port `DocumentKey` to non-container types of `Model/*` (#930)

* naively remove FSTPath import and source/test files.

* port FieldPath, part I

* port FieldPath, part II

* port ResourcePath, part I

* port ResourcePath, part II

* the grand commit to fix build errors

* use testutil:: helper instead of those from FSTHelpers

* fix test and lint

* use c_str in errmsg directly

* fix

* fix

* make code clean

* fix integration test I missed

* fix to avoid naming collision in preprocessor

* address changes

* address changes

* address changes

* fix: fieldMask are actually shared with different context.

* address changes

* add converter function between two DocumentKey implementations

* add unit test

* address changes

* fix lint

* using DocumentKey in model except for the container types `FSTDocumentDictionary`, `FSTDocumentKeySet`, and  `FSTDocumentVersionDictionary`

* change other place w.r.t. the use of `DocumentKey` in model

* update parameter of test helpers from NSString to string_view

* revert a temporary change used in debug

* address changes

8 files changed, 99 insertions, 63 deletions

diff --git a/Firestore/Source/Model/FSTDocument.h b/Firestore/Source/Model/FSTDocument.h
index 36237fd..47e4d28 100644
--- a/Firestore/Source/Model/FSTDocument.h
+++ b/Firestore/Source/Model/FSTDocument.h
@@ -16,9 +16,9 @@
 
 #import <Foundation/Foundation.h>
 
+#include "Firestore/core/src/firebase/firestore/model/document_key.h"
 #include "Firestore/core/src/firebase/firestore/model/field_path.h"
 
-@class FSTDocumentKey;
 @class FSTFieldValue;
 @class FSTObjectValue;
 @class FSTSnapshotVersion;
@@ -31,14 +31,14 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @interface FSTMaybeDocument : NSObject <NSCopying>
 - (id)init __attribute__((unavailable("Abstract base class")));
+- (const firebase::firestore::model::DocumentKey &)key;
 
-@property(nonatomic, strong, readonly) FSTDocumentKey *key;
 @property(nonatomic, readonly) FSTSnapshotVersion *version;
 @end
 
 @interface FSTDocument : FSTMaybeDocument
 + (instancetype)documentWithData:(FSTObjectValue *)data
-                             key:(FSTDocumentKey *)key
+                             key:(firebase::firestore::model::DocumentKey)key
                          version:(FSTSnapshotVersion *)version
                hasLocalMutations:(BOOL)mutations;
 
@@ -50,7 +50,8 @@ NS_ASSUME_NONNULL_BEGIN
 @end
 
 @interface FSTDeletedDocument : FSTMaybeDocument
-+ (instancetype)documentWithKey:(FSTDocumentKey *)key version:(FSTSnapshotVersion *)version;
++ (instancetype)documentWithKey:(firebase::firestore::model::DocumentKey)key
+                        version:(FSTSnapshotVersion *)version;
 @end
 
 /** An NSComparator suitable for comparing docs using only their keys. */
diff --git a/Firestore/Source/Model/FSTDocument.mm b/Firestore/Source/Model/FSTDocument.mm
index ca66da1..9898c2a 100644
--- a/Firestore/Source/Model/FSTDocument.mm
+++ b/Firestore/Source/Model/FSTDocument.mm
@@ -16,33 +16,38 @@
 
 #import "Firestore/Source/Model/FSTDocument.h"
 
+#include <utility>
+
 #import "Firestore/Source/Core/FSTSnapshotVersion.h"
-#import "Firestore/Source/Model/FSTDocumentKey.h"
 #import "Firestore/Source/Model/FSTFieldValue.h"
 #import "Firestore/Source/Util/FSTAssert.h"
 
+#include "Firestore/core/src/firebase/firestore/model/document_key.h"
 #include "Firestore/core/src/firebase/firestore/model/field_path.h"
 #include "Firestore/core/src/firebase/firestore/util/string_apple.h"
 
 namespace util = firebase::firestore::util;
+using firebase::firestore::model::DocumentKey;
 using firebase::firestore::model::FieldPath;
 
