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Chapter.4 Ecto and Changesets

now we can replace our hard-coded data with a real DB without changing controller code

  • Ecto is Elixir framework for data management; here we'll update Accounts context to use Ecto backed by PgSQL

Understanding Ecto

  • is a wrapper primarily intended for RDBMS

  • has encapsulated Query lang used to build layered queries

  • has changesets holding all changes desired to perform on Records encapsulating receiving external data, casting & validating it

  • by default we have repo.ex generated Ecto Repository alongwith config for connection to default PgSQL adapter at ./config/ under config :videologue, Videologue.Repo, ..

  • mix ecto.create creates the DB schema if not already there; mix ecto.migrate to apply migrations

Defining the User Schema and Migration

  • can specify Struct with individual fields tied to DB:Table fields through a DSL

  • update Videologue.Accounts.User bare user struct to define Schema

DSL used for Schema is built with schema & field Macros; allowing us to define underlying DB:Table & Elixir Struct together

  • now generate a migration for DB to reflect the same structure mix ecto.gen.migration create_users which generates a migration file with timestamp; let's add following to its change/0 action
..
  def change do
    create table(:users) do
      add :name, :string
      add :username, :string, null: false
      add :password_hash, :string

      timestamps()
    end
  end
  • then generate a migration to add index to users, we could have added this to earlier migration but it's just a better practice to have seemingly single step migrations
mix ecto.gen.migration create_users_index_username
* creating priv/repo/migrations/20210527193448_create_users_index_username.exs

add following to change here create unique_index(:users, [:username])

  • here we used create & add macros available via use Ecto.Migration API

  • timestamps() add auto-generated inserted_at & updated_at fields

  • let's migrate DB with mix ecto.migrate

  • Repo are ready to use through Accounts context; Repository service need to be up & running... Phoenix ensures that by having it in child_spec list under application.ex

Using the Repository to Add Data

  • can add data using IEx as
iex(1)> alias Videologue.Repo
Videologue.Repo

iex(2)> alias Videologue.Accounts.User
Videologue.Accounts.User

iex(3)> %User{username: "sherlock", name: "Sherlock Holmes"} |> Repo.insert()    
[debug] QUERY OK db=2.5ms decode=1.7ms queue=0.8ms idle=377.5ms
INSERT INTO "users" ("name","username","inserted_at","updated_at") VALUES ($1,$2,$3,$4) RETURNING "id" ["Sherlock Holmes", "sherlock", ~N[2021-05-28 06:51:43], ~N[2021-05-28 06:51:43]]
{:ok,
 %Videologue.Accounts.User{
   __meta__: #Ecto.Schema.Metadata<:loaded, "users">,
   id: 1,
   inserted_at: ~N[2021-05-28 06:51:43],
   name: "Sherlock Holmes",
   updated_at: ~N[2021-05-28 06:51:43],
   username: "sherlock"
 }}

iex(4)> %User{username: "moriarty", name: "James Moriarty"} |> Repo.insert() 

iex(5)> %User{username: "watson", name: "John Watson"} |> Repo.insert()     

iex(6)> Repo.all(User) |> Enum.filter(fn x -> x.username =~ "lock" end) 
[debug] QUERY OK source="users" db=8.6ms queue=1.6ms idle=1886.3ms
SELECT u0."id", u0."name", u0."username", u0."inserted_at", u0."updated_at" FROM "users" AS u0 []
[
  %Videologue.Accounts.User{
    __meta__: #Ecto.Schema.Metadata<:loaded, "users">,
    id: 1,
    inserted_at: ~N[2021-05-28 06:51:43],
    name: "Sherlock Holmes",
    updated_at: ~N[2021-05-28 06:51:43],
    username: "sherlock"
  }
]

iex(7)> Repo.get(User, 2)                                              
[debug] QUERY OK source="users" db=2.4ms queue=2.2ms idle=1155.0ms
SELECT u0."id", u0."name", u0."username", u0."inserted_at", u0."updated_at" FROM "users" AS u0 WHERE (u0."id" = $1) [2]
%Videologue.Accounts.User{
  __meta__: #Ecto.Schema.Metadata<:loaded, "users">,
  id: 2,
  inserted_at: ~N[2021-05-28 06:52:11],
  name: "James Moriarty",
  updated_at: ~N[2021-05-28 06:52:11],
  username: "moriarty"
}
  • can update Videologue.Accounts with Repo.all/1, Repo.get/2, Repo.get!/2, Repo.get_by/2 replacing hard-coding from alias Videologue.Repo

Building Forms

use Phoenix form builder to enable new user creation

  • add new user account action to VideologueWeb.UserController

  • add changeset/2 function to Videologue.Accounts.User doing cast/2, validate_required/1 & validate_length imported from Schema module

  • add change_user/1 function to Videologue.Accounts.User which gets utilized in our new action in Controller

  • Videologue.Accounts module itsef shall be only public API our Controllers shall call

  • way of piping in required validations enables flexible stream

  • in router.ex, add resources "/users", UserController, only: [:index, :show, :new, :create] replacing other /users... routes

  • doing mix phx.routes gives fillowing routes relevant to us

user_path  GET   /users           VideologueWeb.UserController :index
user_path  GET   /users/new       VideologueWeb.UserController :new
user_path  GET   /users/:id       VideologueWeb.UserController :show
user_path  POST  /users           VideologueWeb.UserController :create
page_path  GET   /                VideologueWeb.PageController :index

not limiting resources with only: will give couple more routes for PUT, PATCH, DELETE, etc.

  • add new.html.eex; here using Form Builder to generate required fields in association with Model struct

  • this introduces a CSRF token for input fields; add proper IDs, Names to them

  • available at /users/new.html

Creating Resources

  • our Form submits data for new user and expects VideologueWeb.UserController.create/2

  • add Videologue.Accounts create_user/1 to do Repo.insert; then add VideologueWeb.UserController.create/2 to call this and redirect to Listing on success else render errors on create page if any

  • {:ok, user} gets returned on successful Repo insert; {:error, changeset} on erroneous with validation errors

  • :action field of changeset indicates action tried to perform on it; rendering form with any action we know it had validation errors

  • error_tag gets used in form to display these errors at frontend

  • in addition to validation errors; Changesets track changes; lets see some IEx work on it
iex(1)> alias Videologue.Accounts.User
Videologue.Accounts.User

iex(2)> changeset = User.changeset(%User{username: "hudson", name: "Mrs Hudson"}, %{})
#Ecto.Changeset<action: nil, changes: %{}, errors: [],
 data: #Videologue.Accounts.User<>, valid?: true>

iex(3)> import Ecto.Changeset
Ecto.Changeset

iex(4)> cset = put_change(changeset, :username, "mrshudson")
#Ecto.Changeset<
  action: nil,
  changes: %{username: "mrshudson"},
  errors: [],
  data: #Videologue.Accounts.User<>,
  valid?: true
>

iex(5)> cset.changes                                                                  
%{username: "mrshudson"}

iex(6)> get_change(cset, :username)     
"mrshudson"

iex(7)> get_change(cset, :name)    
nil

Wrapping Up

chapter-05