title: How to fix error: A project ID is required for this service but could not be determined
body: <p>I'm trying to insert data to Google Datastore from AppEngine and I'm getting an error:</p>
  
  <pre><code>java.lang.IllegalArgumentException: A project ID is required for this service but could not be determined from the builder or the environment.  Please set a project ID using the builder.
      at com.google.common.base.Preconditions.checkArgument(Preconditions.java:92)
      at com.google.cloud.ServiceOptions.&lt;init&gt;(ServiceOptions.java:324)
      at com.google.cloud.datastore.DatastoreOptions.&lt;init&gt;(DatastoreOptions.java:85)
      at com.google.cloud.datastore.DatastoreOptions.&lt;init&gt;(DatastoreOptions.java:32)
      at com.google.cloud.datastore.DatastoreOptions$Builder.build(DatastoreOptions.java:75)
      at com.google.cloud.datastore.DatastoreOptions.defaultInstance(DatastoreOptions.java:123)
  </code></pre>
  
  <p>Here's my code:</p>
  
  <pre><code>Datastore datastore = DatastoreOptions.defaultInstance().service();     
               KeyFactory keyFactory = datastore.newKeyFactory().kind("keyKind"); 
               Key key = keyFactory.newKey("keyName"); 
               Entity entity = Entity.builder(key) 
                   .set("name", "John Doe") 
                   .set("age", 30) 
                   .set("access_time", DateTime.now()) 
                   .build(); 
               datastore.put(entity); 
  </code></pre>
  
  <p>How do I fix this error?</p>