ecto-nested-associations

Ecto Nested Associations

RULES — Follow these with no exceptions

1. Use cast_assoc/3 for has_many/has_one — never manually insert children in a separate step; let Ecto manage the relationship
2. Use Ecto.Multi for operations spanning multiple unrelated tables — not nested changesets; Multi provides explicit rollback control
3. Set on_delete explicitly in migrations — :delete_all for owned children, :nothing for references to independent entities
4. Always create indexes on foreign key columns — missing FK indexes cause slow joins and lookups on the child table
5. Use on_replace: :delete in cast_assoc for list management — allows removing items by omitting them from the input
6. Preload associations before updating them — cast_assoc/3 raises (attempting to cast or change association ... that was not loaded) if the association isn't preloaded; preload before update

cast_assoc for Nested Creates

Create parent and children in a single operation. Ecto sets foreign keys automatically.

# Schema definitions
defmodule MyApp.Blog.Post do
  use Ecto.Schema
  import Ecto.Changeset

  schema "posts" do
    field :title, :string
    has_many :comments, MyApp.Blog.Comment

    timestamps()
  end

  def changeset(post, attrs) do
    post
    |> cast(attrs, [:title])
    |> validate_required([:title])
    |> cast_assoc(:comments, with: &MyApp.Blog.Comment.changeset/2)
  end
end

defmodule MyApp.Blog.Comment do
  use Ecto.Schema
  import Ecto.Changeset

  schema "comments" do
    field :body, :string
    belongs_to :post, MyApp.Blog.Post

    timestamps()
  end

  # Do NOT require :post_id — cast_assoc sets it automatically
  def changeset(comment, attrs) do
    comment
    |> cast(attrs, [:body])
    |> validate_required([:body])
  end
end

# Usage — create post with comments in one operation
Blog.create_post(%{
  title: "My Post",
  comments: [
    %{body: "First comment"},
    %{body: "Second comment"}
  ]
})