Ecto

Adding a Ecto.Repo in an elixir program

This can be done in 3 steps :

1. You must define an elixir module which use Ecto.Repo and register your app as an otp_app.

   defmodule Repo do
     use Ecto.Repo, otp_app: :custom_app
   end

2. You must also define some config for the Repo which will allow you to connect to the database. Here is an example with postgres.

   config :custom_app, Repo,
      adapter: Ecto.Adapters.Postgres,
      database: "ecto_custom_dev",
      username: "postgres_dev",
      password: "postgres_dev",
      hostname: "localhost",
     # OR use a URL to connect instead
     url: "postgres://postgres_dev:postgres_dev@localhost/ecto_custom_dev"

3. Before using Ecto in your application, you need to ensure that Ecto is started before your app is started. It can be done with registering Ecto in lib/custom_app.ex as a supervisor.

       def start(_type, _args) do
         import Supervisor.Spec
   
         children = [
          supervisor(Repo, [])
         ]
   
         opts = [strategy: :one_for_one, name: MyApp.Supervisor]
         Supervisor.start_link(children, opts)
       end

“and” clause in a Repo.get_by/3

If you have an Ecto.Queryable, named Post, which has a title and an description.

You can fetch the Post with title: “hello” and description : “world” by performing :

 MyRepo.get_by(Post, [title: "hello", description: "world"])

All of this is possible because Repo.get_by expects in second argument a Keyword List.

Querying with dynamic fields

To query a field which name is contained in a variable, use the field function.

some_field = :id
some_value = 10

from p in Post, where: field(p, ^some_field) == ^some_value

Add custom data types to migration and to schema

(From this answer)

The example below adds an enumerated type to a postgres database.

First, edit the migration file (created with mix ecto.gen.migration):

def up do
  # creating the enumerated type
  execute("CREATE TYPE post_status AS ENUM ('published', 'editing')")

  # creating a table with the column
  create table(:posts) do
    add :post_status, :post_status, null: false
  end
end

def down do
  drop table(:posts)
  execute("DROP TYPE post_status")
end

Second, in the model file either add a field with an Elixir type :

schema "posts" do
  field :post_status, :string
end

or implement the Ecto.Type behaviour.

A good example for the latter is the ecto_enum package and it can be used as a template. Its usage is well documented on its github page.

This commit shows an example usage in a Phoenix project from adding enum_ecto to the project and using the enumerated type in views and models.