Phoenix Ecto Preload - Elixir

Phoenix is a web development framework written in Elixir. Ecto is a database wrapper for Elixir that supports SQL databases. Preloading is a technique in Ecto to load associations of a record in a single database query.

Why Preload Associations?

Preloading associations reduces the number of queries to the database, improving performance. Without preloading, loading associated data requires separate database queries for each record.

Using Preload in Phoenix Ecto

To use preload in Ecto, we need to define associations in the schema of Ecto models. For instance, let's say we have two Ecto models: User and Post. A user can have many posts. The schema for these models can be defined as follows:

defmodule MyApp.User do
  use Ecto.Schema

  schema "users" do
    field :name, :string
    has_many :posts, MyApp.Post
  end
end

defmodule MyApp.Post do
  use Ecto.Schema

  schema "posts" do
    field :title, :string
    field :body, :string
    belongs_to :user, MyApp.User
  end
end

In these models, we defined a has_many association from User to Post, and a belongs_to association from Post to User.

We can now use preload to load all the posts of a user in a single query. For instance, to load all the posts of a user with id 1, we can use the following code:

user = MyApp.Repo.get!(MyApp.User, 1) |> MyApp.Repo.preload(:posts)

Here, we use preload to load all the posts associated with the user, in a single query.

We can also preload nested associations. For instance, to load all the posts of a user with their associated comments, we can use the following code:

user = MyApp.Repo.get!(MyApp.User, 1) |> MyApp.Repo.preload(posts: :comments)

Here, we use preload to load all the comments associated with the posts associated with the user, in a single query.

Conclusion

Preloading associations in Phoenix Ecto can dramatically improve the performance of applications. By defining associations in Ecto models, and using preload to load associations in a single query, we can minimize the number of database queries needed to load associated data.