summaryrefslogtreecommitdiff
path: root/src/mongo/db/pipeline/document.h
blob: a8ebfd1c4d26af305192becc3430ac59149c88a1 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
/**
 * Copyright 2011 (c) 10gen Inc.
 *
 * This program is free software: you can redistribute it and/or  modify
 * it under the terms of the GNU Affero General Public License, version 3,
 * as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 * As a special exception, the copyright holders give permission to link the
 * code of portions of this program with the OpenSSL library under certain
 * conditions as described in each individual source file and distribute
 * linked combinations including the program with the OpenSSL library. You
 * must comply with the GNU Affero General Public License in all respects for
 * all of the code used other than as permitted herein. If you modify file(s)
 * with this exception, you may extend this exception to your version of the
 * file(s), but you are not obligated to do so. If you do not wish to do so,
 * delete this exception statement from your version. If you delete this
 * exception statement from all source files in the program, then also delete
 * it in the license file.
 */

#pragma once

#include "mongo/db/pipeline/document_internal.h"

#include <boost/functional/hash.hpp>
#include <boost/intrusive_ptr.hpp>

#include "mongo/base/string_data.h"
#include "mongo/base/string_data_comparator_interface.h"
#include "mongo/bson/util/builder.h"

namespace mongo {
class BSONObj;
class FieldIterator;
class FieldPath;
class Value;
class MutableDocument;

/** An internal class that represents the position of a field in a document.
 *
 *  This is a low-level class that you usually don't need to worry about.
 *
 *  The main use of this class for clients is to allow refetching or
 *  setting a field without looking it up again. It has a default
 *  constructor that represents a field not being in a document. It also
 *  has a method 'bool found()' that tells you if a field was found.
 *
 *  For more details see document_internal.h
 */
class Position;

/** A Document is similar to a BSONObj but with a different in-memory representation.
 *
 *  A Document can be treated as a const std::map<std::string, const Value> that is
 *  very cheap to copy and is Assignable.  Therefore, it is acceptable to
 *  pass and return by Value. Note that the data in a Document is
 *  immutable, but you can replace a Document instance with assignment.
 *
 *  See Also: Value class in Value.h
 */
class Document {
public:
    /**
     * Operator overloads for relops return a DeferredComparison which can subsequently be evaluated
     * by a DocumentComparator.
     */
    struct DeferredComparison {
        enum class Type {
            kLT,
            kLTE,
            kEQ,
            kGT,
            kGTE,
            kNE,
        };

        DeferredComparison(Type type, const Document& lhs, const Document& rhs)
            : type(type), lhs(lhs), rhs(rhs) {}

        Type type;
        const Document& lhs;
        const Document& rhs;
    };

    static constexpr StringData metaFieldTextScore = "$textScore"_sd;
    static constexpr StringData metaFieldRandVal = "$randVal"_sd;
    static constexpr StringData metaFieldSortKey = "$sortKey"_sd;

    static const std::vector<StringData> allMetadataFieldNames;

    /// Empty Document (does no allocation)
    Document() {}

    /// Create a new Document deep-converted from the given BSONObj.
    explicit Document(const BSONObj& bson);

    /**
     * Create a new document from key, value pairs. Enables constructing a document using this
     * syntax:
     * auto document = Document{{"hello", "world"}, {"number": 1}};
     */
    Document(std::initializer_list<std::pair<StringData, ImplicitValue>> initializerList);

    void swap(Document& rhs) {
        _storage.swap(rhs._storage);
    }

    /// Look up a field by key name. Returns Value() if no such field. O(1)
    const Value operator[](StringData key) const {
        return getField(key);
    }
    const Value getField(StringData key) const {
        return storage().getField(key);
    }

    /// Look up a field by Position. See positionOf and getNestedField.
    const Value operator[](Position pos) const {
        return getField(pos);
    }
    const Value getField(Position pos) const {
        return storage().getField(pos).val;
    }

    /**
     * Returns the Value stored at the location given by 'path', or Value() if no such path exists.
     * If 'positions' is non-null, it will be filled with a path suitable to pass to
     * MutableDocument::setNestedField().
     */
    const Value getNestedField(const FieldPath& path,
                               std::vector<Position>* positions = nullptr) const;

    /// Number of fields in this document. O(n)
    size_t size() const {
        return storage().size();
    }

    /// True if this document has no fields.
    bool empty() const {
        return !_storage || storage().iterator().atEnd();
    }

