Ecto: How to validate unique combinations of two fields.

branch.ex

defmodule VetMapper.Branch do
  use VetMapper.Web, :model
  
  schema "branches" do
    field :name, :string
    field :country, :string
    field :state, :string
    field :city, :string
    field :street_name, :string
    field :street_number, :integer
    
    belongs_to :clinic, VetMapper.Clinic
  end  
  
  def changeset(struct, params \\ %{}) do
    struct
      # Note that the constaint is placed on ONE of the fields.
      |> unique_constraint(:street_number, name: :index_branches_on_address)
  end

end

clinic.ex

defmodule VetMapper.Clinic do
  use VetMapper.Web, :model
  
  schema "clinics" do
    field :name, :string
    has_many :branches, VetMapper.Branch
  end

end

Migration can be found here: https://gist.github.com/Ivor/815f9128fc73b162118dd6cdcc0b48e9
Blog post: https://medium.com/@ivorpaul/ecto-how-to-validate-unique-combinations-of-multiple-fields-8002c74c6757#.f1iwh3dzf