Query all reviews for a book

Your app can now attach reviews to books, but it has no way to read them back yet. You'll add a query for that.

Define the query

Add the following Rell code to src/main/queries.rell:

query get_all_reviews_for_book(isbn: text) {  val reviews = book_review @* { .book.isbn == isbn } (    .reviewer_name,    .review,    .rating  );  return reviews;}

Breakdown of the query

1. Input parameter: You pass an isbn to choose the book whose reviews are returned.
2. Join operation: The expression val reviews = book_review @* { .book.isbn == isbn }; selects every book_review entity whose book.isbn equals the isbn you supplied.
3. Attributes: The following part controls which attributes appear in the result:

   (  .reviewer_name,  .review,  .rating);

Given a book, the query returns each of its reviews with the reviewer's name, the review text, and the rating.

Test it on a local Chromia node

You test against a local node with the Chromia CLI:

1. Update or start the local Chromia node:

   • If your node is running, apply the changes with:

     chr node update

   • If your node is not running, launch it with:

     chr node start

2. Execute the query: Open another terminal window or tab and fetch the reviews for a book with:

   chr query get_all_reviews_for_book "isbn=ISBN1234"

   You should see output like this:

   [  {    "rating": 5,    "review": "It was a great book",    "reviewer_name": "Alice"  }]

Later lessons cover more of Rell, including validation, shaping query results, and running transactions that require signatures.