add concat()

Author: cherylEnkidu

Firestore/Swift/Source/ExpressionImplementation.swift

@@ -894,4 +894,9 @@ public extension Expression {
   func equivalent(_ other: Sendable) -> BooleanExpression {
     return BooleanExpression("equivalent", [self, Helper.sendableToExpr(other)])
   }
+
+  func concat(_ values: [Sendable]) -> FunctionExpression {
+    let exprs = [self] + values.map { Helper.sendableToExpr($0) }
+    return FunctionExpression("concat", exprs)
+  }
 }

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

@@ -1564,4 +1564,15 @@ public protocol Expression: Sendable {
   /// - Parameter other: The value to compare against.
   /// - Returns: A `BooleanExpr` that can be used in `where` clauses.
   func equivalent(_ other: Sendable) -> BooleanExpression
+
+  /// Creates an expression that concatenates multiple sequenceable types together.
+  ///
+  /// ```swift
+  /// // Concatenate the firstName and lastName with a space in between.
+  /// Field("firstName").concat([" ", Field("lastName")])
+  /// ```
+  ///
+  /// - Parameter values: The values to concatenate.
+  /// - Returns: A new `FunctionExpression` representing the concatenated result.
+  func concat(_ values: [Sendable]) -> FunctionExpression
 }

Firestore/Swift/Tests/Integration/PipelineTests.swift

@@ -1869,6 +1869,30 @@ class PipelineIntegrationTests: FSTIntegrationTestCase {
     TestHelper.compare(snapshot: snapshot, expected: expectedResults, enforceOrder: true)
   }
 
+  func testConcatWorks() async throws {
+    let collRef = collectionRef(withDocuments: [
+      "doc1": ["s": "a", "b": "b", "c": "c"],
+      "doc2": ["s": "x", "b": "y", "c": "z"],
+    ])
+    let db = collRef.firestore
+
+    let pipeline = db.pipeline()
+      .collection(collRef.path)
+      .select([
+        Field("s").concat([Field("b"), Field("c"), " "]).as("concatenated"),
+      ])
+      .sort([Field("concatenated").ascending()])
+
+    let snapshot = try await pipeline.execute()
+
+    let expectedResults: [[String: Sendable]] = [
+      ["concatenated": "abc "],
+      ["concatenated": "xyz "],
+    ]
+
+    TestHelper.compare(snapshot: snapshot, expected: expectedResults, enforceOrder: true)
+  }
+
   func testStartsWith() async throws {
     let collRef = collectionRef(withDocuments: bookDocs)
     let db = collRef.firestore