Repositories » Reading Aggregates
Repositories have a powerful API for composing data into nested structures, which we call aggregates, where the root is constructed from the data provided by the root relation, and additional relations provide data for child nodes.
This document uses
rom-sql which provides support for defining canonical database
associations in relation schemas, which are used to simplify reading aggregates.
If an adapter doesn't support association in schemas, you can still load aggregates
which is more advanced and flexible.
First, we want to define our canonical associations. We call them canonical since they are defined by your database schema. It's common to diverge from canonical associations and compose data in different ways, depending on your application needs; however, in the early days of every project, using canonical associations is all you need, so let's start with that!
We're going to define
:users that have many
:tasks. To keep things simpler
let's define relations using block-style setup:
require 'rom' rom = ROM.container(:sql, 'sqlite::memory') do |conf| conf.default.create_table(:users) do primary_key :id column :name, String, null: false column :email, String, null: false end conf.default.create_table(:tasks) do primary_key :id foreign_key :user_id, :users column :title, String, null: false end conf.relation(:users) do schema(infer: true) do associations do has_many :tasks end end end conf.relation(:tasks) do schema(infer: true) do associations do belongs_to :user end end end end
With associations defined in the relation schemas we established common queries that will be automatically used for composing relations into aggregates. Let's see how we can leverage that in repositories.
Let's say we'd like to expose an aggregate where a user is loaded with its tasks.
We need to define a root repository with
:users set up as the root and provide
class UserRepo < ROM::Repository[:users] # we configure this explicitly so we can see that this repo will work with tasks relation too relations :tasks end
Now loading an aggregate is as simple as this:
user_repo = UserRepo.new(rom) user_repo.aggregate(:tasks).one # => #<ROM::Struct[User] id=1 name="jane" email="email@example.com" tasks=[#<ROM::Struct[Task] id=1 user_id=1 title="Jane Task">]>
We can do the other way around, starting with
:tasks relation as the root, which
means we're going to load a task with its user:
class TaskRepo < ROM::Repository[:tasks] relations :users end task_repo = TaskRepo.new(rom) task_repo.aggregate(:user).one # => #<ROM::Struct[Task] id=1 user_id=1 title="Jane Task" user=#<ROM::Struct[User] id=1 name="jane" email="firstname.lastname@example.org" task_id=1>>
Notice that in this case
User struct is loaded as a child object where
is a parent, thus
task_id assigned, which is a virtual foreign key
that doesn't really exist in our schema.
Loading aggregates using wrapping
An alternative, and faster, way of loading parent objects is to use
class TaskRepo < ROM::Repository[:tasks] def by_id_with_user(id) tasks.by_pk(id).wrap_parent(user: task_repo.users).one end end task_repo.by_id_with_user(id) # => #<ROM::Struct[Task] id=1 user_id=1 title="Jane Task" user=#<ROM::Struct[User] id=1 name="jane" email="email@example.com">>
Notice that unlike with
aggregate method, the parent object does not include a virtual foreign key.
Performance and availability
wrap_parentis only available in rom-sql, which uses a join instead of eager-loading and it's significantly faster than using
aggregate. Other adapters can implement this interface too, assuming it makes sense in case of a given data source.
Loading aggregates with repositories can be achieved in many different ways, for detailed information about invidual methods please refer to the API documentation: