Go Datastore Error: Nested Structs Slices of Slices
In an effort to leverage Go for enhanced performance, you have encountered an error while attempting to retrieve entities from the AppEngine datastore defined in Python. The error message "datastore: flattening nested structs leads to a slice of slices: field "Messages"" indicates a structural mismatch between your Go and Python project models.
Go Model Definitions and Datastore Compatibility
The Go datastore package has certain limitations regarding the structure of your data models. It does not support nested slices within slices, such as the "Messages" field in your ModelA definition. This means that although you can have a slice of ModelB entities in ModelA, ModelB itself cannot have any slices within its own fields.
Alternative Options to Resolve the Error
To resolve this error, you have several options:
Example: PropertyLoaderSaver for Custom Deserialization
If you choose the custom deserializer approach, you can define a PropertyLoaderSaver interface implementation for ModelA to handle the deserialization of the "Messages" field. Here's an example:
<code class="go">import ( "appengine_internal/datastore" "code.google.com/p/goprotobuf/proto" pb "appengine_internal/datastore" ) type ModelA struct { DateJoin time.Time `datastore:"date_join,"` Name string `datastore:"name,"` OwnerSalutation string `datastore:"owner_salutation,noindex"` OwnerEmailAddress string `datastore:"owner_email_address,"` LogoURL string `datastore:"logo_url,noindex"` Messages []ModelB `datastore:"-"` } // Load implements the PropertyLoaderSaver interface. func (seller *ModelA) Load(c <-chan datastore.Property) error { f := make(chan datastore.Property, 100) for p := range c { if p.Name == "bm" { var val pb.EntityProto err := proto.Unmarshal([]byte(p.Value.(string)), &val) if err != nil { return err } // TODO: Store the result as a new ModelB instance. } else { f <- p } } close(f) return datastore.LoadStruct(seller, f) }</code>
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