Adding example with nested map

google-cloud-firestore/src/main/java/com/google/cloud/firestore/BasePath.java

@@ -76,4 +76,13 @@ B append(BasePath<B> path) {
   abstract String[] splitChildPath(String path);
 
   abstract B createPathWithSegments(ImmutableList<String> segments);
+
+  boolean isPrefixOf(BasePath<B> field) {
+    if (field.getSegments().size() < this.getSegments().size()) {
+      return false;
+    }
+
+    return this.equals(
+        createPathWithSegments(field.getSegments().subList(0, this.getSegments().size())));
+  }
 }

google-cloud-firestore/src/main/java/com/google/cloud/firestore/UpdateBuilder.java

@@ -57,10 +57,19 @@ private static Map<String, Object> expandObject(Map<FieldPath, Object> data) {
 
     SortedSet<FieldPath> sortedFields = new TreeSet<>(data.keySet());
 
+    FieldPath lastField = null;
+
     for (FieldPath field : sortedFields) {
       List<String> segments = field.getSegments();
       Object value = data.get(field);
 
+      if (lastField != null) {
+        if (lastField.isPrefixOf(field)) {
+          throw new IllegalArgumentException(
+              String.format("Detected ambiguous definition for field '%s'.", lastField));
+        }
+      }
+
       Map<String, Object> currentMap = result;
 
       for (int i = 0; i < segments.size(); ++i) {
@@ -71,17 +80,11 @@ private static Map<String, Object> expandObject(Map<FieldPath, Object> data) {
             currentMap.put(segments.get(i), new HashMap<>());
           }
 
-          Object nestedMap = currentMap.get(segments.get(i));
-
-          if (!(nestedMap instanceof Map)) {
-            throw new IllegalArgumentException(
-                String.format(
-                    "Detected ambiguous definition for field '%s'.",
-                    FieldPath.of(segments.subList(0, i + 1).toArray(new String[] {}))));
-          }
-          currentMap = (Map<String, Object>) nestedMap;
+          currentMap = (Map<String, Object>) currentMap.get(segments.get(i));
         }
       }
+
+      lastField = field;
     }
 
     return result;

google-cloud-firestore/src/test/java/com/google/cloud/firestore/DocumentReferenceTest.java

@@ -601,6 +601,24 @@ public void updateConflictingFields() throws Exception {
     } catch (IllegalArgumentException e) {
       assertEquals(e.getMessage(), "Detected ambiguous definition for field 'a.b'.");
     }
+
+    try {
+      documentReference
+          .update("a.b", SINGLE_FIELD_MAP, "a", SINGLE_FIELD_MAP)
+          .get();
+      fail();
+    } catch (IllegalArgumentException e) {
+      assertEquals(e.getMessage(), "Detected ambiguous definition for field 'a'.");
+    }
+
+    try {
+      documentReference
+          .update("a.b", SINGLE_FIELD_MAP, "a.b.c", SINGLE_FIELD_MAP)
+          .get();
+      fail();
+    } catch (IllegalArgumentException e) {
+      assertEquals(e.getMessage(), "Detected ambiguous definition for field 'a.b'.");
+    }
   }
 
   @Test