If a Pointer represents an object, mixins represent data about the object. Mixins collate optional additional information about an object. Different types of object will typically make use of different mixins. You can see these as aspects of the data if you like.

A mixin table starts with an id column which references Pointer and forms the default primary key. It is up to the user to choose which other fields go in the table, and thus what the mixin is for.

Use of a mixin is typically through has_one:

has_one :my_mixin, MyMixin, foreign_key: :id, references: :id

Sometimes, the user may wish to add fields to the primary key by using the primary_key: true option to add in their migrations. This is permitted and in such case we call the resulting mixin a multimixin. Use becomes has_many:

has_many :my_mixin, MyMixin, foreign_key: :id, references: :id

Thus the choice of single or multi comes down to how many times you want to store that data for the object. A user's profile naturally lends itself to a regular single mixin, whereas an object's appearance in a feed would naturally lend itself to being a multimixin since the object may appear in many feeds.

Declaring a mixin table type

defmodule My.Mixin do

  use Needle.Mixin,
    otp_app: :my_app,
    source: "postgres_table_name"

  mixin_schema do
    field :is_awesome, :boolean
  end
end