Defining the book review entity

Now that we have created the book entity, the next step is to introduce the book_review entity. This entity will allow us to record book reviews. The model is defined as follows:

Adding the book review entity

To define the book_review entity, open the file src/main/entities.rell and add the following code:

src/main/entities.rell

entity book_review {
  index book: book;
  reviewer_name: text;
  review: text;
  rating: integer;
}

info

Linking & implicit joins

This entity includes a reference to book. Under the hood, Rell stores the referenced row’s rowid, creating a link between the review and its corresponding book.

• When a query or operation starts from book_review and accesses fields of .book
  (such as .book.title, .book.to_struct(), or @sort .book.author), Rell performs an implicit join between the book_review and book entities to retrieve those values.
• If you only access the fields of book_review, no join occurs.
• If the expression begins from the book entity, you are already positioned in that entity — no extra join is necessary.

tip

Each reference you dereference adds its own implicit join. If an entity has many references and you retrieve them all (e.g., via .to_struct()), the expression will perform multiple joins, which can affect performance on large datasets.

Breakdown of the code

• entity keyword: Defines a new entity, similar to creating a table in a SQL database.
• index book: book;: The index keyword is used to optimize queries that involve the book relationship. This attribute refers to a book, linking the book to its reviews.
• reviewer_name: text;: Allows users to enter their name or alias as a reviewer.
• review: text;: Stores the content of the review as text.
• rating: integer;: Captures the numerical rating assigned to the book review.

With these attributes, you can now begin recording book reviews in your Chromia-based app.