add collectionId()

Author: cherylEnkidu

Firestore/Swift/Source/ExpressionImplementation.swift

@@ -587,6 +587,10 @@ public extension Expression {
     return FunctionExpression("document_id", [self])
   }
 
+  func collectionId() -> FunctionExpression {
+    return FunctionExpression("collection_id", [self])
+  }
+
   func ifError(_ catchExpression: Expression) -> FunctionExpression {
     return FunctionExpression("if_error", [self, catchExpression])
   }

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

@@ -1644,8 +1644,23 @@ public protocol Expression: Sendable {
   /// - Returns: A new "FunctionExpression" representing the bitwise right shift operation.
   func bitRightShift(_ numberExpression: Expression) -> FunctionExpression
 
+  /// Creates an expression that returns the document ID from a path.
+  ///
+  /// - Note: This API is in beta.
+  ///
+  /// ```swift
+  /// // Get the document ID from a path.
+  /// Field(FieldPath.documentID()).documentId()
+  /// ```
+  ///
+  /// - Returns: A new `FunctionExpression` representing the documentId operation.
   func documentId() -> FunctionExpression
 
+  /// Gets the collection id (kind) of a given document (either an absolute or
+  /// namespace relative reference).  Throw error if the input is the
+  /// root itself.
+  func collectionId() -> FunctionExpression
+
   /// Creates an expression that returns the result of `catchExpr` if this expression produces an
   /// error during evaluation,
   /// otherwise returns the result of this expression.

Firestore/Swift/Tests/Integration/PipelineTests.swift

@@ -2027,6 +2027,42 @@ class PipelineIntegrationTests: FSTIntegrationTestCase {
     TestHelper.compare(pipelineSnapshot: snapshot, expected: expectedResults, enforceOrder: true)
   }
 
+  func testCollectionIdWorks() async throws {
+    let collRef = collectionRef()
+    let docRef = collRef.document("doc")
+    try await docRef.setData(["foo": "bar"])
+
+    let pipeline = db.pipeline()
+      .collection(collRef.path)
+      .select([
+        Field(FieldPath.documentID()).collectionId().as("collectionId"),
+      ])
+
+    let snapshot = try await pipeline.execute()
+
+    let expectedResults: [[String: Sendable]] = [
+      ["collectionId": collRef.collectionID],
+    ]
+
+    TestHelper.compare(pipelineSnapshot: snapshot, expected: expectedResults, enforceOrder: true)
+  }
+
+//  func testCollectionIdOnRootThrowsError() async throws {
+//    let db = firestore()
+//    let pipeline = db.pipeline()
+//      .database()
+//      .select([
+//        Field(FieldPath.documentID()).collectionId().as("collectionId"),
+//      ])
+//
+//    do {
+//      _ = try await pipeline.execute()
+//      XCTFail("Should have thrown an error")
+//    } catch {
+//      // Expected error
+//    }
+//  }
+
   func testComparisonOperators() async throws {
     let collRef = collectionRef(withDocuments: bookDocs)
     let db = collRef.firestore