Query every book

Queries let you read data from the blockchain. This lesson walks you through writing a query and then testing it.

Define the query

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

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

Breakdown of the query

• query keyword: Declares a new query. It plays a role comparable to a SELECT statement in SQL and returns data from the blockchain.
• @* operand: Writing book @* tells Rell the query can return any number of book objects, including none.
• Filter criteria: You place query conditions inside the curly braces {}. They are empty here, so every entry is returned.
• Attributes: The part after the curly braces lists the attributes to return: .isbn, .title, and .author.

The result is a collection of every book, where each item carries its isbn, title, and author.

Update the unit tests

Next, extend the tests so they verify the query's behavior.

Step 1: Update imports

In src/test/book_review_test.rell, change the import statement to the following:

import main.{ book, create_book, get_all_books };

Step 2: Add the new test function

Add this test function to the same file:

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() builds a transaction for testing.
• Adding operations: The transaction receives two create_book operations.
• Execution: Calling run() executes the transaction.
• Query and assertions: get_all_books() fetches the books, and the test asserts that exactly 2 were returned.

From your project folder, run the tests with this terminal command:

chr test

Test on a local Chromia node

To check the query against a local Chromia node:

1. Update or start the local Chromia node:

   • If your node is running, update it with:

     chr node update

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

     chr node start

2. Run the query: Open another terminal window or tab and run:

   chr query get_all_books

   The output lists the books stored so far, such as any you added earlier with the create_book operation. If you completed the previous steps, the result should resemble this:

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

   This output shows that the query returns the stored book from the blockchain as expected.

You have now stored entities on your test node's blockchain and written a query that returns a collection of all books.