Serializing Associations

Learning Goals

Serialize nested data using has_many and belongs_to

Introduction

We have seen how ActiveModel::Serializer can be used to easily customize the JSON being returned for a single model. But what if we have multiple associated models? As it turns out, that situation can also be handled easily with AMS, by using the Active Record macros has_many and belongs_to that we're already familiar with. In this lesson, we'll learn how to implement serializers for associated models in our Movie app.

To enable us to do this, we've expanded our movie app to include two more models. Specifically, we made the following changes:

Instead of including director as an attribute of our Movie instances, we created a separate Director class.
We modified our app to include movie reviews using a Review class.

The relationships we want to model look like this:

Director -< Movies -< Reviews

To implement the Director class, we made the following changes to our code:

Removed director and female_director as attributes in our movie migration file; added a director_id attribute
Added a new migration for our director model with three attributes: name, birthplace and female_director
Added the belongs_to :director macro to the Movie model and the has_many :movies macro to the Director model
Added index and show routes for the Director model in config/routes.rb
Added a DirectorsController and created the index and show actions

To implement the Review class, we made the following changes:

Added a new migration with four attributes: author, date, url, and movie_id
Added the has_many :reviews macro to the Movie model and the belongs_to :movie macro to the Review model
Added an index route for the Review model in config/routes.rb
Created a ReviewsController and added the index action

Spend a few minutes looking through the code to familiarize yourself with how everything is set up.

Using ActiveModel::Serializer with Associated Data

Let's see the updated version of our app in action. To set it up, run:

$ bundle install
$ rails db:migrate db:seed
$ rails s

The setup for Movie has not changed: you should still be able to navigate to its index and show routes, as well as the custom /movies/:id/summary and movie_summaries routes we created in the last lesson.

Take a look at the new index and show routes for Director in the browser. You'll see that the JSON for the directors includes two attributes that we don't want: created_at and updated_at. Luckily we know how to fix this — we simply need to create a serializer for director as we did for movies:

$ rails g serializer director

We can then add the desired attributes to the director_serializer file:

# app/serializers/director_serializer.rb
class DirectorSerializer < ActiveModel::Serializer
  attributes :id, :name, :birthplace, :female_director
end

Now if you navigate to /directors or /directors/:id you will see that we're only displaying the desired attributes.

Serializing a One-to-Many Association

Let's take a look at our new Movie index route. Now that we've removed the director and female_director attributes, the JSON for movies no longer includes any information about director. We need to figure out how to add the information about each movie's associated director to the JSON being returned by the movies serializer. AMS allows us to do this using the same macros in the serializers that we use to set up associations in our model files. In this case, we want our serializer to reflect the fact that Movie belongs to Director, so we'll update the serializer as follows:

# serializers/movie_serializer.rb
class MovieSerializer < ActiveModel::Serializer
  attributes :id, :title, :year, :length, :description, :poster_url, :category, :discount

  belongs_to :director
end

Now if you navigate to localhost:3000/movies/1, you should see the following:

{
  "id": 1,
  "title": "The Color Purple",
  "year": 1985,
  "length": 154,
  "description": "Whoopi Goldberg brings Alice Walker's Pulitzer Prize-winning feminist novel to life as Celie, a Southern woman who suffered abuse over decades. A project brought to a hesitant Steven Spielberg by producer Quincy Jones, the film marks Spielberg's first female lead.",
  "poster_url": "https://pisces.bbystatic.com/image2/BestBuy_US/images/products/3071/3071213_so.jpg",
  "category": "Drama",
  "discount": false,
  "director": {
    "id": 1,
    "name": "Steven Spielberg",
    "birthplace": "Cincinnati, OH",
    "female_director": false
  }
}

We once again can see the director information for our movie!

We can also set up the relationship in the other direction, by adding the corresponding macro in our DirectorSerializer:

# serializers/director_serializer.rb
class DirectorSerializer < ActiveModel::Serializer
  attributes :id, :name, :birthplace, :female_director

  has_many :movies
end

Because we have included the has_many macro in the Director serializer, when we navigate to localhost:3000/directors/:id, we can see the list of movies that belong to that particular director:

{
  "id": 1,
  "name": "Steven Spielberg",
  "birthplace": "Cincinnati, OH",
  "female_director": false,
  "movies": [
    {
      "id": 1,
      "title": "The Color Purple",
      "year": 1985,
      "length": 154,
      "description": "Whoopi Goldberg brings Alice Walker's Pulitzer Prize-winning feminist novel to life as Celie, a Southern woman who suffered abuse over decades. A project brought to a hesitant Steven Spielberg by producer Quincy Jones, the film marks Spielberg's first female lead.",
      "poster_url": "https://pisces.bbystatic.com/image2/BestBuy_US/images/products/3071/3071213_so.jpg",
      "category": "Drama",
      "discount": false
    }
  ]
}

IMPORTANT: You should only add macros to your serializers if you're sure you need the data! The level of complexity ramps up quickly as you add more macros, so keeping them to a minimum will save you headaches in the long run. It's also good to consider how much data is being sent with each request, since adding more data means running more SQL code to access that info from different tables in the database, which will make our responses slower.

