npm package: @nova-odm/mapper
Description: A schema-based data mapper for Amazon DynamoDB
Author: Arseny Yankovski
Latest version: 0.8.7

# Nova ODM / Amazon DynamoDB DataMapper

[![Apache 2 License](https://img.shields.io/github/license/ArsenyYankovsky/nova-odm.svg?style=flat)](https://www.apache.org/licenses/LICENSE-2.0)

This library provides a `DataMapper` class that allows easy interoperability
between your application's domain classes and their persisted form in Amazon
DynamoDB. Powered by the `@nova-odm/marshaller` and
`@nova-odm/expressions` packages, using `DataMapper` lets you define each
object's persisted representation once and then load, save, scan, and query your
tables using the vocabulary of your application domain rather than its
representation in DynamoDB.

## Getting started

To use the `DataMapper` with a given JavaScript class, you will need to add a
couple properties to the prototype of the class you would like to map to a
DynamoDB table. Specifically, you will need to provide a schema and the name of
the table:

```typescript
import {DynamoDbSchema, DynamoDbTable} from '@nova-odm/mapper';

class MyDomainModel {
    // declare methods and properties as normal
}

Object.defineProperties(MyDomainModel.prototype, {
    [DynamoDbTable]: {
        value: 'MyTable'
    },
    [DynamoDbSchema]: {
        value: {
            id: {
                type: 'String',
                keyType: 'HASH'
            },
            foo: {type: 'String'},
            bar: {
                type: 'Set',
                memberType: 'String',
            },
            baz: {
                type: 'Tuple',
                members: [
                    {type: 'Boolean'},
                    {type: 'String'},
                ],
            },
        },
    },
});
```

The schema and table name may be declared as property accessors directly on the
class if the value should be determined dynamically:

```typescript
import {DynamoDbTable} from '@nova-odm/mapper';

class MyOtherDomainClass {
    id: number;

    get [DynamoDbTable]() {
        return this.id % 2 === 0 ? 'evens' : 'odds';
    }
}
```

Next, create an instance of `DataMapper` and use the `MyDomainClass` constructor
defined above to save and load objects from DynamoDB:

```typescript
import {
    DataMapper,
    DynamoDbSchema,
    DynamoDbTable,
} from '@nova-odm/mapper';
import { DynamoDBClient } from '@aws-sdk/client-dynamodb';

const client = new DynamoDBClient({region: 'us-west-2'});
const mapper = new DataMapper({client});

class MyDomainModel {
    id: string;
    foo?: string;
    bar?: Set<string>;
    baz?: [boolean, string];
}

Object.defineProperties(MyDomainModel.prototype, {
    [DynamoDbTable]: {
        value: 'MyTable'
    },
    [DynamoDbSchema]: {
        value: {
            id: {
                type: 'String',
                keyType: 'HASH'
            },
            foo: {type: 'String'},
            bar: {
                type: 'Set',
                memberType: 'String',
            },
            baz: {
                type: 'Tuple',
                members: [
                    {type: 'Boolean'},
                    {type: 'String'},
                ],
            },
        },
    },
});

// delete an object
const toDelete = new MyDomainModel();
toDelete.id = 'DELETE_ME';
mapper.delete(toDelete);

// if that's too verbose, you can write the above as a single expression with
// Object.assign:
mapper.delete(Object.assign(new MyDomainModel(), {id: 'DELETE_ME'}));

// fetch an object
const toGet = new MyDomainModel();
toGet.id = 'ID_TO_FETCH';
const fetched = await mapper.get(toGet);

// this should return a rejected promise, as it's fetching an object that does
// not exist
mapper.get(toDelete)
    .catch(err => console.log('I expected this to happen'));

// put something new into the database
const toPut = new MyDomainModel();
toPut.id = 'NEW_RECORD';
toPut.foo = 'bar';
toPut.bar = new Set<string>(['fizz', 'buzz', 'pop']);
toPut.baz = [true, 'quux'];

mapper.put(toPut).then((persisted: MyDomainModel) => {
    // now change the record a bit
    const toUpdate = new MyDomainModel();
    toUpdate.id = persisted.id;
    toUpdate.baz = [false, 'beep'];
    return mapper.update(toUpdate, {onMissing: 'skip'});
});
```

## Supported operations

### `batchDelete`

Deletes any number of items from one or more tables in batches of 25 or fewer
items. Unprocessed deletes are retried following an exponentially increasing
backoff delay that is applied on a per-table basis.

