aboutsummaryrefslogtreecommitdiff
path: root/src/query.ts
blob: 24db4de569c301ecc31c9d3f62e5f9207f781555 (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
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
/*
 This file is part of TALER
 (C) 2016 GNUnet e.V.

 TALER is free software; you can redistribute it and/or modify it under the
 terms of the GNU General Public License as published by the Free Software
 Foundation; either version 3, or (at your option) any later version.

 TALER 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 General Public License for more details.

 You should have received a copy of the GNU General Public License along with
 TALER; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
 */


/**
 * Database query abstractions.
 * @module Query
 * @author Florian Dold
 */


/**
 * Result of an inner join.
 */
export interface JoinResult<L, R> {
  left: L;
  right: R;
}

/**
 * Result of a left outer join.
 */
export interface JoinLeftResult<L, R> {
  left: L;
  right?: R;
}


/**
 * Definition of an object store.
 */
export class Store<T> {
  constructor(public name: string,
              public storeParams?: IDBObjectStoreParameters,
              public validator?: (v: T) => T) {
  }
}


/**
 * Definition of an index.
 */
export class Index<S extends IDBValidKey, T> {
  /**
   * Name of the store that this index is associated with.
   */
  storeName: string;

  constructor(s: Store<T>, public indexName: string, public keyPath: string | string[]) {
    this.storeName = s.name;
  }

  /**
   * We want to have the key type parameter in use somewhere,
   * because otherwise the compiler complains.  In iterIndex the
   * key type is pretty useful.
   */
  protected _dummyKey: S|undefined;
}

/**
 * Stream that can be filtered, reduced or joined
 * with indices.
 */
export interface QueryStream<T> {
  /**
   * Join the current query with values from an index.
   * The left side of the join is extracted via a function from the stream's
   * result, the right side of the join is the key of the index.
   */
  indexJoin<S, I extends IDBValidKey>(index: Index<I, S>, keyFn: (obj: T) => I): QueryStream<JoinResult<T, S>>;
  /**
   * Join the current query with values from an index, and keep values in the
   * current stream that don't have a match.  The left side of the join is
   * extracted via a function from the stream's result, the right side of the
   * join is the key of the index.
   */
  indexJoinLeft<S, I extends IDBValidKey>(index: Index<I, S>,
                                          keyFn: (obj: T) => I): QueryStream<JoinLeftResult<T, S>>;
  /**
   * Join the current query with values from another object store.
   * The left side of the join is extracted via a function over the current query,
   * the right side of the join is the key of the object store.
   */
  keyJoin<S, I extends IDBValidKey>(store: Store<S>, keyFn: (obj: T) => I): QueryStream<JoinResult<T, S>>;

  /**
   * Only keep elements in the result stream for which the predicate returns
   * true.
   */
  filter(f: (x: T) => boolean): QueryStream<T>;

  /**
   * Reduce the stream, resulting in a single value.
   */
  reduce<S>(f: (v: T, acc: S) => S, start?: S): Promise<S>;

  /**
   * Map each element of the stream using a function, resulting in another
   * stream of a different type.
   */
  map<S>(f: (x: T) => S): QueryStream<S>;

  /**
   * Map each element of the stream to a potentially empty array, and collect
   * the result in a stream of the flattened arrays.
   */
  flatMap<S>(f: (x: T) => S[]): QueryStream<S>;

  /**
   * Collect the stream into an array and return a promise for it.
   */
  toArray(): Promise<T[]>;

  /**
   * Get the first value of the stream.
   */
  first(): QueryValue<T>;

  then(onfulfill: any, onreject: any): any;
}


/**
 * Query result that consists of at most one value.
 */
export interface QueryValue<T> {
  /**
   * Apply a function to a query value.
   */
  map<S>(f: (x: T) => S): QueryValue<S>;
  /**
   * Conditionally execute either of two queries based
   * on a property of this query value.
   *
   * Useful to properly implement complex queries within a transaction (as
   * opposed to just computing the conditional and then executing either
   * branch).  This is necessary since IndexedDB does not allow long-lived
   * transactions.
   */
  cond<R>(f: (x: T) => boolean, onTrue: (r: QueryRoot) => R, onFalse: (r: QueryRoot) => R): Promise<void>;
}


abstract class BaseQueryValue<T> implements QueryValue<T> {

  constructor(public root: QueryRoot) {
  }

  map<S>(f: (x: T) => S): QueryValue<S> {
    return new MapQueryValue<T, S>(this, f);
  }

