How to Unmarshal Nested JSON without Knowing its Structure?

Unmarshal Nested JSON without Knowing Structure

When working with a key-value store that contains JSON documents with varying schemas, it becomes challenging to unmarshal the JSON data into specific structs without knowing their structure beforehand. This article explores methods to address this issue and propose solutions.

Method 1: Unmarshal to Interface

To handle JSON documents with unknown structures, you can unmarshal them into an interface type, such as map[string]interface{}. This allows you to access the underlying data as a dictionary without needing to define a specific struct.

By unmarshalling the top-level foo namespace into a map[string]*json.RawMessage, you can retrieve the nested JSON data. However, you still need to determine the type of the data within the foo namespace to correctly unmarshal it into a struct.

Method 2: Regex Type Extraction

An alternative method is to extract the type string from the JSON data and use a regular expression to determine the struct type. Once the type is known, you can unmarshal the json.RawMessage into the appropriate struct.

Repeated Unmarshals

Whether repeated unmarshals are a concern depends on the performance requirements of your application. If speed is critical, it may be more efficient to unmarshal into a specific struct directly instead of using an intermediary interface. However, for small datasets or occasional use, the performance penalty may not be significant.

Update

Based on the update provided, two possible solutions are proposed:

a) Copy and Unmarshal

• Make a copy of the json.RawMessage.
• Unmarshal the copy into an interface.
• Once the type is known, unmarshal the original json.RawMessage into the appropriate struct.

b) Regex and Unmarshal

• Use a regular expression to extract the type string.
• Programmatically create a struct type based on the extracted type string.
• Unmarshal the json.RawMessage into the newly created struct.

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