Returns an async iterable of items that have been deleted (deleted items are
yielded when the delete has been accepted by DynamoDB). The results can be
consumed with a `for-await-of` loop. If you are using TypeScript, you will need
to include `esnext.asynciterable` in your `lib` declaration (as well as enabling
`downlevelIteration` if targeting ES5 or lower). Please refer to [the TypeScript
release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-3.html#async-iteration)
for more information.

Takes one parameter:

* An iterable (synchronous or asynchronous) of items to delete. Each item must
    be an instance of a class with a table name accessible via a property
    identified with the `DynamoDbTable` symbol and a schema accessible via a
    property identified with the `DynamoDbSchema` symbol.

### `batchGet`

Fetches any number of items from one or more tables in batches of 100 or fewer
items. Unprocessed reads are retried following an exponentially increasing
backoff delay that is applied on a per-table basis.

Takes two parameters:

* An iterable (synchronous or asynchronous) of items to fetch. Each item must be
    an instance of a class with a table name accessible via a property
    identified with the `DynamoDbTable` symbol and a schema accessible via a
    property identified with the `DynamoDbSchema` symbol.

* (Optional) An object specifying any of the following options:

    * `readConsistency` - Specify `'strong'` to perform a strongly consistent
        read. Specify `'eventual'` (the default) to perform an eventually
        consistent read.

    * `perTableOptions` - An object whose keys are table names and whose values
        are objects specifying any of the following options:

        * `readConsistency` - Specify `'strong'` to perform a strongly
            consistent read. Specify `'eventual'` (the default) to perform an
            eventually consistent read.

        * `projection` - A projection expression directing DynamoDB to return a
            subset of the fetched item's attributes. Please refer to the
            documentation for the `@nova-odm/expressions` package for
            guidance on creating projection expression objects.

        * `projectionSchema` - The schema to use when mapping the supplied
            `projection` option to the attribute names used in DynamoDB.

            This parameter is only necessary if a batch contains items from
            multiple classes that map to the *same* table using *different*
            property names to represent the same DynamoDB attributes.

            If not supplied, the schema associated with the first item
            associated with a given table will be used in its place.

### `batchPut`

Puts any number of items to one or more tables in batches of 25 or fewer items.
Unprocessed puts are retried following an exponentially increasing backoff delay
that is applied on a per-table basis.

Returns an async iterable of items that have been put (put items are yielded
when the put has been accepted by DynamoDB). The results can be consumed with a
`for-await-of` loop. If you are using TypeScript, you will need to include
`esnext.asynciterable` in your `lib` declaration (as well as enabling
`downlevelIteration` if targeting ES5 or lower). Please refer to [the TypeScript
release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-3.html#async-iteration)
for more information.

Takes one parameter:

* An iterable (synchronous or asynchronous) of items to put. Each item must be
    an instance of a class with a table name accessible via a property
    identified with the `DynamoDbTable` symbol and a schema accessible via a
    property identified with the `DynamoDbSchema` symbol.

### `batchWrite`

Puts or deletes any number of items to one or more tables in batches of 25 or
fewer items. Unprocessed writes are retried following an exponentially
increasing backoff delay that is applied on a per-table basis.

Returns an async iterable of tuples of the string 'put'|'delete' and the item on
which the specified write action was performed. The results can be consumed with
a `for-await-of` loop. If you are using TypeScript, you will need to include
`esnext.asynciterable` in your `lib` declaration (as well as enabling
`downlevelIteration` if targeting ES5 or lower). Please refer to [the TypeScript
release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-3.html#async-iteration)
for more information.

Takes one parameter:

* An iterable (synchronous or asynchronous) of tuples of the string
    'put'|'delete' and the item on which to perform the specified write action.
    Each item must be an instance of a class with a table name accessible via a
    property identified with the `DynamoDbTable` symbol and a schema accessible
    via a property identified with the `DynamoDbSchema` symbol.

### `delete`

Removes an item from a DynamoDB table. Takes two parameters:

* The item to be deleted. Must be an instance of a class with a table name
    accessible via a property identified with the `DynamoDbTable` symbol and a
    schema accessible via a property identified with the `DynamoDbSchema`
    symbol.

* (Optional) An object specifying any of the following options:

    * `condition` - A condition expression whose assertion must be satisfied in
        order for the delete operation to be executed. Please refer to the
        documentation for the `@nova-odm/expressions` package for guidance
        on creating condition expression objects.