  cond<R>(f: (x: T) => boolean, onTrue: (r: QueryRoot) => R, onFalse: (r: QueryRoot) => R): Promise<void> {
    return new Promise<void>((resolve, reject) => {
      this.subscribeOne((v, tx) => {
        if (f(v)) {
          onTrue(new QueryRoot(this.root.db));
        } else {
          onFalse(new QueryRoot(this.root.db));
        }
      });
      resolve();
    });
  }

  abstract subscribeOne(f: SubscribeOneFn): void;
}

class FirstQueryValue<T> extends BaseQueryValue<T> {
  private gotValue = false;
  private s: QueryStreamBase<T>;

  constructor(stream: QueryStreamBase<T>) {
    super(stream.root);
    this.s = stream;
  }

  subscribeOne(f: SubscribeOneFn): void {
    this.s.subscribe((isDone, value, tx) => {
      if (this.gotValue) {
        return;
      }
      if (isDone) {
          f(undefined, tx);
      } else {
        f(value, tx);
      }
      this.gotValue = true;
    });
  }
}

class MapQueryValue<T, S> extends BaseQueryValue<S> {
  constructor(private v: BaseQueryValue<T>, private mapFn: (x: T) => S) {
    super(v.root);
  }

  subscribeOne(f: SubscribeOneFn): void {
    this.v.subscribeOne((v, tx) => this.mapFn(v));
  }
}


/**
 * Exception that should be thrown by client code to abort a transaction.
 */
export const AbortTransaction = Symbol("abort_transaction");

/**
 * Get an unresolved promise together with its extracted resolve / reject
 * function.
 */
export function openPromise<T>(): any {
  let resolve: ((value?: T | PromiseLike<T>) => void) | null = null;
  let reject: ((reason?: any) => void) | null = null;
  const promise = new Promise<T>((res, rej) => {
    resolve = res;
    reject = rej;
  });
  if (!(resolve && reject)) {
    // Never happens, unless JS implementation is broken
    throw Error();
  }
  return {resolve, reject, promise};
}


abstract class QueryStreamBase<T> implements QueryStream<T>, PromiseLike<void> {
  abstract subscribe(f: (isDone: boolean,
                         value: any,
                         tx: IDBTransaction) => void): void;
  constructor(public root: QueryRoot) {
  }

  first(): QueryValue<T> {
    return new FirstQueryValue(this);
  }

  then<R>(onfulfilled: (value: void) => R | PromiseLike<R>,
          onrejected: (reason: any) => R | PromiseLike<R>): PromiseLike<R>  {
    return this.root.then(onfulfilled, onrejected);
  }

  flatMap<S>(f: (x: T) => S[]): QueryStream<S> {
    return new QueryStreamFlatMap<T, S>(this, f);
  }

  map<S>(f: (x: T) => S): QueryStream<S> {
    return new QueryStreamMap(this, f);
  }

  indexJoin<S, I extends IDBValidKey>(index: Index<I, S>,
                                      keyFn: (obj: T) => I): QueryStream<JoinResult<T, S>> {
    this.root.addStoreAccess(index.storeName, false);
    return new QueryStreamIndexJoin<T, S>(this, index.storeName, index.indexName, keyFn);
  }

  indexJoinLeft<S, I extends IDBValidKey>(index: Index<I, S>,
                                          keyFn: (obj: T) => I): QueryStream<JoinLeftResult<T, S>> {
    this.root.addStoreAccess(index.storeName, false);
    return new QueryStreamIndexJoinLeft<T, S>(this, index.storeName, index.indexName, keyFn);
  }

  keyJoin<S, I extends IDBValidKey>(store: Store<S>,
                                    keyFn: (obj: T) => I): QueryStream<JoinResult<T, S>> {
    this.root.addStoreAccess(store.name, false);
    return new QueryStreamKeyJoin<T, S>(this, store.name, keyFn);
  }

  filter(f: (x: any) => boolean): QueryStream<T> {
    return new QueryStreamFilter(this, f);
  }

  toArray(): Promise<T[]> {
    const {resolve, promise} = openPromise();
    const values: T[] = [];

    this.subscribe((isDone, value) => {
      if (isDone) {
        resolve(values);
        return;
      }
      values.push(value);
    });

    return Promise.resolve()
                  .then(() => this.root.finish())
                  .then(() => promise);
  }

  reduce<A>(f: (x: any, acc?: A) => A, init?: A): Promise<any> {
    const {resolve, promise} = openPromise();
    let acc = init;

    this.subscribe((isDone, value) => {
      if (isDone) {
        resolve(acc);
        return;
      }
      acc = f(value, acc);
    });

