How to Handle Nested Struct Errors When Loading Datastore Entities from Python to Go?

Loading Datastore Entities from Python to Go: Understanding Nested Struct Errors

In Google AppEngine, an attempt to read datastore entities defined in Python from a Go project may result in the error:

datastore: flattening nested structs leads to a slice of slices: field "Messages"

Root Cause

The error stems from incompatible data structures between the Python and Go programming languages. In the provided Python models, ModelA contains a repeated property messages of type ModelB. In Go, both ModelA and ModelB are defined with slice fields, leading to a nested slice structure.

Solution

The Go datastore package is restricted in supporting nested slices. To resolve this issue, one must adjust the data structures in either Python or Go:

Python Model Adjustment

• Change messages in ModelA to be a single ModelB type instead of a slice.

Go Model Adjustment

• Remove the slice field from ModelA (i.e., make messages simply ModelB).
• Implement custom decoding logic in the Load method of ModelA to handle the nested structure (as demonstrated in the provided Go code).

Considerations

• Choosing to resolve the issue in Python requires rewriting existing data.
• Implementing custom decoding in Go may be complex and error-prone.

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