Add support for the parent expression

Author: yvonnep165

google-cloud-firestore/src/main/java/com/google/cloud/firestore/pipeline/expressions/Expression.java

@@ -4364,6 +4364,39 @@ public static Expression collectionId(String pathFieldName) {
     return collectionId(field(pathFieldName));
   }
 
+  /**
+   * Creates an expression that returns the parent document of a document reference.
+   *
+   * @param documentPath An expression that evaluates to a document path.
+   * @return A new {@link Expression} representing the parent operation.
+   */
+  @BetaApi
+  public static Expression parent(Expression documentPath) {
+    return new FunctionExpression("parent", ImmutableList.of(documentPath));
+  }
+
+  /**
+   * Creates an expression that returns the parent document of a document reference.
+   *
+   * @param documentPath The string representation of the document path.
+   * @return A new {@link Expression} representing the parent operation.
+   */
+  @BetaApi
+  public static Expression parent(String documentPath) {
+    return parent(constant(documentPath));
+  }
+
+  /**
+   * Creates an expression that returns the parent document of a document reference.
+   *
+   * @param docRef The {@link DocumentReference}.
+   * @return A new {@link Expression} representing the parent operation.
+   */
+  @BetaApi
+  public static Expression parent(DocumentReference docRef) {
+    return parent(constant(docRef));
+  }
+
   // Type Checking Functions
   /**
    * Creates an expression that checks if a field exists.
@@ -7016,6 +7049,16 @@ public final Expression collectionId() {
     return collectionId(this);
   }
 
+  /**
+   * Creates an expression that returns the parent document of a document reference.
+   *
+   * @return A new {@link Expression} representing the parent operation.
+   */
+  @BetaApi
+  public final Expression parent() {
+    return parent(this);
+  }
+
   /**
    * Creates an expression that returns a string indicating the type of the value this expression
    * evaluates to.

google-cloud-firestore/src/test/java/com/google/cloud/firestore/it/ITPipelineTest.java

@@ -72,6 +72,7 @@
 import static com.google.cloud.firestore.pipeline.expressions.Expression.notEqual;
 import static com.google.cloud.firestore.pipeline.expressions.Expression.nullValue;
 import static com.google.cloud.firestore.pipeline.expressions.Expression.or;
+import static com.google.cloud.firestore.pipeline.expressions.Expression.parent;
 import static com.google.cloud.firestore.pipeline.expressions.Expression.pow;
 import static com.google.cloud.firestore.pipeline.expressions.Expression.rand;
 import static com.google.cloud.firestore.pipeline.expressions.Expression.regexMatch;
@@ -109,6 +110,7 @@
 import com.google.cloud.Timestamp;
 import com.google.cloud.firestore.Blob;
 import com.google.cloud.firestore.CollectionReference;
+import com.google.cloud.firestore.DocumentReference;
 import com.google.cloud.firestore.Firestore;
 import com.google.cloud.firestore.FirestoreOptions;
 import com.google.cloud.firestore.GeoPoint;
@@ -4292,4 +4294,28 @@ public void disallowDuplicateAliasesAcrossStages() {
             });
     assertThat(exception).hasMessageThat().contains("Duplicate alias or field name");
   }
+
+  @Test
+  public void testSupportsParent() throws Exception {
+    DocumentReference docRef = collection.document("book4").collection("reviews").document("review1");
+
+    Pipeline pipeline =
+        firestore
+            .pipeline()
+            .collection(collection.getPath())
+            .limit(1)
+            .select(
+                parent(docRef).as("parentRefStatic"),
+                constant(docRef).parent().as("parentRefInstance"))
+            .select(
+                field("parentRefStatic").documentId().as("parentIdStatic"),
+                field("parentRefInstance").documentId().as("parentIdInstance"));
+
+    List<PipelineResult> results = pipeline.execute().get().getResults();
+    assertThat(results).hasSize(1);
+    Map<String, Object> data = results.get(0).getData();
+
+    assertThat(data.get("parentIdStatic")).isEqualTo("book4");
+    assertThat(data.get("parentIdInstance")).isEqualTo("book4");
+  }
 }