Count loader for graphql-batch

When implemented a GraphQL API using GraphQL Ruby it is beneficial to ensure that executed queries don’t trigger any N+1 query problems.

One (of many) solutions to this is graphql-batch which provides a means of delcaring loaders to perform batched execution, avoding N+1 query problems.

A record and assocation loader is provided by default and these perform their jobs admirably, though alone they do not cover the gamut of N+1 issues that can be encountered.

In this post we’ll address the issue of counting generically using CountLoader, and more explicitly associations using AssociationCountLoader.

The following example type if fetched multiple times in a query (e.g. as a collection) would cause multiple queries in counting:

class Types::UserType < Types::BaseObject
  field :name, String, null: false
  field :post_count, Integer, "blog posts authored by this user", null: false

  def post_count
    object.posts.count
  end
end

We can resolve this issue by implementing a CountLoader and updating the typing accordingly.

class CountLoader < GraphQL::Batch::Loader
  def initialize(model, field)
    super()
    @model = model
    @field = field
  end

  def perform(ids)
    counts = @model.where(@field => ids).group(@field).count

    counts.each { |id, count| fulfill(id, count) }
    ids.each { |id| fulfill(id, 0) unless fulfilled?(id) }
  end
end

class Types::UserType < Types::BaseObject
  field :name, String, null: false
  field :post_count, Integer, "blog posts authored by this user", null: false

  def post_count
    CountLoader.for(Post, :user_id).load(object.id)
  end
end

Using the CountLoader above the multiple queries as a result of counting have been resolved. As can be seen the structure of CountLoader is very similar to the provided RecordLoader.

Counting ActiveRecord associations

Reddit user u/Owumaro suggested using an ActiveRecord association name rather than having to specify the join model and foriegn key back.

The provided AssociationCountLoader reflects on the provided relationship to alleviate the need for specifying these extra implementation details (it should be less britle).

class AssociationCountLoader < GraphQL::Batch::Loader
  def initialize(model, association_name)
    super()
    @model = model
    @association_name = association_name
  end

  def perform(records)
    reflection = @model.reflect_on_association(@association_name)
    reflection.check_preloadable!

    klass = reflection.klass
    field = reflection.join_primary_key
    counts = klass.where(field => records).group(field).count

    records.each do |record|
      record_key = record[reflection.active_record_primary_key]
      fulfill(record, counts[record_key] || 0)
    end
  end
end

Using AssociationCountLoader post_count in Types::UserType changes to:

def post_count
  AssociationCountLoader.for(User, :posts).load(object)
end

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2021-04-14 Update: Implementations of the loaders shown here are being maintained at https://github.com/jamesbrooks/graphql-batch-loaders