    /// Create a new FieldIterator that can be used to examine the Document's fields in order.
    FieldIterator fieldIterator() const;

    /// Convenience type for dealing with fields. Used by FieldIterator.
    typedef std::pair<StringData, Value> FieldPair;

    /** Get the approximate storage size of the document and sub-values in bytes.
     *  Note: Some memory may be shared with other Documents or between fields within
     *        a single Document so this can overestimate usage.
     */
    size_t getApproximateSize() const;

    /**
     * Compare two documents. Most callers should prefer using DocumentComparator instead. See
     * document_comparator.h for details.
     *
     *  BSON document field order is significant, so this just goes through
     *  the fields in order.  The comparison is done in roughly the same way
     *  as strings are compared, but comparing one field at a time instead
     *  of one character at a time.
     *
     *  Pass a non-null StringData::ComparatorInterface if special string comparison semantics are
     *  required. If the comparator is null, then a simple binary compare is used for strings. This
     *  comparator is only used for string *values*; field names are always compared using simple
     *  binary compare.
     *
     *  Note: This does not consider metadata when comparing documents.
     *
     *  @returns an integer less than zero, zero, or an integer greater than
     *           zero, depending on whether lhs < rhs, lhs == rhs, or lhs > rhs
     *  Warning: may return values other than -1, 0, or 1
     */
    static int compare(const Document& lhs,
                       const Document& rhs,
                       const StringData::ComparatorInterface* stringComparator);

    std::string toString() const;

    friend std::ostream& operator<<(std::ostream& out, const Document& doc) {
        return out << doc.toString();
    }

    /** Calculate a hash value.
     *
     * Meant to be used to create composite hashes suitable for
     * hashed container classes such as unordered_map.
     */
    void hash_combine(size_t& seed, const StringData::ComparatorInterface* stringComparator) const;

    /**
     * Serializes this document to the BSONObj under construction in 'builder'. Metadata is not
     * included. Throws a AssertionException if 'recursionLevel' exceeds the maximum allowable
     * depth.
     */
    void toBson(BSONObjBuilder* builder, size_t recursionLevel = 1) const;
    BSONObj toBson() const;

    /**
     * Like toBson, but includes metadata at the top-level.
     * Output is parseable by fromBsonWithMetaData
     */
    BSONObj toBsonWithMetaData() const;

    /**
     * Like Document(BSONObj) but treats top-level fields with special names as metadata.
     * Special field names are available as static constants on this class with names starting
     * with metaField.
     */
    static Document fromBsonWithMetaData(const BSONObj& bson);

    /**
     * Given a BSON object that may have metadata fields added as part of toBsonWithMetadata(),
     * returns the same object without any of the metadata fields.
     */
    static BSONObj stripMetadataFields(const BSONObj& bsonWithMetadata);

    // Support BSONObjBuilder and BSONArrayBuilder "stream" API
    friend BSONObjBuilder& operator<<(BSONObjBuilderValueStream& builder, const Document& d);

    /** Return the abstract Position of a field, suitable to pass to operator[] or getField().
     *  This can potentially save time if you need to refer to a field multiple times.
     */
    Position positionOf(StringData fieldName) const {
        return storage().findField(fieldName);
    }

    /** Clone a document.
     *
     *  This should only be called by MutableDocument and tests
     *
     *  The new document shares all the fields' values with the original.
     *  This is not a deep copy.  Only the fields on the top-level document
     *  are cloned.
     */
    Document clone() const {
        return Document(storage().clone().get());
    }

    bool hasTextScore() const {
        return storage().hasTextScore();
    }
    double getTextScore() const {
        return storage().getTextScore();
    }

    bool hasRandMetaField() const {
        return storage().hasRandMetaField();
    }
    double getRandMetaField() const {
        return storage().getRandMetaField();
    }

    bool hasSortKeyMetaField() const {
        return storage().hasSortKeyMetaField();
    }
    BSONObj getSortKeyMetaField() const {
        return storage().getSortKeyMetaField();
    }

    /// members for Sorter
    struct SorterDeserializeSettings {};  // unused
    void serializeForSorter(BufBuilder& buf) const;
    static Document deserializeForSorter(BufReader& buf, const SorterDeserializeSettings&);
    int memUsageForSorter() const {
        return getApproximateSize();
    }
    Document getOwned() const {
        return *this;
    }

