Ecto for Elixir: Foreign Key

Question

I have a user profile model with the following schema:

  schema "user_profiles" do
    field :birthday, Ecto.Date
    field :gender, :string
    field :first_name, :string
    field :last_name, :string
    field :description, :string
    field :username, :string

    # Associations
    belongs_to :user, User

    timestamps
  end

I also have a location model with the following schema:

  schema "locations" do
    field :name, :string
    field :admin_1, :string
    field :admin_2, :string
    field :country, :string
    field :country_abbreviation, :string
    field :code, :string
    field :latitude, :decimal
    field :longitude, :decimal

    timestamps
  end

A location is a city like Paris, France. Many user profiles can be associated with the same location. But a location doesn't really need to be aware that user profiles exist.

I want to add a location association to the user profile schema. I do not want to user has_one because the relationship is not one-to-one. I do not want to add any associations on the location schema to user profile, either.

I have looked at the documentation for Ecto and I am at a loss for how to express this association.

I have consulted the following page: https://hexdocs.pm/ecto/Ecto.Schema.html

I am probably missing something basic but I did try more than a few things before asking this question.

Answer 1

One solution is to do a simple has_many and belongs_to relationship between the two models where the profile will belong to a relationship.

This relationship will mean that a location can have many profiles associated with it, whereas a profile can only have one location.

E.g.

Works:

Profile1:London √

Profile2:London √

Wrong:

Profile1:London X

Profile1:France X

This is how you would go about implementing it.

#web/models/location.ex
schema "location" do
  has_many :profiles, MyApp.Profile
end

#web/models/profile.ex
schema "profile" do
  belongs_to :location, MyApp.Location
end

Then on your profile_table migration you would have:

add :location_id, references(:locations)

You don't need a foreign key on the location model, only the profile model.