Get Firebase items belonging to category

问题

In my app I will have items. Each item belongs to at least one category, but multiple categories are possible. I think the following is the best way to structure the data:

items
    item1
         title: apple
         categories:
             food: true
             fruit: true
    item2
         title: spinach
         categories:
             food: true
             veggie: true
    item3
         title: triceratops
         categories:
             dino: true

How do I query Firebase to return all items matching a category? For example: 'food' should return item1 and item2 and 'veggie' item2. I'm using Angular Fire and tried the following which gives me an error:

$firebaseArray ref.child('items').orderByChild("categories/food")

来源：https://stackoverflow.com/questions/32922271/get-firebase-items-belonging-to-category