> For the complete documentation index, see [llms.txt](https://openai.gitbook.io/code-cheatsheets/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://openai.gitbook.io/code-cheatsheets/all/elixir/phoenix/db_ecto/relations.md).

# 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)

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

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

#### Usage

```elixir
# 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`](https://hexdocs.pm/ecto/Ecto.Changeset.html)'s section on related data for more info.

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

## Create/Update Relations

* [`cast_assoc`](https://hexdocs.pm/ecto/Ecto.Changeset.html#cast_assoc/3)

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

```elixir
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

```elixir
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

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

## Example Schema

```elixir
  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
```