module InflateBelongsTo
  # when you have an array of models, but want to include (or even map)
  # an associated resource, this method puts together a single query to
  # fetch the data, then appends it to the model, cutting down on total
  # database calls.
  # this does the exact same work as eager loading when you first query
  # for a set of models, but is useful for nested resources deeper than
  # one level, especially if you wish to get results conditionally
  #
  # models: the array of models whose foreign key relationship you wish
  #         to inflate
  # key: the instance method used to get the foreign record
  #      key is used to construct two methods:
  #        - `#{key}_id` for getting foreign record's id
  #        - `#{key}=` for assignment once we've fetched
  #
  # returns an array of all resources
  def inflate(models, key)
    key_id = :"#{key}_id"
    models = Array(models)

    resource_ids = models
      .select { |m| m.respond_to?(key_id) }
      .map { |m| m.send(key_id) }
      .uniq
      .compact

    resources = where(id: resource_ids)

    resource_map = resources.map { |r| [r.id, r] }.to_h

    models
      .select { |m| m.respond_to?(:"#{key}=") }
      .map { |m| m.send(:"#{key}=", resource_map[m.send(key_id)]) }

    resources
  end
end