 NS_ASSUME_NONNULL_BEGIN
 
 @interface FSTMaybeDocument ()
 
-- (instancetype)initWithKey:(FSTDocumentKey *)key
+- (instancetype)initWithKey:(DocumentKey)key
                     version:(FSTSnapshotVersion *)version NS_DESIGNATED_INITIALIZER;
 
 @end
 
-@implementation FSTMaybeDocument
+@implementation FSTMaybeDocument {
+  DocumentKey _key;
+}
 
-- (instancetype)initWithKey:(FSTDocumentKey *)key version:(FSTSnapshotVersion *)version {
+- (instancetype)initWithKey:(DocumentKey)key version:(FSTSnapshotVersion *)version {
   FSTAssert(!!version, @"Version must not be nil.");
   self = [super init];
   if (self) {
-    _key = key;
+    _key = std::move(key);
     _version = version;
   }
   return self;
@@ -53,23 +58,29 @@ NS_ASSUME_NONNULL_BEGIN
   return self;
 }
 
+- (const DocumentKey &)key {
+  return _key;
+}
+
 @end
 
 @implementation FSTDocument
 
 + (instancetype)documentWithData:(FSTObjectValue *)data
-                             key:(FSTDocumentKey *)key
+                             key:(DocumentKey)key
                          version:(FSTSnapshotVersion *)version
                hasLocalMutations:(BOOL)mutations {
-  return
-      [[FSTDocument alloc] initWithData:data key:key version:version hasLocalMutations:mutations];
+  return [[FSTDocument alloc] initWithData:data
+                                       key:std::move(key)
+                                   version:version
+                         hasLocalMutations:mutations];
 }
 
 - (instancetype)initWithData:(FSTObjectValue *)data
-                         key:(FSTDocumentKey *)key
+                         key:(DocumentKey)key
                      version:(FSTSnapshotVersion *)version
            hasLocalMutations:(BOOL)mutations {
-  self = [super initWithKey:key version:version];
+  self = [super initWithKey:std::move(key) version:version];
   if (self) {
     _data = data;
     _localMutations = mutations;
@@ -100,7 +111,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 - (NSString *)description {
   return [NSString stringWithFormat:@"<FSTDocument: key:%s version:%@ localMutations:%@ data:%@>",
-                                    self.key.path.CanonicalString().c_str(), self.version,
+                                    self.key.ToString().c_str(), self.version,
                                     self.localMutations ? @"YES" : @"NO", self.data];
 }
 
@@ -112,8 +123,8 @@ NS_ASSUME_NONNULL_BEGIN
 
 @implementation FSTDeletedDocument
 
-+ (instancetype)documentWithKey:(FSTDocumentKey *)key version:(FSTSnapshotVersion *)version {
-  return [[FSTDeletedDocument alloc] initWithKey:key version:version];
++ (instancetype)documentWithKey:(DocumentKey)key version:(FSTSnapshotVersion *)version {
+  return [[FSTDeletedDocument alloc] initWithKey:std::move(key) version:version];
 }
 
 - (BOOL)isEqual:(id)other {
diff --git a/Firestore/Source/Model/FSTDocumentSet.h b/Firestore/Source/Model/FSTDocumentSet.h
index 022e900..b5521e7 100644
--- a/Firestore/Source/Model/FSTDocumentSet.h
+++ b/Firestore/Source/Model/FSTDocumentSet.h
@@ -18,8 +18,9 @@
 
 #import "Firestore/Source/Model/FSTDocumentDictionary.h"
 
+#include "Firestore/core/src/firebase/firestore/model/document_key.h"
+
 @class FSTDocument;
-@class FSTDocumentKey;
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -41,10 +42,10 @@ NS_ASSUME_NONNULL_BEGIN
 - (BOOL)isEmpty;
 
 /** Returns YES if this set contains a document with the given key. */
-- (BOOL)containsKey:(FSTDocumentKey *)key;
+- (BOOL)containsKey:(const firebase::firestore::model::DocumentKey &)key;
 
