Google Datastore (NoSQL) backend for PHP CMS

I am working on a personal project where I am creating a small CMS using PHP as the developing language, but I want to be able to deploy it to the Google App Engine and I want to use its Google Datastore (NoSQL). The question I have is how to do relationship between kinds? **For example:** - Kind 1: call category - Kind 2: call tags - Kind 3: call post I want the posts to be able to have one category, but multiple tags. I am having trouble visualizing it. I am coming from SQL world so having the ID of the category and the IDs of the tags to them JOIN them will take care of the same issue. **Note:** if there is some tutorial or reading that you could share I will appreciate it.