feat(firestore): Add ifNull and coalesce expressions

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

@@ -277,6 +277,100 @@ public static Expression ifAbsent(String ifFieldName, Object elseValue) {
     return ifAbsent(field(ifFieldName), toExprOrConstant(elseValue));
   }
 
+  /**
+   * Creates an expression that returns a default value if an expression evaluates to null.
+   *
+   * <p>Note: This function provides a fallback for both absent and explicit null values. In
+   * contrast, {@link ifAbsent} only triggers for missing fields.
+   *
+   * @param ifExpr The expression to check.
+   * @param elseExpression The default value.
+   * @return A new {@link Expression} representing the ifNull operation.
+   */
+  @BetaApi
+  public static Expression ifNull(Expression ifExpr, Expression elseExpression) {
+    return new FunctionExpression("if_null", ImmutableList.of(ifExpr, elseExpression));
+  }
+
+  /**
+   * Creates an expression that returns a default value if an expression evaluates to null.
+   *
+   * <p>Note: This function provides a fallback for both absent and explicit null values. In
+   * contrast, {@link ifAbsent} only triggers for missing fields.
+   *
+   * @param ifExpr The expression to check.
+   * @param elseValue The default value.
+   * @return A new {@link Expression} representing the ifNull operation.
+   */
+  @BetaApi
+  public static Expression ifNull(Expression ifExpr, Object elseValue) {
+    return ifNull(ifExpr, toExprOrConstant(elseValue));
+  }
+
+  /**
+   * Creates an expression that returns a default value if a field is null.
+   *
+   * <p>Note: This function provides a fallback for both absent and explicit null values. In
+   * contrast, {@link ifAbsent} only triggers for missing fields.
+   *
+   * @param ifFieldName The field to check.
+   * @param elseExpression The default value.
+   * @return A new {@link Expression} representing the ifNull operation.
+   */
+  @BetaApi
+  public static Expression ifNull(String ifFieldName, Expression elseExpression) {
+    return ifNull(field(ifFieldName), elseExpression);
+  }
+
+  /**
+   * Creates an expression that returns a default value if a field is null.
+   *
+   * <p>Note: This function provides a fallback for both absent and explicit null values. In
+   * contrast, {@link ifAbsent} only triggers for missing fields.
+   *
+   * @param ifFieldName The field to check.
+   * @param elseValue The default value.
+   * @return A new {@link Expression} representing the ifNull operation.
+   */
+  @BetaApi
+  public static Expression ifNull(String ifFieldName, Object elseValue) {
+    return ifNull(field(ifFieldName), toExprOrConstant(elseValue));
+  }
+
+  /**
+   * Returns the first non-null, non-absent argument, without evaluating the rest of the arguments.
+   * When all arguments are null or absent, returns the last argument.
+   *
+   * @param expression The first expression to check for null.
+   * @param replacement The fallback expression or value if the first one is null.
+   * @param others Optional additional expressions to check if previous ones are null.
+   * @return A new {@link Expression} representing the coalesce operation.
+   */
+  @BetaApi
+  public static Expression coalesce(Expression expression, Object replacement, Object... others) {
+    ImmutableList.Builder<Expression> args = ImmutableList.builder();
+    args.add(expression);
+    args.add(toExprOrConstant(replacement));
+    for (Object other : others) {
+      args.add(toExprOrConstant(other));
+    }
+    return new FunctionExpression("coalesce", args.build());
+  }
+
+  /**
+   * Returns the first non-null, non-absent argument, without evaluating the rest of the arguments.
+   * When all arguments are null or absent, returns the last argument.
+   *
+   * @param firstFieldName The name of the first field to check for null.
+   * @param replacement The fallback expression or value if the first one is null.
+   * @param others Optional additional expressions to check if previous ones are null.
+   * @return A new {@link Expression} representing the coalesce operation.
+   */
+  @BetaApi
+  public static Expression coalesce(String firstFieldName, Object replacement, Object... others) {
+    return coalesce(field(firstFieldName), replacement, others);
+  }
+
   /**
    * Creates an expression that joins the elements of an array into a string.
    *
@@ -4206,6 +4300,33 @@ public Expression ifAbsent(Object elseValue) {
     return Expression.ifAbsent(this, elseValue);
   }
 