    /// only for testing
    const void* getPtr() const {
        return _storage.get();
    }

private:
    friend class FieldIterator;
    friend class ValueStorage;
    friend class MutableDocument;
    friend class MutableValue;

    explicit Document(const DocumentStorage* ptr) : _storage(ptr){};

    const DocumentStorage& storage() const {
        return (_storage ? *_storage : DocumentStorage::emptyDoc());
    }
    boost::intrusive_ptr<const DocumentStorage> _storage;
};

//
// Comparison API.
//
// Document instances can be compared either using Document::compare() or via operator overloads.
// Most callers should prefer operator overloads. Note that the operator overloads return a
// DeferredComparison, which must be subsequently evaluated by a DocumentComparator. See
// document_comparator.h for details.
//

inline Document::DeferredComparison operator==(const Document& lhs, const Document& rhs) {
    return Document::DeferredComparison(Document::DeferredComparison::Type::kEQ, lhs, rhs);
}

inline Document::DeferredComparison operator!=(const Document& lhs, const Document& rhs) {
    return Document::DeferredComparison(Document::DeferredComparison::Type::kNE, lhs, rhs);
}

inline Document::DeferredComparison operator<(const Document& lhs, const Document& rhs) {
    return Document::DeferredComparison(Document::DeferredComparison::Type::kLT, lhs, rhs);
}

inline Document::DeferredComparison operator<=(const Document& lhs, const Document& rhs) {
    return Document::DeferredComparison(Document::DeferredComparison::Type::kLTE, lhs, rhs);
}

inline Document::DeferredComparison operator>(const Document& lhs, const Document& rhs) {
    return Document::DeferredComparison(Document::DeferredComparison::Type::kGT, lhs, rhs);
}

inline Document::DeferredComparison operator>=(const Document& lhs, const Document& rhs) {
    return Document::DeferredComparison(Document::DeferredComparison::Type::kGTE, lhs, rhs);
}

/** This class is returned by MutableDocument to allow you to modify its values.
 *  You are not allowed to hold variables of this type (enforced by the type system).
 */
class MutableValue {
public:
    void operator=(const Value& v) {
        _val = v;
    }

    void operator=(Value&& v) {
        _val = std::move(v);
    }

    /** These are designed to allow things like mutDoc["a"]["b"]["c"] = Value(10);
     *  It is safe to use even on nonexistent fields.
     */
    MutableValue operator[](StringData key) {
        return getField(key);
    }
    MutableValue operator[](Position pos) {
        return getField(pos);
    }

    MutableValue getField(StringData key);
    MutableValue getField(Position pos);

private:
    friend class MutableDocument;

    /// can only be constructed or copied by self and friends
    MutableValue(const MutableValue& other) : _val(other._val) {}
    explicit MutableValue(Value& val) : _val(val) {}

    /// Used by MutableDocument(MutableValue)
    const RefCountable*& getDocPtr() {
        if (_val.getType() != Object || _val._storage.genericRCPtr == NULL) {
            // If the current value isn't an object we replace it with a Object-typed Value.
            // Note that we can't just use Document() here because that is a NULL pointer and
            // Value doesn't refcount NULL pointers. This led to a memory leak (SERVER-10554)
            // because MutableDocument::newStorage() would set a non-NULL pointer into the Value
            // without setting the refCounter bit. While allocating a DocumentStorage here could
            // result in an allocation where none is needed, in practice this is only called
            // when we are about to add a field to the sub-document so this just changes where
            // the allocation is done.
            _val = Value(Document(new DocumentStorage()));
        }

        return _val._storage.genericRCPtr;
    }

    MutableValue& operator=(const MutableValue&);  // not assignable with another MutableValue

    Value& _val;
};

/** MutableDocument is a Document builder that supports both adding and updating fields.
 *
 *  This class fills a similar role to BSONObjBuilder, but allows you to
 *  change existing fields and more easily write to sub-Documents.
 *
 *  To preserve the immutability of Documents, MutableDocument will
 *  shallow-clone its storage on write (COW) if it is shared with any other
 *  Documents.
 */
class MutableDocument {
    MONGO_DISALLOW_COPYING(MutableDocument);

public:
    /** Create a new empty Document.
     *
     *  @param expectedFields a hint at what the number of fields will be, if known.
     *         this can be used to increase memory allocation efficiency. There is
     *         no impact on correctness if this field over or under estimates.
     *
     *  TODO: find some way to convey field-name sizes to make even more efficient
     */
    MutableDocument() : _storageHolder(NULL), _storage(_storageHolder) {}
    explicit MutableDocument(size_t expectedFields);

