Infer database URL from Project ID (firebase#1898)

Author: schmidt-sebastian

firebase-database/CHANGELOG.md

@@ -1,5 +1,6 @@
 # Unreleased
-
+- [changed] The SDK can now infer a default database URL if none is provided in
+ the config.
 - [changed] Added internal HTTP header to the WebChannel connection.
 - [feature] Realtime Database now supports connecting to a local emulator via
  `FirebaseDatabase#useEmulator()`

firebase-database/src/androidTest/java/com/google/firebase/database/FirebaseDatabaseTest.java

@@ -119,6 +119,21 @@ public void getInstanceForAppWithHttpsUrl() {
  assertEquals("https://tests.fblocal.com:9000", db.getReference().toString());
  }
 
+ @Test
+ public void canInferDatabaseUrlFromProjectId() {
+ FirebaseApp app =
+ FirebaseApp.initializeApp(
+ InstrumentationRegistry.getInstrumentation().getTargetContext(),
+ new FirebaseOptions.Builder()
+ .setApplicationId("appid")
+ .setApiKey("apikey")
+ .setProjectId("abc123")
+ .build(),
+ "canInferDatabaseUrlFromProjectId");
+ FirebaseDatabase db = FirebaseDatabase.getInstance(app);
+ assertEquals("https://abc123-default-rtdb.firebaseio.com", db.getReference().toString());
+ }
+
  @Test
  public void getDifferentInstanceForAppWithUrl() {
  FirebaseApp app =

firebase-database/src/main/java/com/google/firebase/database/FirebaseDatabase.java

@@ -83,7 +83,16 @@ public static FirebaseDatabase getInstance(@NonNull String url) {
  */
  @NonNull
  public static FirebaseDatabase getInstance(@NonNull FirebaseApp app) {
- return getInstance(app, app.getOptions().getDatabaseUrl());
+ String databaseUrl = app.getOptions().getDatabaseUrl();
+ if (databaseUrl == null) {
+ if (app.getOptions().getProjectId() == null) {
+ throw new DatabaseException(
+ "Failed to get FirebaseDatabase instance: Can't determine Firebase Database URL. "
+ + "Be sure to include a Project ID in your configuration.");
+ }
+ databaseUrl = "https://" + app.getOptions().getProjectId() + "-default-rtdb.firebaseio.com";
+ }
+ return getInstance(app, databaseUrl);
  }
 
  /**