How to Unmarshal JSON Arrays of Unknown Size in Go?

Unmarshalling JSON with Arrays of Unknown Size

When retrieving data via JSON, it's often encountered that the returned JSON may or may not have an array. For example, when requesting a list of smart meters, it might be received as a single element under "gwrcmd." However, when requesting electricity usage, it might be returned as an array of "gwrcmds."

This ambiguity presents a marshalling challenge as Go structs require explicit definitions for arrays and single elements. To address this, json.Unmarshal can be used to return either a single struct { } or an array []struct { } based on the JSON structure.

However, json.Unmarshal may encounter an error if the expected type conflicts with the JSON structure. To resolve this issue, a second struct can be created that duplicates the first with the exception of using a slice for "Gwrcmd." Alternatively, a more robust approach involves using json.RawMessage to capture the JSON value of unknown type and perform further unmarshalling based on its contents. This avoids the guesswork of determining the array type based solely on the first byte.

Examples of both approaches are demonstrated in the provided Go code snippets. Additionally, relevant resources are listed for further exploration.

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