Rails automatically uses the appropriate serializer, based on naming conventions, to display the associated data for each of our models. We can see that in the example above: Rails has used the MovieSerializer to render the movie JSON, so all of the attributes we listed in that serializer are rendered in the Director's index and show routes.

Serializing a Many-to-Many Association

Our Movie example uses a one-to-many association (directors have many movies and movies belong to a director), but you you can also use Active Model Serializers with a many-to-many association.

For example, if we had an app with Article and Tag models, we could create a join table and set up has_many :through associations for both models:

# app/models/article.rb
class Article < ApplicationRecord
  has_many :article_tags
  has_many :tags, through: :article_tags
end

# app/models/article_tag.rb
class ArticleTag < ApplicationRecord
  belongs_to :article
  belongs_to :tag
end

# app/models/tag.rb
class Tag < ApplicationRecord
  has_many :article_tags
  has_many :articles, through: :article_tags
end

Then, if we want the JSON for Article to include a list of the article's tags, we would simply use has_many :tags in our ArticleSerializer:

# app/serializers/article_serializer.rb
class ArticleSerializer < ActiveModel::Serializer
  attributes :id, :title, :author, :content

  has_many :tags
end

Because the has_many :through association is defined in the model files, Rails will know to nest a list of each article's tags in the JSON that's being returned.

Adding Custom Serializers

Let's return to our Movie example. We have successfully set up our Director serializer to include a list of the director's movies in the JSON that's returned:

{
  "id": 1,
  "name": "Steven Spielberg",
  "birthplace": "Cincinnati, OH",
  "female_director": false,
  "movies": [
    {
      "id": 1,
      "title": "The Color Purple",
      "year": 1985,
      "length": 154,
      "description": "Whoopi Goldberg brings Alice Walker's Pulitzer Prize-winning feminist novel to life as Celie, a Southern woman who suffered abuse over decades. A project brought to a hesitant Steven Spielberg by producer Quincy Jones, the film marks Spielberg's first female lead.",
      "poster_url": "https://pisces.bbystatic.com/image2/BestBuy_US/images/products/3071/3071213_so.jpg",
      "category": "Drama",
      "discount": false
    }
  ]
}

With only one Steven Spielberg movie in our data, including all that information isn't too unreasonable. But what happens when we add the rest of his movies to our database? We may decide we don't need to include all the details of every movie in this view.

To fix this, we can simply create a new, streamlined serializer:

$ rails g serializer director_movie

Here we'll include just the title and year of each of the director's movies:

class DirectorMovieSerializer < ActiveModel::Serializer
  attributes :title, :year
end

Now, if you refresh the page... nothing changes. Why not?

Recall that Rails automatically selects the serializer based on naming conventions, so it's still using the DirectorSerializer to render the data. To fix this, we need to tell the DirectorSerializer that it should be using this new serializer instead; we need to pass it explicitly:

class DirectorSerializer < ActiveModel::Serializer
  attributes :id, :name, :birthplace, :female_director

  has_many :movies, serializer: DirectorMovieSerializer
end

Rails is still using DirectorSerializer to render the JSON for the director, but now DirectorSerializer is passing along the request for the director's movies to the new, simplified serializer.

Now if you refresh the page, you should see the following:

{
  "id": 1,
  "name": "Steven Spielberg",
  "birthplace": "Cincinnati, OH",
  "female_director": false,
  "movies": [
    {
      "title": "The Color Purple",
      "year": 1985
    }
  ]
}

Deeply Nested Models

Now that we've got the JSON set up the way we want for our associated Movie and Director models, we can turn our attention to the Review model. Let's take another look at our model relationships:

Director -< Movies -< Reviews

We've already set up the association in the Movie and Review model files:

# app/models/movie.rb
class Movie < ApplicationRecord
  belongs_to :director
  has_many :reviews
end

# app/models/review.rb
class Review < ApplicationRecord
  belongs_to :movie
end

Next, we'll create our review serializer:

$ rails g serializer review

We can also specify the attributes we want to include:

# app/serializers/review_serializer.rb
class ReviewSerializer < ActiveModel::Serializer
  attributes :id, :author, :date, :url
end

We can now go to localhost:3000/reviews and see our reviews listed. However, viewing a list of reviews separately from the information about the movies they're associated with is not particularly helpful.