+  /**
+   * Creates an expression that returns a default value if this expression evaluates null.
+   *
+   * <p>Note: This function provides a fallback for both absent and explicit null values. In
+   * contrast, {@link ifAbsent} only triggers for missing fields.
+   *
+   * @param elseValue The default value.
+   * @return A new {@link Expression} representing the ifNull operation.
+   */
+  @BetaApi
+  public Expression ifNull(Object elseValue) {
+    return Expression.ifNull(this, elseValue);
+  }
+
+  /**
+   * Returns the first non-null, non-absent argument, without evaluating the rest of the arguments.
+   * When all arguments are null or absent, returns the last argument.
+   *
+   * @param second The next expression or literal to evaluate.
+   * @param others Additional expressions or literals to evaluate.
+   * @return A new {@link Expression} representing the coalesce operation.
+   */
+  @BetaApi
+  public Expression coalesce(Object second, Object... others) {
+    return Expression.coalesce(this, second, others);
+  }
+
   /**
    * Creates an expression that joins the elements of this array expression into a string.
    *

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

@@ -2662,6 +2662,92 @@ public void testIfAbsent() throws Exception {
     assertThat(data(results)).containsExactly(map("res", "Frank Herbert"));
   }
 
+  @Test
+  public void testIfNull() throws Exception {
+    List<PipelineResult> results =
+        firestore
+            .pipeline()
+            .collection(collection.getPath())
+            .limit(1)
+            .replaceWith(Expression.map(map("title", "foo", "name", null)))
+            .select(
+                Expression.ifNull("title", "default title").as("staticMethod"),
+                field("title").ifNull("default title").as("instanceMethod"),
+                field("name").ifNull(field("title")).as("nameOrTitle"),
+                field("name").ifNull("default name").as("fieldIsNull"),
+                field("absent").ifNull("default name").as("fieldIsAbsent"))
+            .execute()
+            .get()
+            .getResults();
+
+    assertThat(data(results))
+        .containsExactly(
+            map(
+                "staticMethod", "foo",
+                "instanceMethod", "foo",
+                "nameOrTitle", "foo",
+                "fieldIsNull", "default name",
+                "fieldIsAbsent", "default name"));
+  }
+
+  @Test
+  public void testCoalesce() throws Exception {
+    assumeFalse(
+        "Coalesce is not supported against the emulator.",
+        isRunningAgainstFirestoreEmulator(firestore));
+
+    List<PipelineResult> results =
+        firestore
+            .pipeline()
+            .collection(collection.getPath())
+            .limit(1)
+            .replaceWith(
+                Expression.map(
+                    map(
+                        "numberValue",
+                        1L,
+                        "stringValue",
+                        "hello",
+                        "booleanValue",
+                        false,
+                        "nullValue",
+                        null,
+                        "nullValue2",
+                        null)))
+            .select(
+                Expression.coalesce(field("numberValue"), field("stringValue")).as("staticMethod"),
+                field("numberValue").coalesce(field("stringValue")).as("instanceMethod"),
+                Expression.coalesce(field("nullValue"), field("stringValue")).as("firstIsNull"),
+                Expression.coalesce(field("nullValue"), field("nullValue2"), field("booleanValue"))
+                    .as("lastIsNotNull"),
+                Expression.coalesce(field("nullValue"), field("nullValue2")).as("allFieldsNull"),
+                Expression.coalesce(field("nullValue"), field("nullValue2"), constant("default"))
+                    .as("allFieldsNullWithDefault"),
+                Expression.coalesce(field("absentField"), field("numberValue"), constant("default"))
+                    .as("withAbsentField"))
+            .execute()
+            .get()
+            .getResults();
+
+    assertThat(data(results))
+        .containsExactly(
+            map(
+                "staticMethod",
+                1L,
+                "instanceMethod",
+                1L,
+                "firstIsNull",
+                "hello",
+                "lastIsNotNull",
+                false,
+                "allFieldsNull",
+                null,
+                "allFieldsNullWithDefault",
+                "default",
+                "withAbsentField",
+                1L));
+  }
+
   @Test
   public void testJoin() throws Exception {
     // Test join with a constant delimiter