This obscure little feature in Ecto/Elixir is something we've been using at Slab.com for about a year. While the blog post only covers the feature itself, I wanted to give a better overview of *WHY* we're using it.

Slab is a multi-tenant Phoenix app where all tables in our database, including pivot tables, have an `org_id` foreign key that references the team the data belongs to. Using association defaults enforces that the correct `org_id` value is automatically set for all resources.

We combine this with an `OrgRepo` module that wraps around our default `Repo`, which can only write or read other resources with the same `org_id`, ensuring that data for one team has no possibility of accidentally leaking to another team (using `Repo` is forbidden in our codebase).

An added bonus is; when we eventually have to scale our databases via something like sharding and have to vertically partition the data based on `org_id`, we would already have this system in place to support us.