# Param-based count loader (for params and foreign keys).
#
# Example:
#   CountLoader.for(Category).load(category_id)
#
class CountLoader < GraphQL::Batch::Loader
  def initialize(model, param: :id, scope: nil)
    @model = model
    @param = param
    @scope = case scope
    when Symbol
      scope.to_proc
    when Hash
      ->(mod) { mod.where(scope) }
    when ActiveRecord::Relation
      ->(_) { scope }
    end
  end

  def perform(ids)
    scope = @scope ? @scope.call(@model) : @model
    scope.where(@param => ids).group(@param).count
      .each_pair { |record_id, count| fulfill(record_id.to_s, count) }
    ids.each { |id|
      id_ = id.to_s
      fulfill(id_, nil) unless fulfilled?(id_)
    }
  end
end