Improve return value semantices for tm().load() (#576)

Since we rarely (if ever) want to check the result of a single element load,
make TransactionManager.load(VKey) return non-optional, non-nullable and just
throw a NoSuchElementException if the entity is not found.

Also add a maybeLoad() that does return an optional in case we ever want to do
this (exists() should suffice for an existence check).

core/src/main/java/google/registry/model/ofy/DatastoreTransactionManager.java

@@ -22,6 +22,7 @@ import com.googlecode.objectify.Key;
 import google.registry.persistence.VKey;
 import google.registry.persistence.transaction.TransactionManager;
 import java.util.Iterator;
+import java.util.NoSuchElementException;
 import java.util.Optional;
 import java.util.function.Supplier;
 import java.util.stream.StreamSupport;
@@ -136,10 +137,19 @@ public class DatastoreTransactionManager implements TransactionManager {
   // VKey instead of by ofy Key.  But ideally, there should be one set of TransactionManager
   // interface tests that are applied to both the datastore and SQL implementations.
   @Override
-  public <T> Optional<T> load(VKey<T> key) {
+  public <T> Optional<T> maybeLoad(VKey<T> key) {
     return Optional.of(getOfy().load().key(key.getOfyKey()).now());
   }
 
+  @Override
+  public <T> T load(VKey<T> key) {
+    T result = getOfy().load().key(key.getOfyKey()).now();
+    if (result == null) {
+      throw new NoSuchElementException(key.toString());
+    }
+    return result;
+  }
+
   @Override
   public <T> ImmutableList<T> load(Iterable<VKey<T>> keys) {
     Iterator<Key<T>> iter =

core/src/main/java/google/registry/persistence/transaction/JpaTransactionManagerImpl.java

@@ -232,12 +232,23 @@ public class JpaTransactionManagerImpl implements JpaTransactionManager {
   }
 
   @Override
-  public <T> Optional<T> load(VKey<T> key) {
+  public <T> Optional<T> maybeLoad(VKey<T> key) {
     checkArgumentNotNull(key, "key must be specified");
     assertInTransaction();
     return Optional.ofNullable(getEntityManager().find(key.getKind(), key.getSqlKey()));
   }
 
+  @Override
+  public <T> T load(VKey<T> key) {
+    checkArgumentNotNull(key, "key must be specified");
+    assertInTransaction();
+    T result = getEntityManager().find(key.getKind(), key.getSqlKey());
+    if (result == null) {
+      throw new NoSuchElementException(key.toString());
+    }
+    return result;
+  }
+
   @Override
   public <T> ImmutableList<T> load(Iterable<VKey<T>> keys) {
     checkArgumentNotNull(keys, "keys must be specified");

core/src/main/java/google/registry/persistence/transaction/TransactionManager.java

@@ -108,7 +108,10 @@ public interface TransactionManager {
   <T> boolean checkExists(VKey<T> key);
 
   /** Loads the entity by its id, returns empty if the entity doesn't exist. */
-  <T> Optional<T> load(VKey<T> key);
+  <T> Optional<T> maybeLoad(VKey<T> key);
+
+  /** Loads the entity by its id, throws NoSuchElementException if it doesn't exist. */
+  <T> T load(VKey<T> key);
 
   /**
    * Leads the set of entities by their key id.