 /** Returns the document from this set with the given key if it exists or nil if it doesn't. */
-- (FSTDocument *_Nullable)documentForKey:(FSTDocumentKey *)key;
+- (FSTDocument *_Nullable)documentForKey:(const firebase::firestore::model::DocumentKey &)key;
 
 /**
  * Returns the first document in the set according to its built in ordering, or nil if the set
@@ -62,7 +63,7 @@ NS_ASSUME_NONNULL_BEGIN
  * Returns the index of the document with the provided key in the document set. Returns NSNotFound
  * if the key is not present.
  */
-- (NSUInteger)indexOfKey:(FSTDocumentKey *)key;
+- (NSUInteger)indexOfKey:(const firebase::firestore::model::DocumentKey &)key;
 
 - (NSEnumerator<FSTDocument *> *)documentEnumerator;
 
@@ -79,7 +80,7 @@ NS_ASSUME_NONNULL_BEGIN
 - (instancetype)documentSetByAddingDocument:(FSTDocument *_Nullable)document;
 
 /** Returns a new FSTDocumentSet that excludes any document associated with the given key. */
-- (instancetype)documentSetByRemovingKey:(FSTDocumentKey *)key;
+- (instancetype)documentSetByRemovingKey:(const firebase::firestore::model::DocumentKey &)key;
 @end
 
 NS_ASSUME_NONNULL_END
diff --git a/Firestore/Source/Model/FSTDocumentSet.mm b/Firestore/Source/Model/FSTDocumentSet.mm
index 6f44799..2f0b42b 100644
--- a/Firestore/Source/Model/FSTDocumentSet.mm
+++ b/Firestore/Source/Model/FSTDocumentSet.mm
@@ -20,6 +20,10 @@
 #import "Firestore/Source/Model/FSTDocumentKey.h"
 #import "Firestore/third_party/Immutable/FSTImmutableSortedSet.h"
 
+#include "Firestore/core/src/firebase/firestore/model/document_key.h"
+
+using firebase::firestore::model::DocumentKey;
+
 NS_ASSUME_NONNULL_BEGIN
 
 /**
@@ -119,12 +123,12 @@ typedef FSTImmutableSortedSet<FSTDocument *> SetType;
   return [self.index isEmpty];
 }
 
-- (BOOL)containsKey:(FSTDocumentKey *)key {
-  return [self.index objectForKey:key] != nil;
+- (BOOL)containsKey:(const DocumentKey &)key {
+  return [self.index objectForKey:(FSTDocumentKey *)key] != nil;
 }
 
-- (FSTDocument *_Nullable)documentForKey:(FSTDocumentKey *)key {
-  return [self.index objectForKey:key];
+- (FSTDocument *_Nullable)documentForKey:(const DocumentKey &)key {
+  return [self.index objectForKey:(FSTDocumentKey *)key];
 }
 
 - (FSTDocument *_Nullable)firstDocument {
@@ -135,8 +139,8 @@ typedef FSTImmutableSortedSet<FSTDocument *> SetType;
   return [self.sortedSet lastObject];
 }
 
-- (NSUInteger)indexOfKey:(FSTDocumentKey *)key {
-  FSTDocument *doc = [self.index objectForKey:key];
+- (NSUInteger)indexOfKey:(const DocumentKey &)key {
+  FSTDocument *doc = [self.index objectForKey:(FSTDocumentKey *)key];
   return doc ? [self.sortedSet indexOfObject:doc] : NSNotFound;
 }
 
@@ -171,8 +175,8 @@ typedef FSTImmutableSortedSet<FSTDocument *> SetType;
   return [[FSTDocumentSet alloc] initWithIndex:index set:set];
 }
 