    return Promise.resolve()
                  .then(() => this.root.finish())
                  .then(() => promise);
  }
}

type FilterFn = (e: any) => boolean;
type SubscribeFn = (done: boolean, value: any, tx: IDBTransaction) => void;
type SubscribeOneFn = (value: any, tx: IDBTransaction) => void;

class QueryStreamFilter<T> extends QueryStreamBase<T> {
  constructor(public s: QueryStreamBase<T>, public filterFn: FilterFn) {
    super(s.root);
  }

  subscribe(f: SubscribeFn) {
    this.s.subscribe((isDone, value, tx) => {
      if (isDone) {
        f(true, undefined, tx);
        return;
      }
      if (this.filterFn(value)) {
        f(false, value, tx);
      }
    });
  }
}


class QueryStreamFlatMap<T, S> extends QueryStreamBase<S> {
  constructor(public s: QueryStreamBase<T>, public flatMapFn: (v: T) => S[]) {
    super(s.root);
  }

  subscribe(f: SubscribeFn) {
    this.s.subscribe((isDone, value, tx) => {
      if (isDone) {
        f(true, undefined, tx);
        return;
      }
      const values = this.flatMapFn(value);
      for (const v in values) {
        f(false, v, tx);
      }
    });
  }
}


class QueryStreamMap<S, T> extends QueryStreamBase<T> {
  constructor(public s: QueryStreamBase<S>, public mapFn: (v: S) => T) {
    super(s.root);
  }

  subscribe(f: SubscribeFn) {
    this.s.subscribe((isDone, value, tx) => {
      if (isDone) {
        f(true, undefined, tx);
        return;
      }
      const mappedValue = this.mapFn(value);
      f(false, mappedValue, tx);
    });
  }
}


class QueryStreamIndexJoin<T, S> extends QueryStreamBase<JoinResult<T, S>> {
  constructor(public s: QueryStreamBase<T>, public storeName: string, public indexName: string,
              public key: any) {
    super(s.root);
  }

  subscribe(f: SubscribeFn) {
    this.s.subscribe((isDone, value, tx) => {
      if (isDone) {
        f(true, undefined, tx);
        return;
      }
      const s = tx.objectStore(this.storeName).index(this.indexName);
      const req = s.openCursor(IDBKeyRange.only(this.key(value)));
      req.onsuccess = () => {
        const cursor = req.result;
        if (cursor) {
          f(false, {left: value, right: cursor.value}, tx);
          cursor.continue();
        }
      };
    });
  }
}


class QueryStreamIndexJoinLeft<T, S> extends QueryStreamBase<JoinLeftResult<T, S>> {
  constructor(public s: QueryStreamBase<T>, public storeName: string, public indexName: string,
              public key: any) {
    super(s.root);
  }

  subscribe(f: SubscribeFn) {
    this.s.subscribe((isDone, value, tx) => {
      if (isDone) {
        f(true, undefined, tx);
        return;
      }
      const s = tx.objectStore(this.storeName).index(this.indexName);
      const req = s.openCursor(IDBKeyRange.only(this.key(value)));
      let gotMatch = false;
      req.onsuccess = () => {
        const cursor = req.result;
        if (cursor) {
          gotMatch = true;
          f(false, {left: value, right: cursor.value}, tx);
          cursor.continue();
        } else {
          if (!gotMatch) {
            f(false, {left: value}, tx);
          }
        }
      };
    });
  }
}


class QueryStreamKeyJoin<T, S> extends QueryStreamBase<JoinResult<T, S>> {
  constructor(public s: QueryStreamBase<T>, public storeName: string,
              public key: any) {
    super(s.root);
  }

  subscribe(f: SubscribeFn) {
    this.s.subscribe((isDone, value, tx) => {
      if (isDone) {
        f(true, undefined, tx);
        return;
      }
      const s = tx.objectStore(this.storeName);
      const req = s.openCursor(IDBKeyRange.only(this.key(value)));
      req.onsuccess = () => {
        const cursor = req.result;
        if (cursor) {
          f(false, {left: value, right: cursor.value}, tx);
          cursor.continue();
        } else {
          f(true, undefined, tx);
        }
      };
    });
  }
}


class IterQueryStream<T> extends QueryStreamBase<T> {
  private storeName: string;
  private options: any;
  private subscribers: SubscribeFn[];

  constructor(qr: QueryRoot, storeName: string, options: any) {
    super(qr);
    this.options = options;
    this.storeName = storeName;
    this.subscribers = [];

