Limit firebase query with equal-to using Polymer

Question

I'm using Firebase with Polymer 2.0. My Firebase database contains users and groups. A user can be part of more than one group. A user can have different roles in different groups.

I want to list all groups where user is a member but I cant find a way to limit the firebase query to groups where user is a member. It's listing everything or nothing.

Database structure looks like this:

 {
  "groups" : {
    "$groupId" : {
      "members" : {
        "$userId" : "true",
      },
      "name" : "Group Name A"
    },
  },
  "users" : {
    "$userId" : {
      "groups" : {
        "$groupId" : "true",
      },
      "settings" : {
        "name" : "Linus"
      }
    }
  }
}

Query looks like this:

  <firebase-auth user="{{user}}"></firebase-auth>
  <firebase-query
    id="query"
    path="/groups"
    order-by-child="members/{{user.uid}}"
    equal-to="{{user.uid}}"
    data="{{accounts}}">
  </firebase-query>

Expected result in this case would be to only display groups where $userId is true.