title: Firebase Firestore - Access document within another document of unknown id
body: <p>i don't know how to go about accessing several documents inside a document of unknown name. Here is my Firebase Firestore structure:</p>
  <ul>
  <li>BUSINESS
  <ul>
  <li>unknownbusinessid1
  <ul>
  <li>PROMOTIONS
  <ul>
  <li>unknownpromotionid1</li>
  <li>unknownpromotionid2
  (...)</li>
  </ul>
  </li>
  </ul>
  </li>
  <li>unknownbusinessid2
  (...)</li>
  <li>unknownbusinessid3
  (...)</li>
  </ul>
  </li>
  </ul>
  <p>what I would like to achieve is to retrieve all promotion documents (unknownpromotionid1, unknownpromotionid2, etc.) without knowing the business ids (unknownbusinessid1, unknownbusinessid2, unknownbusinessid3, etc.)</p>
  <p>it should be something like this:</p>
  <pre><code>const getItem = db.doc(`BUSINESS/$whatever`).collection(&quot;PROMOTIONS&quot;)
      .get().then((snapshot) =&gt; {
  
          snapshot.docs.map(doc =&gt; {
              console.log(&quot;this is your promotion&quot;, doc)
          })
  
      return promotionsArray
  })
  </code></pre>
  <p>how can this be accomplished?
  thank you!</p>