title: Firebase push uniqueness scope
body: <p>The docs say Firebase push generates a unique key, I want to know if this key is unique across my whole database (or even a GuId), or just in the node where it was pushed on  </p>
  
  <p>I am structuring my database like this ( I flattened it down, according to their guide ):</p>
  
  <pre><code>users {
      -KIH-uFo_2jW16Ue8JNH { //User-Key
         username : "Johnny Dummy"
         gender : "male"
      }
  }
  items {
      -KIH-uFo_2jW16Ue8JNH { //User-Key
          -KIH6iaw5uAS856i6-u9 : { //Item-Key
              itemname : "Item Dummy"
              count : 3
          }
      }
  }
  </code></pre>
  
  <p>So the uniqueness scope of Item-Key is important to me when i fetch all Items of all users</p>