    * `returnValues` - Specify `'ALL_OLD'` to have the deleted item returned to
        you when the delete operation completes.

    * `skipVersionCheck` - Whether to forgo creating a condition expression
        based on a defined `versionAttribute` in the schema.

### `get`

Fetches an item from a DynamoDB table. If no item with the specified key was
found, the returned promise will be rejected with an error. Takes two
parameters:

* The item to be fetched. Must be an instance of a class with a table name
    accessible via a property identified with the `DynamoDbTable` symbol and a
    schema accessible via a property identified with the `DynamoDbSchema`
    symbol.

    The supplied item will **NOT** be updated in place. Rather, a new item of
    the same class with data from the DynamoDB table will be returned.

* (Optional) An object specifying any of the following options:

    * `readConsistency` - Specify `'strong'` to perform a strongly consistent
        read. Specify `'eventual'` (the default) to perform an eventually
        consistent read.

    * `projection` - A projection expression directing DynamoDB to return a
        subset of the fetched item's attributes. Please refer to the
        documentation for the `@nova-odm/expressions` package for guidance
        on creating projection expression objects.

### `put`

Inserts an item into a DynamoDB table. Takes two parameters:

* The item to be inserted. Must be an instance of a class with a table name
    accessible via a property identified with the `DynamoDbTable` symbol and a
    schema accessible via a property identified with the `DynamoDbSchema`
    symbol.

* (Optional) An object specifying any of the following options:

    * `condition` - A condition expression whose assertion must be satisfied in
        order for the put operation to be executed. Please refer to the
        documentation for the `@nova-odm/expressions` package for guidance
        on creating condition expression objects.

    * `returnValues` - Specify `'ALL_OLD'` to have the overwritten item (if one
        existed) returned to you when the put operation completes.

    * `skipVersionCheck` - Whether to forgo creating a condition expression
        based on a defined `versionAttribute` in the schema.

### `query`

Retrieves multiple values from a table or index based on the primary key
attributes. Queries must target a single partition key value but may read
multiple items with different range keys.

This method is implemented as an async iterator and the results can be consumed
with a `for-await-of` loop. If you are using TypeScript, you will need to
include `esnext.asynciterable` in your `lib` declaration (as well as enabling
`downlevelIteration` if targeting ES5 or lower). Please refer to [the TypeScript
release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-3.html#async-iteration)
for more information.

Takes three parameters:

* The constructor function to use for any results returned by this operation.
    Must have a prototype with a table name accessible via a property identified
    with the `DynamoDbTable` symbol and a schema accessible via a property
    identified with the `DynamoDbSchema` symbol.

* The condition that specifies the key value(s) for items to be retrieved by the
    query operation. You may provide a hash matching key properties to the
    values they must equal, a hash matching keys to
    `ConditionExpressionPredicate`s, or a fully composed `ConditionExpression`.
    If a hash is provided, it may contain a mixture of condition expression
    predicates and exact value matches:

    ```typescript
    import {between} from '@nova-odm/expressions';

    const keyCondition = {
        partitionKey: 'foo',
        rangeKey: between(10, 99),
    };
    ```

    The key condition must target a single value for the partition key.

    Please refer to the documentation for the `@nova-odm/expressions`
    package for guidance on creating condition expression objects.

* (Optional) An object specifying any of the following options:

    * `filter` - A condition expression that DynamoDB applies after the Query
        operation, but before the data is returned to you. Items that do not
        satisfy the `filter` criteria are not returned.

        You cannot define a filter expression based on a partition key or a sort
        key.

        Please refer to the documentation for the `@nova-odm/expressions`
        package for guidance on creating condition expression objects.

    * `indexName` - The name of the index against which to execute this query.
        If not specified, the query will be executed against the base table.

    * `limit` - The maximum number of items to return.

    * `pageSize` - The maximum number of items to return **per page of results**.

    * `projection` - A projection expression directing DynamoDB to return a
        subset of any fetched item's attributes. Please refer to the
        documentation for the `@nova-odm/expressions` package for guidance
        on creating projection expression objects.

    * `readConsistency` - Specify `'strong'` to perform a strongly consistent
        read. Specify `'eventual'` (the default) to perform an eventually
        consistent read.

    * `scanIndexForward` - Specifies the order for index traversal: If true, the
        traversal is performed in ascending order; if false, the traversal is
        performed in descending order.

    * `startKey` - The primary key of the first item that this operation will
        evaluate.