    /// No copy of data yet. Copy-on-write. See storage()
    explicit MutableDocument(Document d) : _storageHolder(NULL), _storage(_storageHolder) {
        reset(std::move(d));
    }

    ~MutableDocument() {
        if (_storageHolder)
            intrusive_ptr_release(_storageHolder);
    }

    /** Replace the current base Document with the argument
     *
     *  All Positions from the passed in Document are valid and refer to the
     *  same field in this MutableDocument.
     */
    void reset(Document d = Document()) {
        reset(std::move(d._storage));
    }

    /** Add the given field to the Document.
     *
     *  BSON documents' fields are ordered; the new Field will be
     *  appended to the current list of fields.
     *
     *  Unlike getField/setField, addField does not look for a field with the
     *  same name and therefore cannot be used to update fields.
     *
     *  It is an error to add a field that has the same name as another field.
     *
     *  TODO: This is currently allowed but getField only gets first field.
     *        Decide what level of support is needed for duplicate fields.
     *        If duplicates are not allowed, consider removing this method.
     */
    void addField(StringData fieldName, const Value& val) {
        storage().appendField(fieldName) = val;
    }

    /** Update field by key. If there is no field with that key, add one.
     *
     *  If the new value is missing(), the field is logically removed.
     */
    MutableValue operator[](StringData key) {
        return getField(key);
    }
    void setField(StringData key, const Value& val) {
        getField(key) = val;
    }
    MutableValue getField(StringData key) {
        return MutableValue(storage().getField(key));
    }

    /// Update field by Position. Must already be a valid Position.
    MutableValue operator[](Position pos) {
        return getField(pos);
    }
    void setField(Position pos, const Value& val) {
        getField(pos) = val;
    }
    MutableValue getField(Position pos) {
        return MutableValue(storage().getField(pos).val);
    }

    /// Logically remove a field. Note that memory usage does not decrease.
    void remove(StringData key) {
        getField(key) = Value();
    }
    void removeNestedField(const std::vector<Position>& positions) {
        getNestedField(positions) = Value();
    }

    /** Gets/Sets a nested field given a path.
     *
     *  All fields along path are created as empty Documents if they don't exist
     *  or are any other type.
     */
    MutableValue getNestedField(const FieldPath& dottedField);
    void setNestedField(const FieldPath& dottedField, const Value& val) {
        getNestedField(dottedField) = val;
    }

    /// Takes positions vector from Document::getNestedField. All fields in path must exist.
    MutableValue getNestedField(const std::vector<Position>& positions);
    void setNestedField(const std::vector<Position>& positions, const Value& val) {
        getNestedField(positions) = val;
    }

    /**
     * Copies all metadata from source if it has any.
     * Note: does not clear metadata from this.
     */
    void copyMetaDataFrom(const Document& source) {
        storage().copyMetaDataFrom(source.storage());
    }

    void setTextScore(double score) {
        storage().setTextScore(score);
    }

    void setRandMetaField(double val) {
        storage().setRandMetaField(val);
    }

    void setSortKeyMetaField(BSONObj sortKey) {
        storage().setSortKeyMetaField(sortKey);
    }

    /** Convert to a read-only document and release reference.
     *
     *  Call this to indicate that you are done with this Document and will
     *  not be making further changes from this MutableDocument.
     *
     *  TODO: there are some optimizations that may make sense at freeze time.
     */
    Document freeze() {
        // This essentially moves _storage into a new Document by way of temp.
        Document ret;
        boost::intrusive_ptr<const DocumentStorage> temp(storagePtr(), /*inc_ref_count=*/false);
        temp.swap(ret._storage);
        _storage = NULL;
        return ret;
    }

    /// Used to simplify the common pattern of creating a value of the document.
    Value freezeToValue() {
        return Value(freeze());
    }

    /** Borrow a readable reference to this Document.
     *
     *  Note that unlike freeze(), this indicates intention to continue
     *  modifying this document. The returned Document will not observe
     *  future changes to this MutableDocument.
     */
    Document peek() {
        return Document(storagePtr());
    }

    size_t getApproximateSize() {
        return peek().getApproximateSize();
    }

private:
    friend class MutableValue;  // for access to next constructor
    explicit MutableDocument(MutableValue mv) : _storageHolder(NULL), _storage(mv.getDocPtr()) {}

    void reset(boost::intrusive_ptr<const DocumentStorage> ds) {
        if (_storage)
            intrusive_ptr_release(_storage);
        _storage = ds.detach();
    }

