Anorm Query Monad

This project aims to provide a lightweight Query type to perform queries in Anorm. We have a sample play application to show how to use it in a real context. This app uses compile time dependency injection.

This example uses the following:

• Play! Framework v2.6.11 as a web framework
• Anorm as data access layer
• PostgreSql as a database embedded in docker

Setup and run

• Deploy the postgres database in docker

docker stack deploy -c stack.yml postgres

• Add the database schema.
• Run the play application

sbt run

This application exposes a REST API. You can open a browser and go to localhost:9000.

Core concepts

The Problem

The problem arises when working with relational databases in a web app. The following are desirable:

• Operations should run in a transaction/connection/context
• We want to avoid writing long functions with multiple queries targeting different tables in the same scope
• Easily compose queries

Solution

This project introduces a new data type Query. Query treats functions as value in the context of a database connection.

val query: Query[Book] = for {
  authorId <- Query(connection => SQL("SELECT id from authors where name = $name").as(SqlParser.scalar[Int].single)
  book <- Query(connection => SQL("SELECT * from books where authorId = $authorId").as(SqlParser.scalar[Book].single)
} yield book

Then you can run the query:

val queryRunner = new QueryRunner(database)
...
queryRunner.run(query)

Everything is in for-comprehensions which makes it easier to compose. The value returned from the for-comprehension is a Query[A], so nothing happens until that Query is run in QueryRunner.

Any functions building on the Database will return Query[A] thus making it very obvious what the context of those functions are.

You can easily isolate queries in their own object/class/functions which improves readability.

To go further:

Query is a lightweight version of the reader monad that we can find in cats or scalaz.

• Cats version
• Scalaz version
• Using Cat Data Reader Monad
• A Small (Real) Example of the Reader and Writer Monads