Alright, let's get Firebase set up in your project. Here's what you need to do:

1. Head over to the Firebase Console and create a new project.
2. Add your app to this Firebase project.
3. Install the Firebase SDK in your project. If you're using npm, just run:

npm install firebase

1. Now, let's initialize Firebase in your app code:

import firebase from "firebase/app";
import "firebase/firestore";

const firebaseConfig = {
  apiKey: "YOUR_API_KEY",
  authDomain: "YOUR_PROJECT_ID.firebaseapp.com",
  projectId: "YOUR_PROJECT_ID",
  storageBucket: "YOUR_PROJECT_ID.appspot.com",
  messagingSenderId: "YOUR_MESSAGING_SENDER_ID",
  appId: "YOUR_APP_ID"
};

firebase.initializeApp(firebaseConfig);

const db = firebase.firestore();

Next, think about how you want to structure your data. You have a few options:

• Nested collections
• Subcollections
• Field arrays within documents

Pick the one that makes the most sense for your data and how you'll be accessing it.

You can use Firestore's console or your app code to create your initial collections and documents. Here's an example:

// Create a root collection `companies`
db.collection('companies').doc('companyA').set({
  name: "Company A",
  industry: "Tech"
});

Now, let's add some hierarchical data. You can do this by creating subcollections or nested documents.

Example 1: Using Subcollections

// Add a subcollection `employees` under `companyA`
db.collection('companies').doc('companyA')
  .collection('employees').doc('employee1').set({
    name: "John Doe",
    position: "Developer"
  });

// Add another document in the subcollection
db.collection('companies').doc('companyA')
  .collection('employees').doc('employee2').set({
    name: "Jane Smith",
    position: "Designer"
  });

Example 2: Using Nested Data

// Add nested fields within the document
db.collection('companies').doc('companyA').set({
  name: "Company A",
  industry: "Tech",
  employees: {
    employee1: {
      name: "John Doe",
      position: "Developer"
    },
    employee2: {
      name: "Jane Smith",
      position: "Designer"
    }
  }
});

To get your hierarchical data, use document references and subcollections.

Retrieving Subcollections

// Fetch all employees of companyA
db.collection('companies').doc('companyA')
  .collection('employees').get().then((querySnapshot) => {
    querySnapshot.forEach((doc) => {
      console.log(doc.id, " => ", doc.data());
    });
  });

Retrieving Nested Data

// Fetch companyA data including nested employees
db.collection('companies').doc('companyA').get().then((doc) => {
  if (doc.exists) {
    console.log("Document data:", doc.data());
  } else {
    console.log("No such document!");
  }
});

Sometimes, you'll need to update or delete your hierarchical data.

Updating Subcollection Documents

// Update an employee document in a subcollection
db.collection('companies').doc('companyA')
  .collection('employees').doc('employee1').update({
    position: "Senior Developer"
  });

Updating Nested Fields

// Update nested field in a document
db.collection('companies').doc('companyA').set({
  "employees.employee1.position": "Senior Developer"
}, { merge: true });

Deleting Subcollection Documents

// Delete an employee document in a subcollection
db.collection('companies').doc('companyA')
  .collection('employees').doc('employee1').delete();

Deleting Entire Subcollections

Firestore doesn't directly support deleting entire subcollections, so you'll need to delete each document one by one.

Deleting a Nested Field

// Delete a nested field in a document
db.collection('companies').doc('companyA').update({
  "employees.employee1": firebase.firestore.FieldValue.delete()
});