#### Query metadata

The iterator returned by `query` will keep track of the number of items yielded
and the number of items scanned via its `count` and `scannedCount` properties:

```typescript
const iterator = mapper.query(
    MyClass, 
    {partitionKey: 'foo', rangeKey: between(0, 10)}
);
for await (const record of iterator) {
    console.log(record, iterator.count, iterator.scannedCount);
}
```

#### Pagination

If you wish to perform a resumable query, you can use the `.pages()` method of
the iterator returned by `query` to access the underlying paginator. The
paginator differs from the iterator in that it yields arrays of unmarshalled
records and has a `lastEvaluatedKey` property that may be provided to a new
call to `mapper.query` to resume the query later or in a separate process:

```typescript
const paginator = mapper.query(
    MyClass,
    {partitionKey: 'foo', rangeKey: between(0, 10)},
    {
        // automatically stop after 25 items or the entire result set has been
        // fetched, whichever is smaller
        limit: 25
    }
).pages();

for await (const page of paginator) {
    console.log(
        paginator.count,
        paginator.scannedCount,
        paginator.lastEvaluatedKey
    );
}

const newPaginator = mapper.query(
    MyClass,
    {partitionKey: 'foo', rangeKey: between(0, 10)},
    {
        // start this new paginator where the previous one stopped
        startKey: paginator.lastEvaluatedKey
    }
).pages();
```

### `scan`

Retrieves all values in a table or index.

This method is implemented as an async iterator and the results can be consumed
with a `for-await-of` loop. If you are using TypeScript, you will need to
include `esnext.asynciterable` in your `lib` declaration (as well as enabling
`downlevelIteration` if targeting ES5 or lower). Please refer to [the TypeScript
release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-3.html#async-iteration)
for more information.

Takes two parameters:

* The constructor function to use for any results returned by this operation.
    Must have a prototype with a table name accessible via a property identified
    with the `DynamoDbTable` symbol and a schema accessible via a property
    identified with the `DynamoDbSchema` symbol.

* (Optional) An object specifying any of the following options:

    * `filter` - A condition expression that DynamoDB applies after the scan
        operation, but before the data is returned to you. Items that do not
        satisfy the `filter` criteria are not returned.

        You cannot define a filter expression based on a partition key or a sort
        key.

        Please refer to the documentation for the `@nova-odm/expressions`
        package for guidance on creating condition expression objects.

    * `indexName` - The name of the index against which to execute this query.
        If not specified, the query will be executed against the base table.

    * `limit` - The maximum number of items to return.

    * `pageSize` - The maximum number of items to return **per page of results**.

    * `projection` - A projection expression directing DynamoDB to return a
        subset of any fetched item's attributes. Please refer to the
        documentation for the `@nova-odm/expressions` package for guidance
        on creating projection expression objects.

    * `readConsistency` - Specify `'strong'` to perform a strongly consistent
        read. Specify `'eventual'` (the default) to perform an eventually
        consistent read.

    * `segment` - The identifier for this segment (if this scan is being
        performed as part of a parallel scan operation).

    * `startKey` - The primary key of the first item that this operation will
        evaluate.

    * `totalSegments` - The number of segments into which this scan has been
        divided (if this scan is being performed as part of a parallel scan
        operation).

#### Scan metadata

The iterator returned by `scan` will keep track of the number of items yielded
and the number of items scanned via its `count` and `scannedCount` properties:

```typescript
const iterator = mapper.scan(MyClass);
for await (const record of iterator) {
    console.log(record, iterator.count, iterator.scannedCount);
}
```

#### Pagination

If you wish to perform a resumable scan, you can use the `.pages()` method of
the iterator returned by `scan` to access the underlying paginator. The
paginator differs from the iterator in that it yields arrays of unmarshalled
records and has a `lastEvaluatedKey` property that may be provided to a new
call to `mapper.scan` to resume the scan later or in a separate process:

```typescript
const paginator = mapper.scan(
    MyClass,
    {
        // automatically stop after 25 items or the entire result set has been
        // fetched, whichever is smaller
        limit: 25
    }
).pages();
for await (const page of paginator) {
    console.log(
        paginator.count,
        paginator.scannedCount,
        paginator.lastEvaluatedKey
    );
}

const newPaginator = mapper.scan(
    MyClass,
    {
        // start this new paginator where the previous one stopped
        startKey: paginator.lastEvaluatedKey
    }
).pages();
```

### `parallelScan`

Retrieves all values in a table by dividing the table into segments, all of
which are scanned in parallel.

