How to Implement Dynamic Properties in the Go App Engine Datastore?

Dynamic Properties in the Go App Engine Datastore

Unlike Python's App Engine, Go does not offer an out-of-the-box mechanism for creating entities with dynamic properties, where attributes can be assigned at runtime. However, using the PropertyLoadSaver interface, it is possible to implement similar functionality.

Using PropertyList for Dynamic Properties

Go's App Engine platform provides the PropertyList type, which implements the PropertyLoadSaver interface and allows you to construct properties dynamically at save time. Consider the following example:

package main

import (
    "context"
    "time"

    "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@example.com"},
    }

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

    // ...
}

In this example, we create an entity named "DynEntity" with two dynamic properties, "time" and "email."

Creating a Custom Type for Dynamic Properties

In some cases, it may be preferable to implement the PropertyLoadSaver interface on a custom type rather than using PropertyList. For instance, you could implement it on a map type:

package main

import (
    "context"

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

type DynEnt map[string]interface{}

func (d *DynEnt) Load(props []datastore.Property) error {
    for _, p := range props {
        (*d)[p.Name] = p.Value
    }
    return nil
}

func (d *DynEnt) Save() ([]datastore.Property, error) {
    var props []datastore.Property
    for k, v := range *d {
        props = append(props, datastore.Property{Name: k, Value: v})
    }
    return props, nil
}

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

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

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

    // ...
}

With this custom type, you can use dynamic properties like any other map in Go, but they can be stored and retrieved from the datastore:

d["active"] = true
datastore.Put(ctx, key, &d)

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