How to map JSON::Any to custom Object in Crystal language? - crystal-lang

Question

How to map parsed JSON as JSON::Any type to custom Object?
In my case, I am working on chat client. Chat API can respond to requests with following JSON:
{"ok" => true,
"result" =>
[{"update_id" => 71058322,
"message" =>
{"message_id" => 20,
"from" => "Benjamin",
"text" => "hey"}}]}
Somewhere inside my API client code I parse these JSON to perform some basic health checks and pass the result to response consumer. In the consumer, I iterate over result array and try to convert each update to appropriate object:
module Types
class Update
JSON.mapping({
update_id: {type: Int32},
message: {type: Message},
})
end
end
module Types
class Message
JSON.mapping({
message_id: Int32,
date: Int32,
text: String,
})
end
end
return unless response["ok"]
response["result"].each do |data|
update = Types::Update.from_json(data)
end
Unfortunately, last line results in compile error:
no overload matches 'JSON::Lexer.new' with type JSON::Any
Apparently, Object.from_json can accept only String JSONs, but not parsed JSON. In my case data is JSON::Any object.
Dirty fix Types::Update.from_json(data.to_json) works, but it looks ridiculous.
What is the proper way to map JSON object to custom type preserving all nested structure?

Answer 1

JSON.mapping doesn't work nicely together with JSON.parse. To solve your problem you can create another mapping Types::Result and parse a hole json using Object.from_json which is even much more convenient to work with:
module Types
class Message
JSON.mapping(
message_id: Int32,
text: String
)
end
class Update
JSON.mapping(
update_id: Int32,
message: Message
)
end
class Result
JSON.mapping(
success: { key: "ok", type: Bool },
updates: { key: "result", type: Array(Update) }
)
end
end
result = Types::Result.from_json string_json
result.success # => true
result.updates.first.message.text # => "hey"