Alright, let's get Firestore up and running! First, you'll need to create a new Firebase project through the Firebase console. Once that's done, install the Firebase SDK in your app and initialize Firestore. Easy peasy, right?

Now, to keep track of different versions of a document, we need a smart data structure. One way to do this is to split each document into two collections: a "document" collection for the active version, and a "versions" sub-collection for all the previous versions. Each version document in the "versions" sub-collection will be a snapshot of the main document at the time of update.

For example, if you have a document called 'post', your Firestore structure would look like this:

posts/{postId}
versions/{versionId}

When a user creates a new document, you'll want to make an entry in both the "document" collection and the "versions" sub-collection. For our 'post' example, it might look like this:

const postRef = db.collection('posts').doc(postId);
await postRef.set(postData);

const versionRef = postRef.collection('versions').doc();
await versionRef.set(postData);

When a user updates a document, you need to update both the "document" collection and add a new document in the "versions" sub-collection. Here's how you can do it:

const postRef = db.collection('posts').doc(postId);
await postRef.update(updatedPostData);

const versionRef = postRef.collection('versions').doc();
await versionRef.set(updatedPostData);

This way, every time a document is updated, a new version is automatically created and saved in the "versions" sub-collection. Pretty neat, huh?

To fetch the version history of a document, just read from the corresponding "versions" sub-collection.

const versionsSnapshot = await db.collection('posts').doc(postId).collection('versions').get();

versionsSnapshot.forEach((doc) => {
  console.log(doc.id, " => ", doc.data());
});

This will give you a list of all the versions of the document in the order they were created. It's like a time machine for your data!

Restoring a previous version of a document means replacing the current document with a copy of the desired version. First, fetch the data of the version you want to restore, then write it to the 'document' collection.

const versionDoc = await db.collection('posts').doc(postId).collection('versions').doc(versionId).get();

const postRef = db.collection('posts').doc(postId);
await postRef.set(versionDoc.data());

Remember, after you do this, a new version of the document is created and saved to the "versions" sub-collection. This new version is an exact copy of the restored version. So, every operation keeps the version history intact.