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diff --git a/Firestore/core/src/firebase/firestore/model/document_key.h b/Firestore/core/src/firebase/firestore/model/document_key.h
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+/*
+ * Copyright 2018 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.
+ */
+
+#ifndef FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_MODEL_DOCUMENT_KEY_H_
+#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_MODEL_DOCUMENT_KEY_H_
+
+#include <initializer_list>
+#include <memory>
+#include <string>
+
+#include "Firestore/core/src/firebase/firestore/model/resource_path.h"
+#include "absl/strings/string_view.h"
+
+namespace firebase {
+namespace firestore {
+namespace model {
+
+/**
+ * DocumentKey represents the location of a document in the Firestore database.
+ */
+class DocumentKey {
+ public:
+  /** Creates a "blank" document key not associated with any document. */
+  DocumentKey() : path_{std::make_shared<ResourcePath>()} {
+  }
+
+  /** Creates a new document key containing a copy of the given path. */
+  explicit DocumentKey(const ResourcePath& path);
+
+  /** Creates a new document key, taking ownership of the given path. */
+  explicit DocumentKey(ResourcePath&& path);
+
+  /**
+   * Creates and returns a new document key using '/' to split the string into
+   * segments.
+   */
+  static DocumentKey FromPathString(const absl::string_view path) {
+    return DocumentKey{ResourcePath::FromString(path)};
+  }
+
+  /** Creates and returns a new document key with the given segments. */
+  static DocumentKey FromSegments(std::initializer_list<std::string> list) {
+    return DocumentKey{ResourcePath{list}};
+  }
+
+  /** Returns a shared instance of an empty document key. */
+  static const DocumentKey& Empty();
+
+  /** Returns true iff the given path is a path to a document. */
+  static bool IsDocumentKey(const ResourcePath& path) {
+    return path.size() % 2 == 0;
+  }
+
+  /** The path to the document. */
+  const ResourcePath& path() const {
+    return path_ ? *path_ : Empty().path();
+  }
+
+ private:
+  // This is an optimization to make passing DocumentKey around cheaper (it's
+  // copied often).
+  std::shared_ptr<const ResourcePath> path_;
+};
+
+inline bool operator==(const DocumentKey& lhs, const DocumentKey& rhs) {
+  return lhs.path() == rhs.path();
+}
+inline bool operator!=(const DocumentKey& lhs, const DocumentKey& rhs) {
+  return lhs.path() != rhs.path();
+}
+inline bool operator<(const DocumentKey& lhs, const DocumentKey& rhs) {
+  return lhs.path() < rhs.path();
+}
+inline bool operator<=(const DocumentKey& lhs, const DocumentKey& rhs) {
+  return lhs.path() <= rhs.path();
+}
+inline bool operator>(const DocumentKey& lhs, const DocumentKey& rhs) {
+  return lhs.path() > rhs.path();
+}
+inline bool operator>=(const DocumentKey& lhs, const DocumentKey& rhs) {
+  return lhs.path() >= rhs.path();
+}
+
+}  // namespace model
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_MODEL_DOCUMENT_KEY_H_