-- (instancetype)documentSetByRemovingKey:(FSTDocumentKey *)key {
-  FSTDocument *doc = [self.index objectForKey:key];
+- (instancetype)documentSetByRemovingKey:(const DocumentKey &)key {
+  FSTDocument *doc = [self.index objectForKey:(FSTDocumentKey *)key];
   if (!doc) {
     return self;
   }
diff --git a/Firestore/Source/Model/FSTMutation.h b/Firestore/Source/Model/FSTMutation.h
index 2d29dc9..4e4357d 100644
--- a/Firestore/Source/Model/FSTMutation.h
+++ b/Firestore/Source/Model/FSTMutation.h
@@ -18,10 +18,10 @@
 
 #include <vector>
 
+#include "Firestore/core/src/firebase/firestore/model/document_key.h"
 #include "Firestore/core/src/firebase/firestore/model/field_path.h"
 
 @class FSTDocument;
-@class FSTDocumentKey;
 @class FSTFieldValue;
 @class FSTMaybeDocument;
 @class FSTObjectValue;
@@ -155,7 +155,7 @@ typedef NS_ENUM(NSUInteger, FSTPreconditionExists) {
 
 - (id)init NS_UNAVAILABLE;
 
-- (instancetype)initWithKey:(FSTDocumentKey *)key
+- (instancetype)initWithKey:(firebase::firestore::model::DocumentKey)key
                precondition:(FSTPrecondition *)precondition NS_DESIGNATED_INITIALIZER;
 
 /**
@@ -215,7 +215,7 @@ typedef NS_ENUM(NSUInteger, FSTPreconditionExists) {
                           baseDocument:(nullable FSTMaybeDocument *)baseDoc
                         localWriteTime:(nullable FIRTimestamp *)localWriteTime;
 
-@property(nonatomic, strong, readonly) FSTDocumentKey *key;
+- (const firebase::firestore::model::DocumentKey &)key;
 
 /** The precondition for this mutation. */
 @property(nonatomic, strong, readonly) FSTPrecondition *precondition;
@@ -230,7 +230,7 @@ typedef NS_ENUM(NSUInteger, FSTPreconditionExists) {
  */
 @interface FSTSetMutation : FSTMutation
 
-- (instancetype)initWithKey:(FSTDocumentKey *)key
+- (instancetype)initWithKey:(firebase::firestore::model::DocumentKey)key
                precondition:(FSTPrecondition *)precondition NS_UNAVAILABLE;
 
 /**
@@ -241,7 +241,7 @@ typedef NS_ENUM(NSUInteger, FSTPreconditionExists) {
  * key.
  * @param precondition The precondition for this mutation.
  */
-- (instancetype)initWithKey:(FSTDocumentKey *)key
+- (instancetype)initWithKey:(firebase::firestore::model::DocumentKey)key
                       value:(FSTObjectValue *)value
                precondition:(FSTPrecondition *)precondition NS_DESIGNATED_INITIALIZER;
 
@@ -263,7 +263,7 @@ typedef NS_ENUM(NSUInteger, FSTPreconditionExists) {
 @interface FSTPatchMutation : FSTMutation
 
 /** Returns the precondition for the given FSTPrecondition. */
-- (instancetype)initWithKey:(FSTDocumentKey *)key
+- (instancetype)initWithKey:(firebase::firestore::model::DocumentKey)key
                precondition:(FSTPrecondition *)precondition NS_UNAVAILABLE;
 
 /**
@@ -277,7 +277,7 @@ typedef NS_ENUM(NSUInteger, FSTPreconditionExists) {
  * to determine the locations at which it should be applied).
  * @param precondition The precondition for this mutation.
  */
-- (instancetype)initWithKey:(FSTDocumentKey *)key
+- (instancetype)initWithKey:(firebase::firestore::model::DocumentKey)key
                   fieldMask:(FSTFieldMask *)fieldMask
                       value:(FSTObjectValue *)value
                precondition:(FSTPrecondition *)precondition NS_DESIGNATED_INITIALIZER;
@@ -306,7 +306,7 @@ typedef NS_ENUM(NSUInteger, FSTPreconditionExists) {
  */
 @interface FSTTransformMutation : FSTMutation
 
