Merge array values in configuration using withFallBack in typesafe config

Question

I have some data available as type safe configuration files in HOCON format.

There is a base-file like this :

"data":{
  "k1":{
    "values": ["v1", "v2"]
  },
  "k2":{
    "values": ["x1"]
  },
  "k3":{
    "values": ["z1"]
  }
}

There could be a file which can be used to make some changes, for example during test, like this:

"data":{
  "k1":{
    "values": ["v9"]
  }
}

I am trying to merge these two files using

fileConfig.withFallback(baseFileConfig)

The end result is:

"data":{
  "k1":{
    "values": ["v9"]  // desired ["v1","v2","v9"]
  },
  "k2":{
    "values": ["x1"]
  },
  "k3":{
    "values": ["z1"]
  }
}

i.e. the array values for "k1" from the fallBack configuration are ignored. Is there a way I can get the concatenated array from two files after the merge?

Answer 1

for do that you need add ref for values concatenation (values: ${data.k1.values} ["v9"]):

lazy val defaultConfig     = ConfigFactory.parseResources("a.conf")
lazy val additionalConfig = ConfigFactory.parseResources("b.conf" )
println(additionalConfig.withFallback(defaultConfig).resolve()) 
// Config(SimpleConfigObject({"data":{"k1":{"values":["v1","v2","v9"]},"k2":{"values":["x1"]},"k3":{"values":["z1"]}}}))

configs:

defaultConfig

data: {
  k1: {
    values: ["v1", "v2"]
  },
  k2: {
    "values": [
      "x1"
    ]
  },
  k3: {
    "values": [
      "z1"
    ]
  }
}

additionalConfig:

data: {
  k1: {
    values: ${data.k1.values} ["v9"]
  }
}