keyDecodingStrategy and keys with custom raw values

Using Swift
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2

Funny that this came up pretty much right after I posted Handling edge cases in JSONEncoder/JSONDecoder key conversion strategies (SR-6629). This is a good data point for discussing the topic, I think.

What you're seeing here is a combination of two things:

  1. The raw value of the key is affected by the key strategy. Anything consuming a CodingKey can only operate on its stringValue/intValue; there's no way for it to know that these values are being provided directly vs. being synthesized. As far as JSONEncoder/JSONDecoder is concerned, there's no difference between case last_name and case surname = "last_name", because there's no way for it to discern the difference
  2. The behavior on decode with the key decoding strategy is described in the linked thread — when you decode(String.self, forKey: .surname), the key in the JSON ("last_name") is converted to camelCase based on the .keyDecodingStrategy, and the string value of .surname ("last_name") is used to look up the value. Because the JSON payload has already been converted to camelCase "last_name" isn't found. Instead, you have to provide the camelCase name

(1) here happens by design (this is the idea behind the key strategy), but we're looking to improve on (2). Please take a look at the linked thread if you get a chance; I'd value your feedback there.