Write a query to retrieve all books

This section covers the basics of working with queries in Chromia blockchain development. Queries are used to fetch data from the blockchain. We'll guide you through the process, showing you how to create and test them.

Define the query

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

src/main/queries.rell

query get_all_books() {
  return book @* { } (
    .isbn,
    .title,
    .author
  );
}

Breakdown of the query

• query keyword: Defines a new query. This works similarly to a SELECT statement in SQL, allowing you to retrieve data from the blockchain.
• @* operand: In book @*, we expect 0 or more objects of the book type from the query.
• Filter criteria: The curly braces {} allow you to specify conditions for the query, but here we keep it simple and fetch all entries.
• Attributes: After the curly braces, we specify which attributes to retrieve: .isbn, .title, and .author.

This query retrieves all books in a collection, with each item including the attributes isbn, title, and author.

Update the unit tests

To ensure our query works correctly, we need to update the tests.

Step 1: Update imports

Open the file src/test/book_review_test.rell and update the import statement as follows:

src/test/book_review_test.rell

import main.{ book, create_book, get_all_books };

Step 2: Add the new test function

Next, add the following test function to the same file:

src/test/book_review_test.rell

function test_get_books() {
    rell.test.tx()
        .op(create_book("123", "Book1", "Author1"))
        .op(create_book("124", "Book2", "Author2"))
        .run();

    val books = get_all_books();
    assert_equals(books.size(), 2);
}

Breakdown of the test

• Transaction creation: rell.test.tx() creates a test transaction.
• Adding operations: Two create_book operations are added to the transaction.
• Execution: run() executes the transaction.
• Query and assertions: The get_all_books() function retrieves the books. We then assert that the number of retrieved books is 2.

To run the tests, ensure you are in your project folder and enter the following command in your terminal:

chr test

Test on a local Chromia node

To validate our query on a local Chromia node, follow these 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 update might take a moment to be reflected. You may need to wait for the following block to be processed before the query becomes available.

2. Run the query: In a new terminal window or tab, execute the following command:

   chr query get_all_books

   You should see the results, including any books you previously inserted, like the one you added with the create_book operation. For instance, if you followed the earlier steps, you should see an output similar to the following:

   [
     {
       "author": "George Orwell",
       "isbn": "ISBN1234",
       "title": "1984"
     }
   ]

   This confirms that the query correctly retrieves the stored book from the blockchain.

Congratulations! You have successfully stored entities on your test node blockchain and created a query to retrieve a collection of all books.