-- (instancetype)initWithKey:(FSTDocumentKey *)key
+- (instancetype)initWithKey:(firebase::firestore::model::DocumentKey)key
                precondition:(FSTPrecondition *)precondition NS_UNAVAILABLE;
 
 /**
@@ -315,7 +315,7 @@ typedef NS_ENUM(NSUInteger, FSTPreconditionExists) {
  * @param key Identifies the location of the document to mutate.
  * @param fieldTransforms A list of FSTFieldTransform objects to perform to the document.
  */
-- (instancetype)initWithKey:(FSTDocumentKey *)key
+- (instancetype)initWithKey:(firebase::firestore::model::DocumentKey)key
             fieldTransforms:(NSArray<FSTFieldTransform *> *)fieldTransforms
     NS_DESIGNATED_INITIALIZER;
 
diff --git a/Firestore/Source/Model/FSTMutation.mm b/Firestore/Source/Model/FSTMutation.mm
index a61ee84..253a853 100644
--- a/Firestore/Source/Model/FSTMutation.mm
+++ b/Firestore/Source/Model/FSTMutation.mm
@@ -16,17 +16,20 @@
 
 #import "Firestore/Source/Model/FSTMutation.h"
 
+#include <utility>
+
 #import "FIRTimestamp.h"
 
 #import "Firestore/Source/Core/FSTSnapshotVersion.h"
 #import "Firestore/Source/Model/FSTDocument.h"
-#import "Firestore/Source/Model/FSTDocumentKey.h"
 #import "Firestore/Source/Model/FSTFieldValue.h"
 #import "Firestore/Source/Util/FSTAssert.h"
 #import "Firestore/Source/Util/FSTClasses.h"
 
+#include "Firestore/core/src/firebase/firestore/model/document_key.h"
 #include "Firestore/core/src/firebase/firestore/model/field_path.h"
 
+using firebase::firestore::model::DocumentKey;
 using firebase::firestore::model::FieldPath;
 
 NS_ASSUME_NONNULL_BEGIN
@@ -246,11 +249,13 @@ NS_ASSUME_NONNULL_BEGIN
 
 #pragma mark - FSTMutation
 
-@implementation FSTMutation
+@implementation FSTMutation {
+  DocumentKey _key;
+}
 
-- (instancetype)initWithKey:(FSTDocumentKey *)key precondition:(FSTPrecondition *)precondition {
+- (instancetype)initWithKey:(DocumentKey)key precondition:(FSTPrecondition *)precondition {
   if (self = [super init]) {
-    _key = key;
+    _key = std::move(key);
     _precondition = precondition;
   }
   return self;
@@ -270,24 +275,28 @@ NS_ASSUME_NONNULL_BEGIN
       [self applyTo:maybeDoc baseDocument:baseDoc localWriteTime:localWriteTime mutationResult:nil];
 }
 
+- (const DocumentKey &)key {
+  return _key;
+}
+
 @end
 
 #pragma mark - FSTSetMutation
 
 @implementation FSTSetMutation
 
-- (instancetype)initWithKey:(FSTDocumentKey *)key
+- (instancetype)initWithKey:(DocumentKey)key
                       value:(FSTObjectValue *)value
                precondition:(FSTPrecondition *)precondition {
-  if (self = [super initWithKey:key precondition:precondition]) {
+  if (self = [super initWithKey:std::move(key) precondition:precondition]) {
     _value = value;
   }
   return self;
 }
 
 - (NSString *)description {
-  return [NSString stringWithFormat:@"<FSTSetMutation key=%@ value=%@ precondition=%@>", self.key,
-                                    self.value, self.precondition];
+  return [NSString stringWithFormat:@"<FSTSetMutation key=%s value=%@ precondition=%@>",
+                                    self.key.ToString().c_str(), self.value, self.precondition];
 }
 
