How to Implement Dynamic Properties in Go\'s Google App Engine Datastore?

Achieving Dynamic Properties in Go on Google App Engine Datastore

In Python's App Engine, the Expando Model offers the convenience of assigning dynamic properties to entities without upfront declaration. How can we replicate this functionality in Go?

Implementing Dynamic Properties

The key to achieving dynamic properties in Go is the PropertyLoadSaver interface. It enables entities to construct properties dynamically before they are saved.

Fortunately, the AppEngine platform provides the PropertyList type, which implements the PropertyLoadSaver interface. Using PropertyList, you can effortlessly add dynamic properties by simply adding them to the list.

Saving an Entity with Dynamic Properties

Let's delve into an example:

import (
    "context"
    "time"

    datastore "google.golang.org/appengine/datastore"
)

func main() {
    ctx := context.Background()

    props := datastore.PropertyList{
        datastore.Property{Name: "time", Value: time.Now()},
        datastore.Property{Name: "email", Value: "example@domain.com"},
    }

    k := datastore.NewIncompleteKey(ctx, "DynEntity", nil)
    key, err := datastore.Put(ctx, k, &props)
    if err != nil {
        // Handle the error
    }
    // ...
}

This code creates an entity named "DynEntity" with two dynamic properties: "time" and "email."

Using a Custom Type for Dynamic Entities

If you need more control over dynamic entities, you can implement the PropertyLoadSaver interface on a custom type. For instance, let's define a custom DynEnt type that wraps a map:

type DynEnt map[string]interface{}

func (d *DynEnt) Load(props []datastore.Property) error {
    // ... Your implementation
}

func (d *DynEnt) Save() (props []datastore.Property, err error) {
    // ... Your implementation
}

Now, you can use DynEnt as follows:

import (
    "context"
    "time"

    datastore "google.golang.org/appengine/datastore"
)

func main() {
    ctx := context.Background()

    d := DynEnt{"email": "example@domain.com", "time": time.Now()}

    k := datastore.NewIncompleteKey(ctx, "DynEntity", nil)
    key, err := datastore.Put(ctx, k, &d)
    if err != nil {
        // Handle the error
    }
    // ...
}

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