Limiting Associations Cascade in Active Model Serializer

Question

I'm having an issue with limiting the level of associations serialized within an active model resource.

For example:

A Game has many Teams which has many Players

class GameSerializer < ActiveModel::Serializer
  attributes :id
  has_many :teams
end

class TeamSerializer < ActiveModel::Serializer
  attributes :id
  has_many :players
end

class PlayerSerializer < ActiveModel::Serializer
  attributes :id, :name
end

When I retrieve the JSON for the Team, it includes all the players in a sub array, as desired.

When I retrieve the JSON for the Game, it includes all the Teams in a sub array, excellent, but also all the players for each Team. This is the expected behaviour but is it possible to limit the level of associations? Have Game only return the serialized Teams without the Players?

score 12 · Accepted Answer · answered Sep 23 '13 at 22:00

12

Another option is to abuse Rails' eager loading to determine which associations to render:

In your rails controller:

def show
  @post = Post.includes(:comments).find(params[:id])
  render json: @post
end

then in AMS land:

class PostSerializer < ActiveModel::Serializer
  attributes :id, :title
  has_many :comments, embed: :id, serializer: CommentSerializer, include: true

  def include_comments?
    # would include because the association is hydrated
    object.association(:comments).loaded?
  end
end

Probably not the cleanest solution, but it works nicely for me!

answered Sep 23 '13 at 22:00

Kieran Huggins

171
1
3

`object.association(:comments).loaded?` This is exactly what I was looking for, thanks! I think this method is cleaner than the accepted answer. From the active_model_serializer docs its recommended to include associations using either a join or includes in the controller to avoid n+1 queries. Where I was stumped was in the serializer how to determine if an association was loaded or to omit it. From the docs: "try to avoid n+1 queries by ensuring that data loads in an optimal fashion, e.g. if you are using ActiveRecord, you might want to use query includes or joins as needed" – Mark Oct 28 '13 at 07:59
2

Where do I have to call the include_comments? method? – Kaspar Dec 08 '14 at 15:21

score 8 · Answer 2 · answered Aug 11 '13 at 22:49

8

You can create another Serializer:

class ShortTeamSerializer < ActiveModel::Serializer
  attributes :id
end

Then:

class GameSerializer < ActiveModel::Serializer
  attributes :id
  has_many :teams, serializer: ShortTeamSerializer
end

Or you can define a include_teams? in GameSerializer:

class GameSerializer < ActiveModel::Serializer
  attributes :id
  has_many :teams

  def include_teams?
    @options[:include_teams]
  end
end

answered Aug 11 '13 at 22:49

Pablo B.

1,823
1
17
27

Thanks Pablo, this is what I ended up doing... I tried to make it a bit rails-y, modelling :index and :show pluralization but having a `TeamsSerializer` and `TeamSerializer`. Special cases get a different serializer. – Greg Olsen Aug 13 '13 at 20:07
4

Where does @options came from? – Samnang Jan 22 '14 at 10:24

Limiting Associations Cascade in Active Model Serializer

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