Make everything reactive

# Making everything reactive!

#### The goal: Display the 10 latest `Books` along with their respective `Authors`

# Example 1.
> Authors are *not* reactive

> The problem with this approach is that once a new book (with a new author) appears,
> the new author is not sent to the client
```javascript
Meteor.publish('booksWithAuthors', function() {
  var books = Books.find({}, { sort: { createdAt: -1 }, limit: 10 });

  return [
    books,
    Authors.find({ _id: { $in: _.uniq(books.map(function(book) { return book.authorId; })) }})
  ];
});
```

> Subscribe:
```javascript
Meteor.subscribe('booksWithAuthors');
```


# Example 2.
> Authors are reactive
> This is basically what reactive-relations automates
```javascript
Meteor.publish('booksWithAuthors', function(authorIds) {
  var books = Books.find({}, { sort: { createdAt: -1 }, limit: 10 });

  > Send through the authors initally
  if (authorIds.length === 0)
    authorIds = books.map(function(book) { return book.authorId; });

  return [
    books,
    Authors.find({ _id: { $in: _.uniq(authorIds) }})
  ];
});
```

> Subscribe on the client, passing through an array containing the authorIds that we require
```javascript
Deps.autorun(function() {
```
  > Books query ends up being duplicated here!
```javascript
  var books = Books.find({}, { sort: { createdAt: -1 }, limit: 10 });
```
  > As does this mapping function
```javascript
  Meteor.subscribe('booksWithAuthors', books.map(function(book) { return book.authorId; }));
});
```