add ifAbsent and error

Author: cherylEnkidu

Firestore/Swift/Source/ExpressionImplementation.swift

@@ -890,6 +890,10 @@ public extension Expression {
     return FunctionExpression("if_error", [self, Helper.sendableToExpr(catchValue)])
   }
 
+  func ifAbsent(_ defaultValue: Sendable) -> FunctionExpression {
+    return FunctionExpression("if_absent", [self, Helper.sendableToExpr(defaultValue)])
+  }
+
   // MARK: Sorting
 
   func ascending() -> Ordering {

Firestore/Swift/Source/SwiftAPI/Pipeline/Expressions/Expression.swift

@@ -1531,6 +1531,21 @@ public protocol Expression: Sendable {
   /// - Returns: A new "FunctionExpression" representing the "ifError" operation.
   func ifError(_ catchValue: Sendable) -> FunctionExpression
 
+  /// Creates an expression that returns the literal `defaultValue` if this expression is
+  /// absent (e.g., a field does not exist in a map).
+  /// Otherwise, returns the result of this expression.
+  ///
+  /// - Note: This API is in beta.
+  ///
+  /// ```swift
+  /// // If the "optionalField" is absent, return "default value".
+  /// Field("optionalField").ifAbsent("default value")
+  /// ```
+  ///
+  /// - Parameter defaultValue: The literal `Sendable` value to return if this expression is absent.
+  /// - Returns: A new "FunctionExpression" representing the "ifAbsent" operation.
+  func ifAbsent(_ defaultValue: Sendable) -> FunctionExpression
+
   // MARK: Sorting
 
   /// Creates an `Ordering` object that sorts documents in ascending order based on this expression.

Firestore/Swift/Source/SwiftAPI/Pipeline/Expressions/FunctionExpressions/ErrorExpression.swift

@@ -0,0 +1,28 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import Foundation
+
+/// An expression that produces an error with a custom error message.
+/// This is primarily used for debugging purposes.
+///
+/// Example:
+/// ```swift
+/// ErrorExpression("This is a custom error message").as("errorResult")
+/// ```
+public class ErrorExpression: FunctionExpression, @unchecked Sendable {
+  public init(_ errorMessage: String) {
+    super.init("error", [Constant(errorMessage)])
+  }
+}

Firestore/Swift/Tests/Integration/PipelineTests.swift

@@ -1677,6 +1677,30 @@ class PipelineIntegrationTests: FSTIntegrationTestCase {
     TestHelper.compare(snapshot: snapshot, expected: expectedResults, enforceOrder: true)
   }
 
+  func testIfAbsentWorks() async throws {
+    let collRef = collectionRef(withDocuments: [
+      "doc1": ["value": 1],
+      "doc2": ["value2": 2],
+    ])
+    let db = collRef.firestore
+
+    let pipeline = db.pipeline()
+      .collection(collRef.path)
+      .select([
+        Field("value").ifAbsent(100).as("value"),
+      ])
+      .sort([Field(FieldPath.documentID()).ascending()])
+
+    let snapshot = try await pipeline.execute()
+
+    let expectedResults: [[String: Sendable]] = [
+      ["value": 100],
+      ["value": 1],
+    ]
+
+    TestHelper.compare(snapshot: snapshot, expected: expectedResults, enforceOrder: true)
+  }
+
 //  func testEquivalentWorks() async throws {
 //    let collRef = collectionRef(withDocuments: [
 //      "doc1": ["value": 1, "value2": 1],
@@ -3172,6 +3196,24 @@ class PipelineIntegrationTests: FSTIntegrationTestCase {
     XCTAssertEqual(snapshot.results.count, 1)
   }
 
+  func testErrorExpressionWorks() async throws {
+    let collRef = collectionRef(withDocuments: ["doc1": ["foo": 1]])
+    let db = collRef.firestore
+    
+    let pipeline = db.pipeline()
+      .collection(collRef.path)
+      .select([
+        ErrorExpression("This is a test error").as("error"),
+      ])
+    
+    do {
+      let _ = try await pipeline.execute()
+      XCTFail("The pipeline should have thrown an error, but it did not.")
+    } catch {
+      XCTAssert(true, "Successfully caught expected error from ErrorExpression.")
+    }
+  }
+
   func testSupportsByteLength() async throws {
     let db = firestore()
     let randomCol = collectionRef()