Relations

• Has_many

One to One

belongs_to indicates a one-to-one or many-to-one association with another schema. Defines a foreign key that is the primary key of the listed table.

• belongs_to (has the id in its table) => has_one has their id in their belongs_to table

• The other schema often has a has_one or a has_many field with same fields, but reverse association. (Both the has macros don't change DB, just allow access)

  schema "organization_subscriptions" do
		belongs_to :organization, ReactPhoenix.Organizations.Organization, type: :string
		# subscriptions has organization_id

  schema "organizations" do
    has_one :subscription, ReactPhoenix.Organizations.Subscription 
    #nothing in table organizations

Usage

# Can Preload the belongs_to post on the comment 
[comment] = Repo.all(from(c in Comment, where: c.id == 42, preload: :post))
comment.post #=> %Post{...}
#Or the post
[post] = Repo.all(from(p in Post, where: p.id == 42, preload: :comments))
post.comments #=> [%Comment{...}, ...]

Updating from parent

You could pass in %{id: 1, child: [1]}

• :on_replace - The action taken on associations when the record is replaced when casting or manipulating parent changeset. May be :raise (default), :mark_as_invalid, :nilify, :update, or :delete. See Ecto.Changeset's section on related data for more info.

    has_many :subevent_breakout_groups, ReactPhoenix.Subevents.SubeventBreakoutGroup,
      on_replace: :delete

Create/Update Relations

• cast_assoc

This function should be used when working with the entire association at once (and not a single element of a many-style association) and receiving data external to the application.

It compares each of param to preloaded

• No id or id not in db => insert

• if id and in db => update

• If no id, but id in db => delete/on_replace

In Creation

def create_board(attrs \\ %{}) do
  %Board{}
  |> Board.changeset(attrs)
  |> Ecto.Changeset.cast_assoc(:activities,
  with: &ReactPhoenix.Activities.Activity.changeset/2
  )
  |> Repo.insert()
end

In Update

event
|> Event.changeset(attrs)
|> Ecto.Changeset.cast_assoc(:subevents, with: &Subevent.changeset/2)
|> Ecto.Changeset.cast_assoc(:registration_event_questions,
with: &RegistrationEventQuestion.changeset/2
)
|> Repo.update()

In Get

Repo.get(Event, id)
|> Repo.preload(registration_event_questions: :registration_question)
|> Repo.preload(event_folder: :events)

Example Schema

  schema "subevents" do
    field :title, :string
    # text 
    field :description, :string
    field :type, :string
    field :type_data, :map
    field :link, :string
    field :start_time, :utc_datetime
    field :end_time, :utc_datetime
    belongs_to :event, ReactPhoenix.Events.Event, type: :string
    has_many :calls, ReactPhoenix.Calls.Call, on_replace: :delete

    has_many :subevent_breakout_groups, ReactPhoenix.Subevents.SubeventBreakoutGroup,
      on_replace: :delete

    timestamps()
  end