CODEWITHSUNDEEP
google-app-enginegogoogle-cloud-datastore
santiaago
santiaago

Reputation: 612

How do I remove a field from a model in Google App Engine with Go?

What is the Go equivalent of Python delattr or Java: Entity.removeProperty?

I am trying to remove a property from the datastore as described here: removing deleted properties from the datastore

Upvotes: 2

Views: 784

Answers (1)

icza
icza

Reputation: 417512

In order to remove a property from a saved entity, you have to load it first, and save it again with the same key but without the property you want to remove. If you want to remove a property from all saved entities (of a kind), you have to load and save each one-by-one. (Of course you may use other means like Query's and datastore.PutMulti() to query and save multiple entities.)

You can remove a property from a saved entity in basically 2 ways:

Using 2 structs:

You can use 2 structs: your old model and the new model (without the property you want to remove):

type Old struct {
    Name     string `datastore:"name"`
    Removeme string `datastore:"removeme"`
}

type New struct {
    Name     string `datastore:"name"`
}

And load the entity and re-save it (with the same key):

c := appengine.NewContext(r)
// Constructing the key, for example:
k := datastore.NewKey(c, "Entity", "stringID", 0, nil)

e := new(Old)
if err = datastore.Get(c, key, e); err != nil {
    // Datastore error.
    return
}

e2 := New{e.Name}
if _, err = datastore.Put(c, k, &e2); err != nil {
    // Datastore error
}

Using PropertyList

Or you can use the datastore.PropertyList to load any entity into it.

It's basically just a slice of Property's:

type PropertyList []Property

Remove the property (or properties) you want to delete from this slice, and re-save it with the same key.

Basically the steps are the same: load an entity with a key, remove unwanted properties, and resave it (with the same key).

Remove an element from a slice:

To delete the element from slice a at index i:

a = append(a[:i], a[i+1:]...)
// or
a = a[:i+copy(a[i:], a[i+1:])]

So basically it looks something like this:

c := appengine.NewContext(r)
// Constructing the key, for example:
k := datastore.NewKey(c, "Entity", "stringID", 0, nil)

e := datastore.PropertyList{}
if err = datastore.Get(c, key, &e); err != nil {
    // Datastore error.
    return
}

// Loop over the properties to find the one we want to remove:
for i, v := range e {
    if v.Name == "removeme" {
        // Found it!
        e = append(e[:i], e[i+1:]...)
        break
    }
}

if _, err = datastore.Put(c, k, &e); err != nil {
    // Datastore error
}

Note: Be careful when removing multiple elements from a slice using for range. Result might be unexpected as when you remove an element, all subsequent elements are shifted and you may skip an element. For details see this answer.

Upvotes: 4

Related Questions