2.3.1. Overview

You can use any database access layer you like with Aura Marshal, such as ...

... or anything else. (In theory, you should be able to retrieve data from XML, CSV, Mongo, or anything else, and load it into Aura Marshal.)

With Aura Marshal, you use the data retrieval tools of your choice and write your own queries to retrieve data from a data source. You then load that result data into an entity type object, and it creates entity and collection objects for you based on a mapping scheme you define for it.

Aura Marshal makes it easy to avoid the N+1 problem when working with a domain model. It also uses an identity map (per type) to avoid retaining multiple copies of the same object.

It is important to remember that Aura Marshal, despite resembling an ORM in many ways, it not an ORM proper:

Those things are outside the scope of the Aura Marshal package. Their absence does provide a great amount of flexibility for power users who write their own hand-tuned SQL and need a way to marshal their result sets into a domain model, especially in legacy codebases.

Aura Marshal works by using Type objects (which define the entity types in the domain model). Each Type has a definition indicating its identity field, how to build entities and collections, and the relationships to other Type objects. The Type objects are accessed through a type Manager. You load data into each Type in the Manager, then you retrieve entities and collections from each Type.

2.3.1. Example Schema

For the rest of this narrative, we will assume the existence of the following SQL tables and columns in a naive multiuser blogging system:

(Note that the primary key and foreign key names are not important; they can be anything at all.)

Each author can have many posts.

Each post belongs to one author, has one summary, and can have many comments.

Posts and tags have a many-to-many relationship; that is, each post can have many tags, and each tag can be applied to many posts. They map to each other through posts_tags.