    const doIt = (tx: IDBTransaction) => {
      const {indexName = void 0, only = void 0} = this.options;
      let s: any;
      if (indexName !== void 0) {
        s = tx.objectStore(this.storeName)
              .index(this.options.indexName);
      } else {
        s = tx.objectStore(this.storeName);
      }
      let kr: IDBKeyRange | undefined;
      if (only !== undefined) {
        kr = IDBKeyRange.only(this.options.only);
      }
      const req = s.openCursor(kr);
      req.onsuccess = () => {
        const cursor: IDBCursorWithValue = req.result;
        if (cursor) {
          for (const f of this.subscribers) {
            f(false, cursor.value, tx);
          }
          cursor.continue();
        } else {
          for (const f of this.subscribers) {
            f(true, undefined, tx);
          }
        }
      };
    };

    this.root.addWork(doIt);
  }

  subscribe(f: SubscribeFn) {
    this.subscribers.push(f);
  }
}


/**
 * Root wrapper around an IndexedDB for queries with a fluent interface.
 */
export class QueryRoot implements PromiseLike<void> {
  private work: Array<((t: IDBTransaction) => void)> = [];
  private stores = new Set();
  private kickoffPromise: Promise<void>;

  /**
   * Some operations is a write operation,
   * and we need to do a "readwrite" transaction/
   */
  private hasWrite: boolean;

  private finishScheduled: boolean;

  private finished: boolean = false;

  constructor(public db: IDBDatabase) {
  }

  then<R>(onfulfilled: (value: void) => R | PromiseLike<R>,
          onrejected: (reason: any) => R | PromiseLike<R>): PromiseLike<R> {
    return this.finish().then(onfulfilled, onrejected);
  }

  private checkFinished() {
    if (this.finished) {
      throw Error("Can't add work to query after it was started");
    }
  }

  /**
   * Get a stream of all objects in the store.
   */
  iter<T>(store: Store<T>): QueryStream<T> {
    this.checkFinished();
    this.stores.add(store.name);
    this.scheduleFinish();
    return new IterQueryStream<T>(this, store.name, {});
  }

  /**
   * Count the number of objects in a store.
   */
  count<T>(store: Store<T>): Promise<number> {
    this.checkFinished();
    const {resolve, promise} = openPromise();

    const doCount = (tx: IDBTransaction) => {
      const s = tx.objectStore(store.name);
      const req = s.count();
      req.onsuccess = () => {
        resolve(req.result);
      };
    };

    this.addWork(doCount, store.name, false);
    return Promise.resolve()
                  .then(() => this.finish())
                  .then(() => promise);

  }

  /**
   * Delete all objects in a store that match a predicate.
   */
  deleteIf<T>(store: Store<T>, predicate: (x: T, n: number) => boolean): QueryRoot {
    this.checkFinished();
    const doDeleteIf = (tx: IDBTransaction) => {
      const s = tx.objectStore(store.name);
      const req = s.openCursor();
      let n = 0;
      req.onsuccess = () => {
        const cursor: IDBCursorWithValue = req.result;
        if (cursor) {
          if (predicate(cursor.value, n++)) {
            cursor.delete();
          }
          cursor.continue();
        }
      };
    };
    this.addWork(doDeleteIf, store.name, true);
    return this;
  }

  iterIndex<S extends IDBValidKey, T>(index: Index<S, T>,
                                      only?: S): QueryStream<T> {
    this.checkFinished();
    this.stores.add(index.storeName);
    this.scheduleFinish();
    return new IterQueryStream<T>(this, index.storeName, {
      indexName: index.indexName,
      only,
    });
  }

  /**
   * Put an object into the given object store.
   * Overrides if an existing object with the same key exists
   * in the store.
   */
  put<T>(store: Store<T>, val: T): QueryRoot {
    this.checkFinished();
    const doPut = (tx: IDBTransaction) => {
      tx.objectStore(store.name).put(val);
    };
    this.scheduleFinish();
    this.addWork(doPut, store.name, true);
    return this;
  }


  putWithResult<T>(store: Store<T>, val: T): Promise<IDBValidKey> {
    this.checkFinished();
    const {resolve, promise} = openPromise();
    const doPutWithResult = (tx: IDBTransaction) => {
      const req = tx.objectStore(store.name).put(val);
      req.onsuccess = () => {
        resolve(req.result);
      };
      this.scheduleFinish();
    };
    this.addWork(doPutWithResult, store.name, true);
    return Promise.resolve()
                  .then(() => this.finish())
                  .then(() => promise);
  }


