如何使用Go从Google App Engine中的模型中删除字段？

Question

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

Answer 1

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.