Shared data

Sometimes you need to access specific pieces of data on numerous pages within your application. For example, you may need to display the current user in the site header. Passing this data manually in each response across your entire application is cumbersome. Thankfully, there is a better option: shared data.

Sharing data

Inertia's Rails adapter comes with the shared_data controller method. This method allows you to define shared data that will be automatically merged with the page props provided in your controller.

class EventsController < ApplicationController
  # share syncronously
  inertia_share app_name: env['app.name']

  # share lazily, evaluated at render time
  inertia_share do
    if logged_in?
      {
        user: logged_in_user,
      }
    end
  end

  # share lazily alternate syntax
  inertia_share user_count: lambda { User.count }
end

NOTE

Shared data should be used sparingly as all shared data is included with every response.

NOTE

Page props and shared data are merged together, so be sure to namespace your shared data appropriately to avoid collisions.

Accessing shared data

Once you have shared the data server-side, you will be able to access it within any of your pages or components. Here's an example of how to access shared data in a layout component.

<template>
  <main>
    <header>You are logged in as: {{ user.name }}</header>
    <content>
      <slot />
    </content>
  </main>
</template>

<script>
export default {
  computed: {
    user() {
      return this.$page.props.auth.user
    },
  },
}
</script>

Flash messages

Another great use-case for shared data is flash messages. These are messages stored in the session only for the next request. For example, it's common to set a flash message after completing a task and before redirecting to a different page.

Here's a simple way to implement flash messages in your Inertia applications. First, share the flash message on each request.

class ApplicationController < ActionController::Base
  inertia_share flash: -> { flash.to_hash }
end

Next, display the flash message in a frontend component, such as the site layout.

<template>
  <main>
    <header></header>
    <content>
      <div v-if="$page.props.flash.alert" class="alert">
        {{ $page.props.flash.alert }}
      </div>
      <div v-if="$page.props.flash.notice" class="notice">
        {{ $page.props.flash.notice }}
      </div>
      <slot />
    </content>
    <footer></footer>
  </main>
</template>

Deep Merging Shared Data

By default, Inertia will shallow merge data defined in an action with the shared data. You might want a deep merge. Imagine using shared data to represent defaults you'll override sometimes.

class ApplicationController
  inertia_share do
    { basketball_data: { points: 50, rebounds: 100 } }
  end
end

Let's say we want a particular action to change only part of that data structure. The renderer accepts a deep_merge option:

class CrazyScorersController < ApplicationController
  def index
    render inertia: 'CrazyScorersComponent',
      props: { basketball_data: { points: 100 } },
      deep_merge: true
  end
end

# The renderer will send this to the frontend:
{
  basketball_data: {
    points: 100,
    rebounds: 100,
  }
}

Deep merging can be set as the project wide default via the InertiaRails configuration:

# config/initializers/some_initializer.rb
InertiaRails.configure do |config|
  config.deep_merge_shared_data = true
end

If deep merging is enabled by default, it's possible to opt out within the action:

class CrazyScorersController < ApplicationController
  inertia_share do
    {
      basketball_data: {
        points: 50,
        rebounds: 10,
      }
    }
  end

  def index
    render inertia: 'CrazyScorersComponent',
      props: { basketball_data: { points: 100 } },
      deep_merge: false
  end
end

# Even if deep merging is set by default, since the renderer has `deep_merge: false`, it will send a shallow merge to the frontend:
{
  basketball_data: {
    points: 100,
  }
}