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Construct custom json using Restkits RKObjectMapping
{"version":"1.0.0",
  "datastreams" : [
    {
      "current_value" : "100",
      "id" : "1"
    }
  ]
}

Datastream *datastream = [[ZCCDatastream alloc] init];
datastream.version = @"1.0.0";
datastream.currentValue = @"100";
datastream.identifier = @"1";
[[RKObjectManager sharedManager] postObject:datastream delegate:self];

[[RKObjectManager sharedManager] postObject: datastream usingBlock: ^(RKObjectLoader *loader) {
    RKObjectMapping* mapping = [RKObjectMapping mappingForClass: [Datastream class]];
    [mapping mapAttributes: @"version", nil];

    //etc.

    loader.delegate = self;
    loader.serializationMapping = mapping inverseMapping];
}];