Ecto gettign started with associations

gistfile1.ex

First step is to create a new project

  mix new ecto_quickstart
  cd ecto_quickstart/
  
Next edit your ./mix.exs to include the dependencies for ecto

  defmodule EctoQuickstart.Mixfile do
    use Mix.Project
  
    def project do
      [ app: :ecto_quickstart,
        version: "0.0.1",
        elixir: "~> 0.12.3",
        deps: deps ]
    end
  
    # Configuration for the OTP application
    def application do
      [mod: { EctoQuickstart, [] }]
    end
  
    # Returns the list of dependencies in the format:
    # { :foobar, git: "https://github.com/elixir-lang/foobar.git", tag: "0.1" }
    #
    # To specify particular versions, regardless of the tag, do:
    # { :barbat, "~> 0.1", github: "elixir-lang/barbat" }
    defp deps do
      #### update for ecto deps here ####
      [{ :postgrex, github: "ericmj/postgrex" },
      { :ecto, github: "elixir-lang/ecto" }]
    end
  end

Now use mix to grab your dependencies

  mix deps.get
  
Now we can use the mix tasks included to make an empty repo

  mix ecto.gen.repo Repo
  
We need to create a database that maps to the Repo we just mixed, refer to the psql docs for help here if you don't understand this.

  sudo -u nobody /usr/local/pgsql/bin/createdb ecto_quickstart_repo_dev

Edit the repo in lib/repo.ex

  defmodule Repo do
    use Ecto.Repo, adapter: Ecto.Adapters.Postgres, env: Mix.env
  
    @doc "The URL to reach the database."
    def url(:dev) do
      #### update your pg url here ####
      "ecto://nobody@localhost/ecto_quickstart_repo_dev"
    end
  
    def url(:test) do
      "ecto://user:pass@localhost/ecto_quickstart_repo_test?size=1&max_overflow=0"
    end
  
    def url(:prod) do
      "ecto://user:pass@localhost/ecto_quickstart_repo_prod"
    end
  
    @doc "The priv directory to load migrations and metadata."
    def priv do
      app_dir(:ecto_quickstart, "priv/repo")
    end
  end

Update the supervisor stub

  defmodule EctoQuickstart.Supervisor do
    use Supervisor.Behaviour
  
    def start_link do
      #### replace start_link method ####
      :supervisor.start_link({ :local, __MODULE__ }, __MODULE__, [])
    end
  
    def init([]) do
      #### replace the init here ####
      tree = [ worker(Repo, []) ]
      supervise(tree, strategy: :one_for_all)
    end
  end

now edit lib/ecto_quickstart.ex

  defmodule EctoQuickstart do
    use Application.Behaviour
  
    # See http://elixir-lang.org/docs/stable/Application.Behaviour.html
    # for more information on OTP Applications
    def start(_type, _args) do
      EctoQuickstart.Supervisor.start_link
    end
  end
  
  #### add models below ####
  
  defmodule Parent do
    use Ecto.Model
    queryable "parent" do
      has_many :children, Child
      field :name, :string
    end
  end
  defmodule Child do
    use Ecto.Model
    queryable "child" do
      field :name, :string
      belongs_to :parent, Parent
    end
  end

Mix a migration stub

  mix ecto.gen.migration Repo create_parent_child
  
edit the migration (time stamp will change for yours) priv/repo/migrations/20140206171819_create_parent_child.exs

  defmodule Repo.Migrations.CreateParentChild do
    use Ecto.Migration
  
    def up do
      #### add table create here, parent must be created first ####
      ["create table if not exists parent(id serial primary key,name text)","create table if not exists child(id serial primary key, name text,parent_id integer references parent(id))"]
    end
  
    def down do
      #### add drop here, the order matters due to the association ####
      [ "drop table child","drop table parent" ]
    end
  end

Mix the data structure in the db

  mix ecto.migrate Repo

launch iex: 

  iex -S mix run
  
Run the following commands in iex, they should create a parent, and child that is associated.  It will then commit them to the db and query them back out.

  parent = Parent.new(name: "foo")
  Repo.create(parent)
  child = Child.new(name: "bar", parent_id: 1)
  Repo.create(child)
  
  
  parent = Repo.get(Parent,1)
  Repo.all(parent.children)
  
  c = c.name("unf")
  Repo.update(c)
  Repo.all(parent.children)