module Mongoid::Criteria::Queryable::Aggregable

def aggregating?
  !!@aggregating
end

def aggregation(operation)
  return self unless operation
  clone.tap do |query|
    unless aggregating?
      query.pipeline.concat(query.selector.to_pipeline)
      query.pipeline.concat(query.options.to_pipeline)
      query.aggregating = true
    end
    yield(query.pipeline)
  end
end

def group(operation)
  aggregation(operation) do |pipeline|
    pipeline.group(operation)
  end
end

def project(operation = nil)
  aggregation(operation) do |pipeline|
    pipeline.project(operation)
  end
end

def unwind(field)
  aggregation(field) do |pipeline|
    pipeline.unwind(field)
  end
end