",
+      [donator_account_number, -amount, changed_date, remark]
+    );
+    await client.query(
+      "INSERT INTO account_changes(account_number, amount, changed_date, remark) VALUES($1, $2, $3, $4)",
+      [receiver_account_number, amount, changed_date, remark]
+    );
+    await client.query("COMMIT");
+    console.log("Transaction completed!");
+  } catch (error) {
+    console.error("Error seeding database:", error);
+  } finally {
+    await client.end();
+  }
+}
+
+seedDatabase(client);
diff --git a/Week4/homework/ex2-transactions/transfer.js b/Week4/homework/ex2-transactions/transfer.js
index e69de29bb..9f25123e5 100644
--- a/Week4/homework/ex2-transactions/transfer.js
+++ b/Week4/homework/ex2-transactions/transfer.js
@@ -0,0 +1,60 @@
+export async function transfer(clientMongo, account, transactionDetails) {
+  const {
+    donator_account_number,
+    receiver_account_number,
+    amount,
+    changed_date,
+    remark,
+  } = transactionDetails;
+  const session = clientMongo.startSession();
+  const transactionOptions = {
+    readPreference: "primary",
+    readConcern: { level: "local" },
+    writeConcern: { w: "majority" },
+  };
+  try {
+    await session.withTransaction(async () => {
+      // Deduct from donator
+      await account.updateOne(
+        { account_number: donator_account_number },
+        { $inc: { balance: -amount } },
+        { session }
+      );
+
+      // Add to receiver
+      await account.updateOne(
+        { account_number: receiver_account_number },
+        { $inc: { balance: amount } },
+        { session }
+      );
+
+      // Log donator change
+      await clientMongo.db().collection("account_changes").insertOne(
+        {
+          account_number: donator_account_number,
+          amount: -amount,
+          changed_date,
+          remark,
+        },
+        { session }
+      );
+
+      // Log receiver change
+      await clientMongo.db().collection("account_changes").insertOne(
+        {
+          account_number: receiver_account_number,
+          amount,
+          changed_date,
+          remark,
+        },
+        { session }
+      );
+    }, transactionOptions);
+    console.log("Transaction committed.");
+  } catch (e) {
+    console.log("Transaction aborted.", e);
+  } finally {
+    await session.endSession();
+    console.log("Transaction completed!");
+  }
+}

From 284886713e525fa92ac47d68d08a947e67e1fb9b Mon Sep 17 00:00:00 2001
From: Yaroslav <1.yaroslav.kazeev@gmail.com>
Date: Mon, 15 Sep 2025 11:50:11 +0200
Subject: [PATCH 11/13] transfer function works

---
 Week4/homework/ex2-transactions/transfer.js | 33 +++++----------------
 1 file changed, 7 insertions(+), 26 deletions(-)

diff --git a/Week4/homework/ex2-transactions/transfer.js b/Week4/homework/ex2-transactions/transfer.js
index 9f25123e5..aff527eab 100644
--- a/Week4/homework/ex2-transactions/transfer.js
+++ b/Week4/homework/ex2-transactions/transfer.js
@@ -14,45 +14,26 @@ export async function transfer(clientMongo, account, transactionDetails) {
   };
   try {
     await session.withTransaction(async () => {
-      // Deduct from donator
       await account.updateOne(
         { account_number: donator_account_number },
-        { $inc: { balance: -amount } },
+        {
+          $inc: { balance: -amount },
+          $push: { account_changes: { amount: -amount, changed_date, remark } },
+        },
         { session }
       );
-
-      // Add to receiver
       await account.updateOne(
         { account_number: receiver_account_number },
         { $inc: { balance: amount } },
-        { session }
-      );
-
-      // Log donator change
-      await clientMongo.db().collection("account_changes").insertOne(
-        {
-          account_number: donator_account_number,
-          amount: -amount,
-          changed_date,
-          remark,
-        },
-        { session }
-      );
-
-      // Log receiver change
-      await clientMongo.db().collection("account_changes").insertOne(
         {
-          account_number: receiver_account_number,
-          amount,
-          changed_date,
-          remark,
+          $push: { account_changes: { amount: amount, changed_date, remark } },
         },
         { session }
       );
     }, transactionOptions);
     console.log("Transaction committed.");
-  } catch (e) {
-    console.log("Transaction aborted.", e);
+  } catch (error) {
+    console.log("Transaction aborted.", error);
   } finally {
     await session.endSession();
     console.log("Transaction completed!");

From 7b422ffad17d1769f9231d4a80365621f48cee1b Mon Sep 17 00:00:00 2001
From: Yaroslav <1.yaroslav.kazeev@gmail.com>
Date: Fri, 19 Sep 2025 22:24:42 +0200
Subject: [PATCH 12/13] the second updateOne bug fix

---
 Week4/homework/ex2-transactions/transfer.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Week4/homework/ex2-transactions/transfer.js b/Week4/homework/ex2-transactions/transfer.js
index aff527eab..5e0a25dd7 100644
--- a/Week4/homework/ex2-transactions/transfer.js
+++ b/Week4/homework/ex2-transactions/transfer.js
@@ -24,8 +24,8 @@ export async function transfer(clientMongo, account, transactionDetails) {
       );
       await account.updateOne(
         { account_number: receiver_account_number },
-        { $inc: { balance: amount } },
         {
+          $inc: { balance: amount },
           $push: { account_changes: { amount: amount, changed_date, remark } },
         },
         { session }

From de1ba06bbe79566be3d49634d6e229129b919d4b Mon Sep 17 00:00:00 2001
From: Yaroslav <1.yaroslav.kazeev@gmail.com>
Date: Fri, 19 Sep 2025 22:32:28 +0200
Subject: [PATCH 13/13] the error is rethrown to catch in index.js

---
 Week4/homework/ex2-transactions/index.js    | 2 +-
 Week4/homework/ex2-transactions/transfer.js | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/Week4/homework/ex2-transactions/index.js b/Week4/homework/ex2-transactions/index.js
index a74409be9..61b11d0df 100644
--- a/Week4/homework/ex2-transactions/index.js
+++ b/Week4/homework/ex2-transactions/index.js
@@ -13,7 +13,7 @@ async function main() {
     };
     await transfer(clientMongo, account, transactionDetails);
   } catch (error) {
-    console.error("Error connecting to MongoDB:", error);
+    console.error("Error:", error);
   } finally {
     await clientMongo.close();
     console.log("Connection to MongoDB closed");
diff --git a/Week4/homework/ex2-transactions/transfer.js b/Week4/homework/ex2-transactions/transfer.js
index 5e0a25dd7..9c28bd2a7 100644
--- a/Week4/homework/ex2-transactions/transfer.js
+++ b/Week4/homework/ex2-transactions/transfer.js
@@ -34,6 +34,7 @@ export async function transfer(clientMongo, account, transactionDetails) {
     console.log("Transaction committed.");
   } catch (error) {
     console.log("Transaction aborted.", error);
+    throw new Error(error);
   } finally {
     await session.endSession();
     console.log("Transaction completed!");