Write a query to retrieve all reviews of a book

Now that we have a mechanism to add reviews connected to books, we need a way to retrieve them. We'll define a query for that.

Define the query

Open the file src/main/queries.rell and add the following Rell code:

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: The query takes an isbn parameter to specify which book's reviews you want to retrieve.
2. Join Operation: val reviews = book_review @* { .book.isbn == isbn }; retrieves all book_review entities where the book.isbn matches the given isbn.
3. Attributes: We specify which attributes to include in the result by adding:

   (
     .reviewer_name,
     .review,
     .rating
   );

This query retrieves all reviews for a specified book, including the reviewer's name, the review text, and the rating.

Test it on a local Chromia node

Testing on a local Chromia node is done using the Chromia CLI. Here are the steps:

1. Update or start the local Chromia node:

   • If the node is already running, use the following command to update it:

     chr node update

   • If the node is not already running, start it with:

     chr node start
     note

     After running chr node update, the query might take a moment to become available. You may need to wait for the following block to be processed before executing the query. If you get a "400 Bad Request" error, please wait a few moments and try again.

2. Execute the query: In a new terminal window or tab, run the following command to fetch all reviews for a book:

   chr query get_all_reviews_for_book "isbn=ISBN1234"

   The expected result should be:

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

Summary

In this lesson, we:

1. Defined a book_review entity in Chromia's Rell.
2. Implemented a query to retrieve all reviews for a specific book.

In upcoming lessons, we'll explore Rell's powerful capabilities further, including handling validation, structuring results from queries and executing transactions with signatures.