    // This is split into 3 functions to speed up the fast-path
    DocumentStorage& storage() {
        if (MONGO_unlikely(!_storage))
            return newStorage();

        if (MONGO_unlikely(_storage->isShared()))
            return clonedStorage();

        // This function exists to ensure this is safe
        return const_cast<DocumentStorage&>(*storagePtr());
    }
    DocumentStorage& newStorage() {
        reset(new DocumentStorage);
        return const_cast<DocumentStorage&>(*storagePtr());
    }
    DocumentStorage& clonedStorage() {
        reset(storagePtr()->clone());
        return const_cast<DocumentStorage&>(*storagePtr());
    }

    // recursive helpers for same-named public methods
    MutableValue getNestedFieldHelper(const FieldPath& dottedField, size_t level);
    MutableValue getNestedFieldHelper(const std::vector<Position>& positions, size_t level);

    // this should only be called by storage methods and peek/freeze
    const DocumentStorage* storagePtr() const {
        dassert(!_storage || typeid(*_storage) == typeid(const DocumentStorage));
        return static_cast<const DocumentStorage*>(_storage);
    }

    // These are both const to prevent modifications bypassing storage() method.
    // They always point to NULL or an object with dynamic type DocumentStorage.
    const RefCountable* _storageHolder;  // Only used in constructors and destructor
    const RefCountable*& _storage;  // references either above member or genericRCPtr in a Value
};

/// This is the public iterator over a document
class FieldIterator {
public:
    explicit FieldIterator(const Document& doc) : _doc(doc), _it(_doc.storage().iterator()) {}

    /// Ask if there are more fields to return.
    bool more() const {
        return !_it.atEnd();
    }

    /// Get next item and advance iterator
    Document::FieldPair next() {
        verify(more());

        Document::FieldPair fp(_it->nameSD(), _it->val);
        _it.advance();
        return fp;
    }

private:
    // We'll hang on to the original document to ensure we keep its storage alive
    Document _doc;
    DocumentStorageIterator _it;
};

/// Macro to create Document literals. Syntax is the same as the BSON("name" << 123) macro.
#define DOC(fields) ((DocumentStream() << fields).done())

/** Macro to create Array-typed Value literals.
 *  Syntax is the same as the BSON_ARRAY(123 << "foo") macro.
 */
#define DOC_ARRAY(fields) ((ValueArrayStream() << fields).done())


// These classes are only for the implementation of the DOC and DOC_ARRAY macros.
// They should not be used for any other reason.
class DocumentStream {
    // The stream alternates between DocumentStream taking a fieldname
    // and ValueStream taking a Value.
    class ValueStream {
    public:
        ValueStream(DocumentStream& builder) : builder(builder) {}

        DocumentStream& operator<<(const Value& val) {
            builder._md[name] = val;
            return builder;
        }

        /// support anything directly supported by a value constructor
        template <typename T>
        DocumentStream& operator<<(const T& val) {
            return *this << Value(val);
        }

        StringData name;
        DocumentStream& builder;
    };

public:
    DocumentStream() : _stream(*this) {}

    ValueStream& operator<<(StringData name) {
        _stream.name = name;
        return _stream;
    }

    Document done() {
        return _md.freeze();
    }

private:
    ValueStream _stream;
    MutableDocument _md;
};

class ValueArrayStream {
public:
    ValueArrayStream& operator<<(const Value& val) {
        _array.push_back(val);
        return *this;
    }

    /// support anything directly supported by a value constructor
    template <typename T>
    ValueArrayStream& operator<<(const T& val) {
        return *this << Value(val);
    }

    Value done() {
        return Value(std::move(_array));
    }

private:
    std::vector<Value> _array;
};

inline void swap(mongo::Document& lhs, mongo::Document& rhs) {
    lhs.swap(rhs);
}

/* ======================= INLINED IMPLEMENTATIONS ========================== */

inline FieldIterator Document::fieldIterator() const {
    return FieldIterator(*this);
}

inline MutableValue MutableValue::getField(Position pos) {
    return MutableDocument(*this).getField(pos);
}
inline MutableValue MutableValue::getField(StringData key) {
    return MutableDocument(*this).getField(key);
}
}