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Showing content from https://github.com/exaspark/batch_loader below:

exAspArk/batch_loader: :zap: Powerful tool for avoiding N+1 DB or HTTP queries

This package provides a generic lazy batching mechanism to avoid N+1 DB queries, HTTP queries, etc.

• Highlights
• Usage
  • Ecto Resolve Association
  • Ecto Load Association
  • Ecto Preload Association
  • DIY Batching
  • Customization
• Installation
• Testing

• Generic utility to avoid N+1 DB queries, HTTP requests, etc.
• Adapted Elixir implementation of the battle-tested tools like Haskell Haxl, JS DataLoader, Ruby BatchLoader.
• Convenient and flexible integration with Ecto Schemas.
• Allows inlining the code without defining extra named functions, unlike Absinthe Batch.
• Allows using batching with any data sources, not just DB, unlike Absinthe DataLoader.

Let's imagine that we have a Post GraphQL type defined with Absinthe:

defmodule MyApp.PostType do
  use Absinthe.Schema.Notation
  alias MyApp.Repo

  object :post_type do
    field :title, :string

    field :user, :user_type do
      resolve(fn post, _, _ ->
        user = post |> Ecto.assoc(:user) |> Repo.one() # N+1 DB requests
        {:ok, user}
      end)
    end
  end
end

This will produce N+1 DB requests if we send this GraphQL request:

query {
  posts {
    title
    user { # N+1 request per each post
      name
    }
  }
}

We can get rid of the N+1 DB requests by loading all Users for all Posts at once in. All we have to do is to use resolve_assoc function by passing the Ecto associations name:

import BatchLoader.Absinthe, only: [resolve_assoc: 1]

field :user, :user_type, resolve: resolve_assoc(:user)

Set the default repo in your config.exs file:

config :batch_loader, :default_repo, MyApp.Repo

And finally, add BatchLoader.Absinthe.Plugin plugin to the GraphQL schema. This will allow to lazily collect information about all users which need to be loaded and then batch them all together:

defmodule MyApp.Schema do
  use Absinthe.Schema
  import_types MyApp.PostType

  def plugins do
    [BatchLoader.Absinthe.Plugin] ++ Absinthe.Plugin.defaults()
  end
end

You can use load_assoc to load Ecto associations in the existing schema:

import BatchLoader.Absinthe, only: [load_assoc: 3]

field :author, :string do
  resolve(fn post, _, _ ->
    load_assoc(post, :user, fn user ->
      {:ok, user.name}
    end)
  end)
end

You can use preload_assoc to preload Ecto associations in the existing schema:

import BatchLoader.Absinthe, only: [preload_assoc: 3]

field :title, :string do
  resolve(fn post, _, _ ->
    preload_assoc(post, :user, fn post_with_user ->
      {:ok, "#{post_with_user.title} - #{post_with_user.user.name}"}
    end)
  end)
end

You can also use BatchLoader to batch in the resolve function manually, for example, to fix N+1 HTTP requests:

field :user, :user_type do
  resolve(fn post, _, _ ->
    BatchLoader.Absinthe.for(post.user_id, &resolved_users_by_user_ids/1)
  end)
end

def resolved_users_by_user_ids(user_ids) do
  MyApp.HttpClient.users(user_ids)                   # load all users at once
  |> Enum.map(fn user -> {user.id, {:ok, user}} end) # return "{user.id, result}" tuples
end

Alternatively, you can simply inline the batch function:

field :user, :user_type do
  resolve(fn post, _, _ ->
    BatchLoader.Absinthe.for(post.user_id, fn user_ids ->
      MyApp.HttpClient.users(user_ids)
      |> Enum.map(fn user -> {user.id, {:ok, user}} end)
    end)
  end)
end

• To specify default resolve Absinthe values:

BatchLoader.Absinthe.for(post.user_id, &resolved_users_by_user_ids/1, default_value: {:error, "NOT FOUND"})

• To use custom callback function:

BatchLoader.Absinthe.for(post.user_id, &users_by_user_ids/1, callback: fn user ->
  {:ok, user.name}
end)

• To use custom Ecto repos:

BatchLoader.Absinthe.resolve_assoc(:user, repo: AnotherRepo)
BatchLoader.Absinthe.preload_assoc(post, :user, &callback/1, repo: AnotherRepo)

• To pass custom options to Ecto.Repo.preload:

BatchLoader.Absinthe.resolve_assoc(:user, preload_opts: [prefix: nil])
BatchLoader.Absinthe.preload_assoc(post, :user, &callback/1, preload_opts: [prefix: nil])

Add batch_loader to your list of dependencies in mix.exs:

def deps do
  [
    {:batch_loader, "~> 0.1.0-beta.6"}
  ]
end

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