ScopedAssociations

A very simple macro to implement scoped associations in your schemas.

Installation

The package can be installed by adding scoped_associations to your list of dependencies in mix.exs:

def deps do
  [{:scoped_associations, github: "jgarcia/scoped_associations", app: false}]
end

Usage

Consider the following schema.

defmodule Customer do
  ...
  schema "customers" do
    ...
    has_many :purchases, Purchase
    has_many :reviews, Review
    ...
  end
end

Supose that in a page you want to display the customer's last purchase and first review.

Modifying your schema like this

defmodule Customer do
  ...
  use ScopedAssociations

  schema "customers" do
    ...
  end

  has_one_scoped :last_purchase, schema: Purchase, foreign_key: "customer_id", scope: :last_purchase
  has_one_scoped :first_review, schema: Review, foreign_key: "customer_id", scope: :first_review

  def last_purchase do
    from p in Purchase,
      join: f in fragment("SELECT MAX(id) AS id, customer_id FROM purchases GROUP BY customer_id"),
      on: p.id == f.id
  end

  def first_review do
    from r in Review,
      join: f in fragment("SELECT MIN(id) AS id, customer_id FROM reviews GROUP BY customer_id"),
      on: r.id == f.id
  end
end

To load those relations you can use the include function along with list of relations to load

customers = Repo.all(Customer)
            |> Customer.include([:last_purchase, :first_review])

Notes

Please keep in mind that this is a first draft, there is some code duplication that I will address in the near future as well as some improvements to the api.

Todo

Add tests with ecto schemas
Remove code duplication in favor of reusable functions to reduce the number functions defined by the macro
Figure out some edge cases
Publish in hex