 - (BOOL)isEqual:(id)other {
@@ -347,11 +356,11 @@ NS_ASSUME_NONNULL_BEGIN
 
 @implementation FSTPatchMutation
 
-- (instancetype)initWithKey:(FSTDocumentKey *)key
+- (instancetype)initWithKey:(DocumentKey)key
                   fieldMask:(FSTFieldMask *)fieldMask
                       value:(FSTObjectValue *)value
                precondition:(FSTPrecondition *)precondition {
-  self = [super initWithKey:key precondition:precondition];
+  self = [super initWithKey:std::move(key) precondition:precondition];
   if (self) {
     _fieldMask = fieldMask;
     _value = value;
@@ -382,8 +391,9 @@ NS_ASSUME_NONNULL_BEGIN
 }
 
 - (NSString *)description {
-  return [NSString stringWithFormat:@"<FSTPatchMutation key=%@ mask=%@ value=%@ precondition=%@>",
-                                    self.key, self.fieldMask, self.value, self.precondition];
+  return [NSString stringWithFormat:@"<FSTPatchMutation key=%s mask=%@ value=%@ precondition=%@>",
+                                    self.key.ToString().c_str(), self.fieldMask, self.value,
+                                    self.precondition];
 }
 
 - (nullable FSTMaybeDocument *)applyTo:(nullable FSTMaybeDocument *)maybeDoc
@@ -401,7 +411,7 @@ NS_ASSUME_NONNULL_BEGIN
   BOOL hasLocalMutations = (mutationResult == nil);
   if (!maybeDoc || [maybeDoc isMemberOfClass:[FSTDeletedDocument class]]) {
     // Precondition applied, so create the document if necessary
-    FSTDocumentKey *key = maybeDoc ? maybeDoc.key : self.key;
+    const DocumentKey &key = maybeDoc ? maybeDoc.key : self.key;
     FSTSnapshotVersion *version = maybeDoc ? maybeDoc.version : [FSTSnapshotVersion noVersion];
     maybeDoc = [FSTDocument documentWithData:[FSTObjectValue objectValue]
                                          key:key
@@ -439,12 +449,13 @@ NS_ASSUME_NONNULL_BEGIN
 
 @implementation FSTTransformMutation
 
-- (instancetype)initWithKey:(FSTDocumentKey *)key
+- (instancetype)initWithKey:(DocumentKey)key
             fieldTransforms:(NSArray<FSTFieldTransform *> *)fieldTransforms {
   // NOTE: We set a precondition of exists: true as a safety-check, since we always combine
   // FSTTransformMutations with a FSTSetMutation or FSTPatchMutation which (if successful) should
   // end up with an existing document.
-  if (self = [super initWithKey:key precondition:[FSTPrecondition preconditionWithExists:YES]]) {
+  if (self = [super initWithKey:std::move(key)
+                   precondition:[FSTPrecondition preconditionWithExists:YES]]) {
     _fieldTransforms = fieldTransforms;
   }
   return self;
@@ -472,8 +483,9 @@ NS_ASSUME_NONNULL_BEGIN
 }
 
 - (NSString *)description {
-  return [NSString stringWithFormat:@"<FSTTransformMutation key=%@ transforms=%@ precondition=%@>",
-                                    self.key, self.fieldTransforms, self.precondition];
+  return [NSString stringWithFormat:@"<FSTTransformMutation key=%s transforms=%@ precondition=%@>",
+                                    self.key.ToString().c_str(), self.fieldTransforms,
+                                    self.precondition];
 }
 
 - (nullable FSTMaybeDocument *)applyTo:(nullable FSTMaybeDocument *)maybeDoc
@@ -584,8 +596,8 @@ NS_ASSUME_NONNULL_BEGIN
 }
 
 - (NSString *)description {
-  return [NSString
-      stringWithFormat:@"<FSTDeleteMutation key=%@ precondition=%@>", self.key, self.precondition];
+  return [NSString stringWithFormat:@"<FSTDeleteMutation key=%s precondition=%@>",
+                                    self.key.ToString().c_str(), self.precondition];
 }
 
