title: Flutter firestore fetch documents with a condition on a map field
body: <p>So I have a document in this form:</p>
  
  <pre><code>Field:
      NestedField: Value
  </code></pre>
  
  <p>How can I use the <code>where()</code> method in Flutter to fetch all documents that satisfy a condition on <code>NestedField</code>? I.e:</p>
  
  <pre><code>Firestore.collection("forms").where("Field.NestedField",isEqualsTo: "Op1").getDocuments(). // This returns null
  </code></pre>