What we really want to do is render the information about a movie's reviews along with the rest of the information about that movie. In fact, we don't really need to render information about reviews at all except as part of the data rendered for a particular movie!

Before we figure out how to get that in place, let's follow good programming practice and delete the code we no longer need: we'll remove the resource for reviews from the routes.rb file and the index action from the ReviewsController.

Once that's done, to get reviews included in the JSON that's returned for a given movie, we'll simply add the appropriate macro to the MovieSerializer:

class MovieSerializer < ActiveModel::Serializer
  attributes :id, :title, :year, :length, :description, :poster_url, :category, :discount

  belongs_to :director
  has_many :reviews
end

Now if we visit localhost:3000/movies/1, we can verify that the reviews are now included in the movie's JSON.

So let's review where we are: the JSON for directors includes their movies, and the JSON for movies includes their reviews. Given that, if we visit localhost:3000/directors/1, will we see the full set of nested data? Unfortunately, no, we won't. Our Director JSON will look just the same as it did before we added the Review model:

{
  "id": 1,
  "name": "Steven Spielberg",
  "birthplace": "Cincinnati, OH",
  "female_director": false,
  "movies": [
    {
      "title": "The Color Purple",
      "year": 1985
    }
  ]
}

This is because, by default, AMS only nests associations one level deep.

This behavior is intended to protect against overly complex JSON that's nested many layers deep. Luckily, we can override the behavior by using the include option in the top-level controller — in this case, the DirectorsController:

# app/controllers/directors_controller.rb
class DirectorsController < ApplicationController

  def index
    directors = Director.all
    render json: directors, include: ['movies', 'movies.reviews']
  end

  def show
    director = Director.find(params[:id])
    render json: director, include: ['movies', 'movies.reviews']
  end

end

Let's take a look at the render statement in our show action:

render json: director, include: ['movies', 'movies.reviews']

This code tells AMS that we want to render information for the director, and to also include information for the movies associated with that director, and for the reviews associated with those movies.

Finally, because we're using our custom DirectorMovieSerializer to render the movies in our Director routes, we also need to add the has_many :reviews macro to that serializer:

class DirectorMovieSerializer < ActiveModel::Serializer
  attributes :title, :year

  has_many :reviews
end

With these changes in place, refresh the page and you should now see this:

{
  "id": 1,
  "name": "Steven Spielberg",
  "birthplace": "Cincinnati, OH",
  "female_director": false,
  "movies": [
    {
      "title": "The Color Purple",
      "year": 1985,
      "reviews": [
        {
          "id": 1,
          "author": "Roger Ebert",
          "date": "December 20, 1985",
          "url": "https://www.rogerebert.com/reviews/the-color-purple-1985"
        },
        {
          "id": 2,
          "author": "Variety Staff",
          "date": "December 31, 1984",
          "url": "https://variety.com/1984/film/reviews/the-color-purple-1200426436/"
        },
        {
          "id": 3,
          "author": "Janet Maslin",
          "date": "December 18, 1985",
          "url": "https://www.nytimes.com/1985/12/18/movies/moviesspecial/the-color-purple.html"
        }
      ]
    }
  ]
}

Nice!

Conclusion

ActiveModel::Serializer provides some powerful yet simple-to-use tools for crafting the JSON our app returns, and it does so in a way that's consistent with Rails conventions.

To summarize:

To customize the JSON returned for a resource, create a serializer for that resource and list the desired attributes.
The serializer is used implicitly by Rails based on naming conventions; to override this, custom serializers can be passed explicitly.
AMS enables the use of the belongs_to and has_many macros in serializers to render associated data; these macros should be used sparingly.
By default, AMS will only nest associations one level deep in the serialized JSON. To override this, the include option can be used.

Check For Understanding

Before you move on, make sure you can answer the following questions:

If we have Recipe and Ingredient resources and we want to nest ingredients under recipes in the json we're returning, in which file would we use the has_many macro? In which file would we use the belongs_to macro?
If we want to specify a custom serializer for the parent resource (Recipe), in which file would we do that? In which file would we specify a custom serializer for the child resource (Ingredient)?
In which file do we use the include keyword to set up deep nesting?

lorna-chepkirui/phase-4-serializing-associations