 - (nullable FSTMaybeDocument *)applyTo:(nullable FSTMaybeDocument *)maybeDoc
diff --git a/Firestore/Source/Model/FSTMutationBatch.h b/Firestore/Source/Model/FSTMutationBatch.h
index 1de79fe..3c82338 100644
--- a/Firestore/Source/Model/FSTMutationBatch.h
+++ b/Firestore/Source/Model/FSTMutationBatch.h
@@ -20,6 +20,8 @@
 #import "Firestore/Source/Model/FSTDocumentKeySet.h"
 #import "Firestore/Source/Model/FSTDocumentVersionDictionary.h"
 
+#include "Firestore/core/src/firebase/firestore/model/document_key.h"
+
 @class FSTMutation;
 @class FIRTimestamp;
 @class FSTMutationResult;
@@ -60,7 +62,7 @@ extern const FSTBatchID kFSTBatchIDUnknown;
  *   their hasLocalMutations flag set.
  */
 - (FSTMaybeDocument *_Nullable)applyTo:(FSTMaybeDocument *_Nullable)maybeDoc
-                           documentKey:(FSTDocumentKey *)documentKey
+                           documentKey:(const firebase::firestore::model::DocumentKey &)documentKey
                    mutationBatchResult:(FSTMutationBatchResult *_Nullable)mutationBatchResult;
 
 /**
@@ -68,7 +70,7 @@ extern const FSTBatchID kFSTBatchIDUnknown;
  * the backend).
  */
 - (FSTMaybeDocument *_Nullable)applyTo:(FSTMaybeDocument *_Nullable)maybeDoc
-                           documentKey:(FSTDocumentKey *)documentKey;
+                           documentKey:(const firebase::firestore::model::DocumentKey &)documentKey;
 
 /**
  * Returns YES if this mutation batch has already been removed from the mutation queue.
diff --git a/Firestore/Source/Model/FSTMutationBatch.mm b/Firestore/Source/Model/FSTMutationBatch.mm
index 07aadbb..6598d27 100644
--- a/Firestore/Source/Model/FSTMutationBatch.mm
+++ b/Firestore/Source/Model/FSTMutationBatch.mm
@@ -24,6 +24,10 @@
 #import "Firestore/Source/Model/FSTMutation.h"
 #import "Firestore/Source/Util/FSTAssert.h"
 
+#include "Firestore/core/src/firebase/firestore/model/document_key.h"
+
+using firebase::firestore::model::DocumentKey;
+
 NS_ASSUME_NONNULL_BEGIN
 
 const FSTBatchID kFSTBatchIDUnknown = -1;
@@ -68,10 +72,11 @@ const FSTBatchID kFSTBatchIDUnknown = -1;
 }
 
 - (FSTMaybeDocument *_Nullable)applyTo:(FSTMaybeDocument *_Nullable)maybeDoc
-                           documentKey:(FSTDocumentKey *)documentKey
+                           documentKey:(const DocumentKey &)documentKey
                    mutationBatchResult:(FSTMutationBatchResult *_Nullable)mutationBatchResult {
   FSTAssert(!maybeDoc || [maybeDoc.key isEqualToKey:documentKey],
-            @"applyTo: key %@ doesn't match maybeDoc key %@", documentKey, maybeDoc.key);
+            @"applyTo: key %s doesn't match maybeDoc key %s", documentKey.ToString().c_str(),
+            maybeDoc.key.ToString().c_str());
   FSTMaybeDocument *baseDoc = maybeDoc;
   if (mutationBatchResult) {
     FSTAssert(mutationBatchResult.mutationResults.count == self.mutations.count,
@@ -94,7 +99,7 @@ const FSTBatchID kFSTBatchIDUnknown = -1;
 }
 
 - (FSTMaybeDocument *_Nullable)applyTo:(FSTMaybeDocument *_Nullable)maybeDoc
-                           documentKey:(FSTDocumentKey *)documentKey {
+                           documentKey:(const DocumentKey &)documentKey {
   return [self applyTo:maybeDoc documentKey:documentKey mutationBatchResult:nil];
 }