Add Firestore snippet for getCollections() (#988)

* Add snippet for Collections()

* Fix test failures

Author: samtstern

firestore/src/main/java/com/example/firestore/snippets/RetrieveDataSnippets.java

@@ -137,4 +137,21 @@ public List<DocumentSnapshot> getAllDocuments() throws Exception {
     // [END fs_get_all_docs]
     return documents;
   }
+
+  /**
+   * Return all subcollections of the cities/SF document.
+   *
+   * @return iterable of collection references.
+   */
+  public Iterable<CollectionReference> getCollections() throws Exception {
+    // [START fs_get_collections]
+    Iterable<CollectionReference> collections =
+        db.collection("cities").document("SF").getCollections();
+
+    for (CollectionReference collRef : collections) {
+      System.out.println("Found subcollection with id: " + collRef.getId());
+    }
+    // [END fs_get_collections]
+    return collections;
+  }
 }

firestore/src/test/java/com/example/firestore/snippets/RetrieveDataSnippetsIT.java

@@ -22,10 +22,13 @@
 import com.example.firestore.snippets.model.City;
 
 import com.google.api.core.ApiFuture;
+import com.google.cloud.firestore.CollectionReference;
 import com.google.cloud.firestore.DocumentSnapshot;
 import com.google.cloud.firestore.Firestore;
 import com.google.cloud.firestore.FirestoreOptions;
 import com.google.cloud.firestore.QuerySnapshot;
+import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
@@ -99,6 +102,25 @@ public void testRetrieveAllDocuments() throws Exception {
             && docIds.contains("BJ"));
   }
 
+  @Test
+  public void testGetSubcollections() throws Exception {
+    // Add a landmark subcollection
+    Map<String, String> data = new HashMap<>();
+    data.put("foo", "bar");
+    db.document("cities/SF/landmarks/example").set(data).get();
+
+    Iterable<CollectionReference> collections =
+        retrieveDataSnippets.getCollections();
+
+    List<CollectionReference> collectionList = new ArrayList<>();
+    for (CollectionReference collRef : collections) {
+      collectionList.add(collRef);
+    }
+
+    assertEquals(collectionList.size(), 1);
+    assertEquals(collectionList.get(0).getId(), "landmarks");
+  }
+
   private static void deleteAllDocuments() throws Exception {
     ApiFuture<QuerySnapshot> future = db.collection("cities").get();
     QuerySnapshot querySnapshot = future.get();