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Chapter.6 Generators and Relationships

contexts continue being business logic API accessed by controllers

Using Generators

Adding Video and Annotations

Videologue will allow users chosing a Video, attach comments to it (annotate) in real-time.. users to be able to play back these videos with comments over time.

  • will be using Generators to build skeletal code alongwith migration, context, controllers and templates

Generating Web Interfaces

  • Mix task phx.gen.html creates simple HTTP CRUD scaffold with HTML pages; phx.gen.json does same for REST-based API using JSON

  • while organizing code, think of Contexts first.. where does Video resource reside; in future Videologue might evolve to different media channel annotations

a Multimedia context gives good place to group Video functionality

  • few fields for Video would be associated user, creation time, URI, title, description
  • generate Multimedia resource as
mix phx.gen.html Multimedia Video videos user_id:references:users url:string title:string description:text

this generates controller lib/videologue_web/controllers/video_controller.ex, view lib/videologue_web/views/video_view.ex, templates lib/videologue_web/templates/video/{edit.html.eex,form.html.eex,index.html.eex,new.html.eex,show.html.eex}, context lib/videologue/multimedia.ex with schema lib/videologue/multimedia/video.ex, db migration priv/repo/migrations/20210529175006_create_videos.exs, alongwith tests boilerplate for controller & context

  • expects to explicitly add resources "/videos", VideoController and run Ecto migrations

  • sometimes Generators are too much, pays to be explicit

  • Phoenix uses Singular forms consistently across schema, controller & views in most cases

  • move UserController.authenticate to VideologueWeb.Auth's authenticate_user for central reuse at User and Multimedia checking sessions

  • add import VideologueWeb.Auth, only: [authenticate_user: 2] to controller & route macros in videologue_web.ex

  • and update the Plug at UserController for new name i.e. authenticate_user

  • create /my scope for managing Videos in router.ex and add :authenticate_user to pipe_through list alongwith :browser to be used for all resources under it

  • add VideoController resource to /my scope

  • now visit /my/videos to add a video

Examining Generated Context, Controller, View

  • updated Show/Index templates to have clickable URL rendered; and validate_length for minimum description length

Generated Migrations

  • are for all fields added with up mix ecto.migrate & down mix ecto.rollback

  • it takes care of adding Foreign Key

Building Relationships

  • default generated Schema sets up user_id of type :id; migration sets it up as Foreign Key

  • replace field :user_id, :id by belongs_to :user, Videologue.Accounts.User allowing Ecto to build right associations

  • it's not cast-able or required; the earlier created Video wouldn't have this field yet

  • creating a new video & accessing video.user will give #Ecto.Association.NotLoaded<association :user is not loaded>

  • Ecto associations are explicit, to fetch need to ask to avoid cascade fetch not needed
  • video = Videologue.Repo.preload(video, :user) makes it load, value yet is nil

  • Repo.preload accepts one name or collection of association names

  • let's update the existing video for user_id via IEx

iex(1)> video = Videologue.Repo.get(Videologue.Multimedia.Video, 1)
%Videologue.Multimedia.Video{
  __meta__: #Ecto.Schema.Metadata<:loaded, "videos">,
  description: "With the Phoenix v1.5 release, learn how easy LiveView makes it to build interactive, real-time applications.",
  id: 1,
...
  user_id: nil
}

iex(2)> video = Videologue.Repo.preload(video, :user)

iex(3)> video.user
nil

iex(4)> user = Videologue.Repo.get(Videologue.Accounts.User, 1)  

iex(7)> video = video |> Ecto.Changeset.change() |> Ecto.Changeset.put_assoc(:user, user) |> Videologue.Repo.update!(changeset)

iex(8)> video.user                                
%Videologue.Accounts.User{
  __meta__: #Ecto.Schema.Metadata<:loaded, "users">,
  id: 1,
...
  username: "sherlock"
}

iex(9)> videoo = Videologue.Repo.get(Videologue.Multimedia.Video, 1)                         
%Videologue.Multimedia.Video{
  __meta__: #Ecto.Schema.Metadata<:loaded, "videos">,
  description: "With the Phoenix v1.5 release, learn how easy LiveView makes it to build interactive, real-time applications.",
  id: 1,
...
  user_id: 1 
}

iex(10)> videoo.user
#Ecto.Association.NotLoaded<association :user is not loaded>

iex(11)> videoo = Videologue.Repo.preload(videoo, :user)                                      

iex(12)> videoo.user                                    
%Videologue.Accounts.User{
  __meta__: #Ecto.Schema.Metadata<:loaded, "users">,
  id: 1,
...
  username: "sherlock"
}

generally avoid cyclic dependencies, so one-way relationship from videos to user

Managing Related Data

  • need to grab current-user conn.assigns.current_user and scope operations to them by passing it in Videologue.Multimedia.create_video/2 (updated from create_video/1)
  def create(conn, %{"video" => video_params}) do
    case Multimedia.create_video(conn.assigns.current_user, video_params) do
      {:ok, video} ->
        conn
...
  • in create_video/2 utilize Ecto.Changeset.put_assoc/2 over changeset to associate User to Video just before Repo.insert/1
  def create_video(%Accounts.User{} = user, attrs \\ %{}) do
    %Video{}
    |> Video.changeset(attrs)
    |> Ecto.Changeset.put_assoc(:user, user)
    |> Repo.insert()
  end
  • similarly update new, edit & update function calls
  • BUT here we'll use current_user nearly in all calls; let's add conn.assigns.current_user to be passed to all Controller functions as an argument by default with a custom action function in VideoController

now we have current_user as 3rd param in all functions which can then be used with list_videos to only show current user videos; change_video to edit/update

  • pipe user_videos_query/2 to append FROM checking list/get/delete is only made possible on Current User's already owned content

user_videos_query/2 ensuring get_user_video! to work only for current-user in-turn ensures edit, update & delete controller task only work for current-user

currently get_user_video! is being used which exposes bunch of errors, as discussed earlier these shall be masked alongwith all other 4XX & 5XX

In-context Relationships

  • defining relation within same context like adding Categories to Videos; don't need to create Controller with View/Template as not often added

  • all Multimedia resources will have Categories; so schema be Multimedia.Category

Schema and Context Generators

  • mix phx.gen.html Multimedia Category categories name:string will generate controller, view, template, context, schema & migration

  • mix phx.gen.context Multimedia Category categories name:string makes context, schema & migration (for pre-existing context it will be injected)

  • mix phx.gen.schema Multimedia.Category categories name:string makes a schema with migration

  • mix ecto.gen.migration create_categories generating new empty migration

Generating Category Migrations

  • mix phx.gen.schema Multimedia Category categories name:string makes a schema with migration

  • in Video schema add belongs_to :category, Videologue.Multimedia.Category and cast :category_id in changeset

  • mix ecto.gen.migration add_category_id_to_video and add following to generated file and the mix ecto.migrate

  alter table(:videos) do
    add :category_id, references(:categories)
  end

now we have Ecto level assoc placed and Foreign Key constraints at DB

Wrapping Up

Chapter-07 Ecto Queries and Constraints