title: FireBase Firestore retrieve multiple documents by list of ID's
body: <p>I've got two collections for example <code>Cities</code> and <code>Houses</code>. Houses are not a nested collection of Cities, but a house contains a reference to a City. </p>
  
  <p>Wat is the most efficient way to get city information for each house in a list?</p>
  
  <p>For Example </p>
  
  <pre><code>class City {
    var name: String = ""
    var property1: String = ""
    var property2: String = ""
  }
  
  class House {
    var cityId: String = ""
    var propertyA: String = ""
    var propertyB: String = ""
  }
  </code></pre>
  
  <p>I've got a list of houses, how do I get for each house in this list the name of the city? Is this possible by query, or should I just get all the cities (or each one seperate) and map them in code.</p>
  
  <p>The cities and houses are just an example, but simular to my real situation.</p>