Firebase Query Double Nested

Question

Given the data structure below in firebase, i want to run a query to retrieve the blog 'efg'. I don't know the user id at this point.

{Users :
     "1234567": {
          name: 'Bob',
          blogs: {
               'abc':{..},
               'zyx':{..}
          }
     },
     "7654321": {
          name: 'Frank',
          blogs: {
               'efg':{..},
               'hij':{..}
          }
     }
}

Answer 1

The Firebase API only allows you to filter children one level deep (or with a known path) with its orderByChild and equalTo methods.

So without modifying/expanding your current data structure that just leaves the option to retrieve all data and filter it client-side:

var ref = firebase.database().ref('Users');
ref.once('value', function(snapshot) {
    snapshot.forEach(function(userSnapshot) {
        var blogs = userSnapshot.val().blogs;
        var daBlog = blogs['efg'];
    });
});

This is of course highly inefficient and won't scale when you have a non-trivial number of users/blogs.

So the common solution to that is to a so-called index to your tree that maps the key that you are looking for to the path where it resides:

{Blogs:
     "abc": "1234567",
     "zyx": "1234567",
     "efg": "7654321",
     "hij": "7654321"
}

Then you can quickly access the blog using:

var ref = firebase.database().ref();
ref.child('Blogs/efg').once('value', function(snapshot) {
    var user = snapshot.val();
    ref.child('Blogs/'+user+'/blogs').once('value, function(blogSnapshot) {
        var daBlog = blogSnapshot.val();
    });
});

You might also want to reconsider if you can restructure your data to better fit your use-case and Firebase's limitations. They have some good documentation on structuring your data, but the most important one for people new to NoSQL/hierarchical databases seems to be "avoid building nests".

Also see my answer on Firebase query if child of child contains a value for a good example. I'd also recommend reading about many-to-many relationships in Firebase, and this article on general NoSQL data modeling.