Possibly incorrect example on the datastore preview in README.md

The following example seems incorrect, as datastore doesn't support partial update:

dataset.save({
  key: blogPostKey,
  data: blogPostData
}, function(err) {
  // `blogPostKey` has been updated with an ID so you can do more operations
  // with it, such as an update:
  dataset.save({
    key: blogPostKey,
    data: {
      isDraft: false
    }
  }, function(err) {
    if (!err) {
      // The blog post is now published!
    }
  });
});

It looks like the second update will overwrite the blog post with a single property entity {isDraft: false}

Datastore does support partial updates in the form of an "upsert mutation" -- https://cloud.google.com/datastore/docs/concepts/entities (best link I could find to explain it). Our save method tries to pick the best one that applies: http://googlecloudplatform.github.io/gcloud-node/#/docs/v0.19.0/datastore/transaction?method=save

An upsert mutation is not a partial update (an upsert is akin to a force write, it will insert the entity or update it if an entity already exists for the provided key).

The example I quoted above is indeed incorrect, the resulting BlogPost entity contains only {isDraft: false}.

Here is a (failing) test to illustrate further:

var key = dataset.key('Test');
dataset.save({
  key: key,
  data: {a: 42, bar: 'foo'}
}, function(err) {
  dataset.save({
    key: key,
    data: {bar: 'baz'}
  }, function(err) {
    dataset.get(key, function(err, entity) {
      assert.equal(entity, {a: 42, bar: 'baz'}); // fails, entity is {bar: 'baz'}
    });
  });
});

Here is an updated getting started example that doesn't have this issue:

dataset.save({
  key: blogPostKey,
  data: blogPostData
}, function(err) {
  // `blogPostKey` has been updated with an ID so you can do more operations
  // with it, such as an update:
  blogPostData.isDraft = false;

  dataset.save({
    key: blogPostKey,
    data: blogPostData
  }, function(err) {
    if (!err) {
      // The blog post is now published!
    }
  });
});

Good to know! Thanks for not letting me go on misunderstanding an upsert. Would you be able to send your example in a PR? Either way, thanks for catching this.