Codec Customisation

Default encoding

Sealed trait members that are case classes

sealed trait TrafficLight
final case class Green(rgb: Int) extends TrafficLight 
final case class Red(rgb: Int) extends TrafficLight
final case class Box(trafficLightColour: TrafficLight)

The default encoding for Box(Green(42)) is:

Map(trafficLightColour -> Map(String(Green) -> Map(String(rgb) -> Number(42))))

Here an intermediate map is used to identify the member of TraficLight using the member class name ie Map(String(Green) -> Map(...))

Sealed trait members that are case objects

sealed trait TrafficLight
case object GREEN extends TrafficLight 
case object RED extends TrafficLight
final case class Box(trafficLightColour: TrafficLight)

The default encoding for Box(GREEN) is:

Map(trafficLightColour -> Map(String(GREEN) -> Null))

Here an intermediate map is used to identify the member of TraficLight ie Map(String(GREEN) -> Null) Note that the Null is used as in this case we do not care about the value.

Alternate encodings

Encodings can be customised through the use of the following annotations @discriminator, @enumOfCaseObjects and @id. These annotations are useful when working with a legacy DynamoDB database.

The alternate encodings do not introduce another map for the purposes of identification and this leads to a more compact encoding that may be more intuitive to work with.

The advantage of the default encoding is that it is more uniform and scalable.

Sealed trait members that are case classes

@discriminator("light_type")
sealed trait TrafficLight
final case class Green(rgb: Int) extends TrafficLight
@id("red_traffic_light")
final case class Red(rgb: Int) extends TrafficLight
final case class Amber(@id("red_green_blue") rgb: Int) extends TrafficLight
final case class Box(trafficLightColour: TrafficLight)

encoding for an instance of Box(Green(42)) would be:

Map(trafficLightColour -> Map(String(rgb) -> Number(42), String(light_type) -> String(Green)))

We can specify the field name used to identify the case class through the @discriminator annotation. The alternate encoding removes the intermediate map and inserts a new field with a name specified by discriminator annotation and a value that identifies the member which defaults to the class name.

This can be further customised using the @id annotation - encoding for an instance of Box(Red(42)) would be:

Map(trafficLightColour -> Map(String(rgb) -> Number(42), String(light_type) -> String(red_traffic_light)))

The encoding for case class field names can also be customised via @id - encoding for an instance of Box(Amber(42)) would be:

Map(trafficLightColour -> Map(String(red_green_blue) -> Number(42), String(light_type) -> String(Amber)))

Sealed trait members that are all case objects

@enumOfCaseObjects
sealed trait TrafficLight
case object GREEN extends TrafficLight 
@id("red_traffic_light")
case object RED extends TrafficLight
final case class Box(trafficLightColour: TrafficLight)

We can get a more compact and intuitive encoding of trait members that are case objects by using the @enumOfCaseObjects annotation which encodes to just a value that is the member name. Encoding for an instance of Box(GREEN) would be:

Map(trafficLightColour -> String(GREEN))

This can be further customised by using the @id annotation again - encoding for Box(RED) would be

Map(trafficLightColour -> String(red_traffic_light))

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