app/serializers/sensor_reading_serializer.rb
class SensorReadingSerializer < ApplicationSerializer
attributes :mode, :pin, :value, :x, :y, :z, :read_at
# This is for legacy support reasons.
# Very old sensor_readings will have a
# read_at value of `nil`, so we pre-populate it
# to `created_at` for the convenience of API users.
def read_at
object.read_at || object.created_at
end
end