  /**
   * Get, modify and store an element inside a transaction.
   */
  mutate<T>(store: Store<T>, key: any, f: (v: T) => T): QueryRoot {
    this.checkFinished();
    const doPut = (tx: IDBTransaction) => {
      const reqGet = tx.objectStore(store.name).get(key);
      reqGet.onsuccess = () => {
        const r = reqGet.result;
        let m: T;
        try {
          m = f(r);
        } catch (e) {
          if (e === AbortTransaction) {
            tx.abort();
            return;
          }
          throw e;
        }

        tx.objectStore(store.name).put(m);
      };
    };
    this.scheduleFinish();
    this.addWork(doPut, store.name, true);
    return this;
  }


  /**
   * Add all object from an iterable to the given object store.
   * Fails if the object's key is already present
   * in the object store.
   */
  putAll<T>(store: Store<T>, iterable: T[]): QueryRoot {
    this.checkFinished();
    const doPutAll = (tx: IDBTransaction) => {
      for (const obj of iterable) {
        tx.objectStore(store.name).put(obj);
      }
    };
    this.scheduleFinish();
    this.addWork(doPutAll, store.name, true);
    return this;
  }

  /**
   * Add an object to the given object store.
   * Fails if the object's key is already present
   * in the object store.
   */
  add<T>(store: Store<T>, val: T): QueryRoot {
    this.checkFinished();
    const doAdd = (tx: IDBTransaction) => {
      tx.objectStore(store.name).add(val);
    };
    this.scheduleFinish();
    this.addWork(doAdd, store.name, true);
    return this;
  }

  /**
   * Get one object from a store by its key.
   */
  get<T>(store: Store<T>, key: any): Promise<T|undefined> {
    this.checkFinished();
    if (key === void 0) {
      throw Error("key must not be undefined");
    }

    const {resolve, promise} = openPromise();

    const doGet = (tx: IDBTransaction) => {
      const req = tx.objectStore(store.name).get(key);
      req.onsuccess = () => {
        resolve(req.result);
      };
    };

    this.addWork(doGet, store.name, false);
    return Promise.resolve()
                  .then(() => this.finish())
                  .then(() => promise);
  }

  /**
   * Get one object from a store by its key.
   */
  getIndexed<I extends IDBValidKey, T>(index: Index<I, T>,
                                       key: I): Promise<T|undefined> {
    this.checkFinished();
    if (key === void 0) {
      throw Error("key must not be undefined");
    }

    const {resolve, promise} = openPromise<void>();

    const doGetIndexed = (tx: IDBTransaction) => {
      const req = tx.objectStore(index.storeName)
                    .index(index.indexName)
                    .get(key);
      req.onsuccess = () => {
        resolve(req.result);
      };
    };

    this.addWork(doGetIndexed, index.storeName, false);
    return Promise.resolve()
                  .then(() => this.finish())
                  .then(() => promise);
  }

  private scheduleFinish() {
    if (!this.finishScheduled) {
      Promise.resolve().then(() => this.finish());
      this.finishScheduled = true;
    }
  }

  /**
   * Finish the query, and start the query in the first place if necessary.
   */
  finish(): Promise<void> {
    if (this.kickoffPromise) {
      return this.kickoffPromise;
    }
    this.kickoffPromise = new Promise<void>((resolve, reject) => {
      // At this point, we can't add any more work
      this.finished = true;
      if (this.work.length === 0) {
        resolve();
        return;
      }
      const mode = this.hasWrite ? "readwrite" : "readonly";
      const tx = this.db.transaction(Array.from(this.stores), mode);
      tx.oncomplete = () => {
        resolve();
      };
      tx.onabort = () => {
        reject(Error("transaction aborted"));
      };
      for (const w of this.work) {
        w(tx);
      }
    });
    return this.kickoffPromise;
  }

  /**
   * Delete an object by from the given object store.
   */
  delete(storeName: string, key: any): QueryRoot {
    this.checkFinished();
    const doDelete = (tx: IDBTransaction) => {
      tx.objectStore(storeName).delete(key);
    };
    this.scheduleFinish();
    this.addWork(doDelete, storeName, true);
    return this;
  }

  /**
   * Low-level function to add a task to the internal work queue.
   */
  addWork(workFn: (t: IDBTransaction) => void,
          storeName?: string,
          isWrite?: boolean) {
    this.work.push(workFn);
    if (storeName) {
      this.addStoreAccess(storeName, isWrite);
    }
  }

  addStoreAccess(storeName: string, isWrite?: boolean) {
    if (storeName) {
      this.stores.add(storeName);
    }
    if (isWrite) {
      this.hasWrite = true;
    }
  }
}