This method is implemented as an async iterator and the results can be consumed
with a `for-await-of` loop. If you are using TypeScript, you will need to
include `esnext.asynciterable` in your `lib` declaration (as well as enabling
`downlevelIteration` if targeting ES5 or lower). Please refer to [the TypeScript
release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-3.html#async-iteration)
for more information.

Takes three parameters:

* The constructor to use for any results returned by this operation. Must have a
    prototype with a table name accessible via a property identified with the
    `DynamoDbTable` symbol and a schema accessible via a property identified
    with the `DynamoDbSchema` symbol.

* The total number of parallel workers to use to scan the table.

* (Optional) An object specifying any of the following options:

    * `filter` - A condition expression that DynamoDB applies after the scan
        operation, but before the data is returned to you. Items that do not
        satisfy the `filter` criteria are not returned.

        You cannot define a filter expression based on a partition key or a sort
        key.

        Please refer to the documentation for the `@nova-odm/expressions`
        package for guidance on creating condition expression objects.

    * `indexName` - The name of the index against which to execute this query.
        If not specified, the query will be executed against the base table.

    * `pageSize` - The maximum number of items to return **per page of results**.

    * `projection` - A projection expression directing DynamoDB to return a
        subset of any fetched item's attributes. Please refer to the
        documentation for the `@nova-odm/expressions` package for guidance
        on creating projection expression objects.

    * `readConsistency` - Specify `'strong'` to perform a strongly consistent
        read. Specify `'eventual'` (the default) to perform an eventually
        consistent read.

    * `startKey` - The primary key of the first item that this operation will
        evaluate.

#### Scan metadata

The iterator returned by `parallelScan` will keep track of the number of items
yielded and the number of items scanned via its `count` and `scannedCount`
properties:

```typescript
const iterator = mapper.parallelScan(MyClass, 4);
for await (const record of iterator) {
    console.log(record, iterator.count, iterator.scannedCount);
}
```

#### Pagination

If you wish to perform a resumable parallel scan, you can use the `.pages()`
method of the iterator returned by `parallelScan` to access the underlying
paginator. The  paginator differs from the iterator in that it yields arrays of
unmarshalled records and has a `scanState` property that may be provided
to a new call to `mapper.parallelScan` to resume the scan later or in a separate
process:

```typescript
const paginator = mapper.parallelScan(
    MyClass,
    4
).pages();
for await (const page of paginator) {
    console.log(
        paginator.count,
        paginator.scannedCount,
        paginator.lastEvaluatedKey
    );

    break;
}

const newPaginator = mapper.parallelScan(
    MyClass,
    4,
    {
        // start this new paginator where the previous one stopped
        scanState: paginator.scanState
    }
).pages();
```

### `update`

Updates an item in a DynamoDB table. Will leave attributes not defined in the
schema in place.

Takes two parameters:

* The item with its desired property state. Must be an instance of a class with
    a table name accessible via a property identified with the `DynamoDbTable`
    symbol and a schema accessible via a property identified with the
    `DynamoDbSchema` symbol.

* (Optional) An object specifying any of the following options:

    * `condition` - A condition expression whose assertion must be satisfied in
        order for the update operation to be executed. Please refer to the
        documentation for the `@nova-odm/expressions` package for guidance
        on creating condition expression objects.

    * `onMissing` - Specify `'remove'` (the default) to treat the absence of a
        value in the supplied `item` as a directive to remove the property from
        the record in DynamoDB. Specify `'skip'` to only update the properties
        that are defined in the supplied `item`.

    * `skipVersionCheck` - Whether to forgo creating a condition expression
        based on a defined `versionAttribute` in the schema.

### `executeUpdateExpression`

Executes a custom update expression. This method will **not** automatically
apply a version check, as the current state of the object being updated is not
known.

Takes four parameters:

* The expression to execute. Please refer to the documentation for the
    `@nova-odm/expressions` package for guidance on creating update
    expression objects.

* The key of the item being updated.

* The constructor for the class mapped to the table against which the expression
    should be run. Must have a prototype with a table name accessible via a
    property identified with the `DynamoDbTable` symbol and a schema accessible
    via a property identified with the `DynamoDbSchema` symbol.

* (Optional) An object specifying any of the following options:

    * `condition` - A condition expression whose assertion must be satisfied in
        order for the update operation to be executed. Please refer to the
        documentation for the `@nova-odm/expressions` package for guidance
        on creating condition expression objects.