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diff --git a/Firestore/Source/Model/FSTDocumentKey.mm b/Firestore/Source/Model/FSTDocumentKey.mm
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+++ b/Firestore/Source/Model/FSTDocumentKey.mm
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+/*
+ * Copyright 2017 Google
+ *
+ * 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 "Firestore/Source/Model/FSTDocumentKey.h"
+
+#import "Firestore/Source/Core/FSTFirestoreClient.h"
+#import "Firestore/Source/Model/FSTPath.h"
+#import "Firestore/Source/Util/FSTAssert.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface FSTDocumentKey ()
+/** The path to the document. */
+@property(strong, nonatomic, readwrite) FSTResourcePath *path;
+@end
+
+@implementation FSTDocumentKey
+
++ (instancetype)keyWithPath:(FSTResourcePath *)path {
+  return [[FSTDocumentKey alloc] initWithPath:path];
+}
+
++ (instancetype)keyWithSegments:(NSArray<NSString *> *)segments {
+  return [FSTDocumentKey keyWithPath:[FSTResourcePath pathWithSegments:segments]];
+}
+
++ (instancetype)keyWithPathString:(NSString *)resourcePath {
+  NSArray<NSString *> *segments = [resourcePath componentsSeparatedByString:@"/"];
+  return [FSTDocumentKey keyWithSegments:segments];
+}
+
+/** Designated initializer. */
+- (instancetype)initWithPath:(FSTResourcePath *)path {
+  FSTAssert([FSTDocumentKey isDocumentKey:path], @"invalid document key path: %@", path);
+
+  if (self = [super init]) {
+    _path = path;
+  }
+  return self;
+}
+
+- (BOOL)isEqual:(id)object {
+  if (self == object) {
+    return YES;
+  }
+  if (![object isKindOfClass:[FSTDocumentKey class]]) {
+    return NO;
+  }
+  return [self isEqualToKey:(FSTDocumentKey *)object];
+}
+
+- (NSUInteger)hash {
+  return self.path.hash;
+}
+
+- (NSString *)description {
+  return [NSString stringWithFormat:@"<FSTDocumentKey: %@>", self.path];
+}
+
+/** Implements NSCopying without actually copying because FSTDocumentKeys are immutable. */
+- (id)copyWithZone:(NSZone *_Nullable)zone {
+  return self;
+}
+
+- (BOOL)isEqualToKey:(FSTDocumentKey *)other {
+  return FSTDocumentKeyComparator(self, other) == NSOrderedSame;
+}
+
+- (NSComparisonResult)compare:(FSTDocumentKey *)other {
+  return FSTDocumentKeyComparator(self, other);
+}
+
++ (NSComparator)comparator {
+  return ^NSComparisonResult(id obj1, id obj2) {
+    return [obj1 compare:obj2];
+  };
+}
+
++ (BOOL)isDocumentKey:(FSTResourcePath *)path {
+  return path.length % 2 == 0;
+}
+
+@end
+
+const NSComparator FSTDocumentKeyComparator =
+    ^NSComparisonResult(FSTDocumentKey *key1, FSTDocumentKey *key2) {
+      return [key1.path compare:key2.path];
+    };
+
+NSString *const kDocumentKeyPath = @"__name__";
+
+NS_ASSUME_NONNULL_END