Custom Keys for Protobuf Maps

[AR]

One of the not-so-great things about proto3 is that maps have to have primitive types (string, int etc)

We can convert keys to custom types using the (scalapb.field).key_type field, but this requires a mapping from the business object to a simpler protobuf supported key. 

What is the practice for defining protobuf messages with typemappers such that the generated code exposes fields of type scala.immutable.Map[ComplexKey, ComplexValue] ?

The only way I see to do that is to have:

option (scalapb.options) = {
  import: "my.namespace.TypeMappers._"
};

message KeyVal {

   Key key = 1;

   Val  val  = 2;
}

message KeyVals {

  repeated KeyVal key_vals = 1;

}

message MyMsg {

  KeyVals data = 1 [(scalapb.field).type = "scala.collection.immutable.Map[Key, Val]"]

}

Is there a way or a practice/pattern to define generic typemappers?
 
We end up creating 2 protobuf classes and one TypeMapper for every map this way. 

[Nadav Samet]

AFAIK, there's currently no way to have map with custom key types unless the custom key type maps to a key type that is allowed for proto3 maps. Looks like we can have a feature that allows the generated code to allow for all types by enabling some special option on repeated fields. Feel free to file a feature request with a proposal.

Best,

Nadav

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-Nadav

[Aditya Rao]

Sounds interesting ...

So basically the Proto message would become

message MyMsg {

  repeated KeyVal myMap= 1; (scalapb.mapfield).keytype=Key, (scalapb.mapfield).valuetype=Value

}

And that automatically creates a Scala case class with a single field 

case class MyMsg(myMap: scala.immutable.Map[Key, Value] = Map.Empty)

?

[Nadav Samet]

As a user you'd have to define a message for KeyVal, because otherwise protoc won't be able to compile this. That makes scalapb.mapfield.keytype and valuetype unnecessary.

So I suggest something like:

message KeyValue {

   option (scalapb.message).map_entry = true;

   Key key = 1;

   Val  value  = 2;
}

message MyMsg {

  repeated KeyVal kv_map = 1; // generates kv_map: scala.collection.immutable.Map[Key, Val]

 

  // but the user can specify custom map types similar to custom collection types:

  repeated KeyVal kv_mutable_map = 2; [(scalapb.field).collection_type="collection.mutable.HashMap"]

}

--

-Nadav

[Aditya Rao]

This sounds great, how do I file a feature request, and what would be the rough eta?

[Nadav Samet]

You can file a feature request https://github.com/scalapb/ScalaPB/issues

ETA can be reduced if you are able to work on it :), otherwise I hope I can get to it at some point in the next few weeks.

-Nadav

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-Nadav