NB!

This module is a fork of (mongo-cursor-pagination)[https://github.com/mixmaxhq/mongo-cursor-pagination] by MixMax. The fork was needed to keep supporting
Node.js v6 as the upstream version started using async..await.

Use the following command to install this module:

npm install mongo-cursor-pagination-node6 --save

And then in your code:

const MongoPaging = require('mongo-cursor-pagination-node6');

mongo-cursor-pagination

This module aids in implementing "cursor-based" pagination using Mongo range queries or relevancy-based search results. **This module is currently used in
production for the Mixmax API to return millions of results a day**.

Background

See this blog post for background on why this library was built.

API Pagination is typically implemented one of two different ways:

1. Offset-based paging. This is traditional paging where skip and limit parameters are passed on the url (or some variation such as page_num and count).
The API would return the results and some indication of whether there is a next page, such as has_more on the response. An issue with this approach is that
it assumes a static data set; if collection changes while querying, then results in pages will shift and the response will be wrong.

2. Cursor-based paging. An improved way of paging where an API passes back a "cursor" (an opaque string) to tell the caller where to query the next or previous
pages. The cursor is usually passed using query parameters next and previous. It's implementation is typically more performant that skip/limit because it
can jump to any page without traversing all the records. It also handles records being added or removed because it doesn't use fixed offsets.

This module helps in implementing #2 - cursor based paging - by providing a method that make it easy to query within a Mongo collection. It also helps by
returning a url-safe string that you can return with your HTTP response (see example below).

Here are some examples of cursor-based APIs:

* Twitter
* Stripe
* Facebook

Install

npm install mongo-cursor-pagination --save

Usage

$3

Find will return ordered and paged results based on a field (paginatedField) that you pass in.

Call find() with the following parameters:

``
Performs a find() query on a passed-in Mongo collection, using criteria you specify. The results
are ordered by the paginatedField.

@param {MongoCollection} collection A collection object returned from the MongoDB library's
db.collection() method.
@param {Object} params
-query {Object} The find query.
-limit {Number} The page size. Must be between 1 and config.MAX_LIMIT.
-fields {Object} Fields to query in the Mongo object format, e.g. {_id: 1, timestamp :1}.
The default is to query all fields.
-paginatedField {String} The field name to query the range for. The field must be:
1. Orderable. We must sort by this value. If duplicate values for paginatedField field
exist, the results will be secondarily ordered by the _id.
2. Indexed. For large collections, this should be indexed for query performance.
3. Immutable. If the value changes between paged queries, it could appear twice.
4. Complete. A value must exist for all documents.
The default is to use the Mongo built-in '_id' field, which satisfies the above criteria.
The only reason to NOT use the Mongo _id field is if you chose to implement your own ids.
-sortAscending {Boolean} True to sort using paginatedField ascending (default is false - descending).
-next {String} The value to start querying the page.
-previous {String} The value to start querying previous page.
@param {Function} done Node errback style function.
`

Example:

`js
var MongoClient = require('mongodb').MongoClient;
var MongoPaging = require('mongo-cursor-pagination');

MongoClient.connect('mongodb://localhost:27017/mydb', (err, db) => {
db.collection('myobjects').insertMany(
[
{
counter: 1
},
{
counter: 2
},
{
counter: 3
},
{
counter: 4
}
],
err => {
// Query the first page.
MongoPaging.find(
db.collection('myobjects'),
{
limit: 2
},
(err, result) => {
console.log(result);

// Query next page.
MongoPaging.find(
db.collection('myobjects'),
{
limit: 2,
next: result.next // This queries the next page
},
(err, result) => {
console.log(result);
}
);
}
);
}
);
});
`

Output:

`sh
page 1 { results:
[ { _id: 580fd16aca2a6b271562d8bb, counter: 4 },
{ _id: 580fd16aca2a6b271562d8ba, counter: 3 } ],
next: 'eyIkb2lkIjoiNTgwZmQxNmFjYTJhNmIyNzE1NjJkOGJhIn0',
hasMore: true }
page 2 { results:
[ { _id: 580fd16aca2a6b271562d8b9, counter: 2 },
{ _id: 580fd16aca2a6b271562d8b8, counter: 1 } ],
previous: 'eyIkb2lkIjoiNTgwZmQxNmFjYTJhNmIyNzE1NjJkOGI5In0',
next: 'eyIkb2lkIjoiNTgwZmQxNmFjYTJhNmIyNzE1NjJkOGI4In0',
hasMore: false }
`

$3

Search uses Mongo's text search feature and will return paged results ordered by search relevancy. As such, and
unlike find(), it does not take a paginatedField parameter.

`
Performs a search query on a Mongo collection and pages the results. This is different from
find() in that the results are ordered by their relevancy, and as such, it does not take
a paginatedField parameter. Note that this is less performant than find() because it must
perform the full search on each call to this function. Also note that results might change

@param {MongoCollection} collection A collection object returned from the MongoDB library's
db.collection() method. This MUST have a Mongo $text index on it.
See https://docs.mongodb.com/manual/core/index-text/.
@param {String} searchString String to search on.
@param {Object} params
-query {Object} The find query.
-limit {Number} The page size. Must be between 1 and config.MAX_LIMIT.
-fields {Object} Fields to query in the Mongo object format, e.g. {title :1}.
The default is to query ONLY _id (note this is a difference from find()).
-next {String} The value to start querying the page. Defaults to start at the beginning of
the results.
@param {Function} done Node errback style function.
`

Example:

`js
var MongoClient = require('mongodb').MongoClient;
var MongoPaging = require('mongo-cursor-pagination');

MongoClient.connect('mongodb://localhost:27017/mydb', (err, db) => {
db.collection('myobjects').ensureIndex(
{
mytext: 'text'
},
err => {
db.collection('myobjects').insertMany(
[
{
mytext: 'dogs'
},
{
mytext: 'dogs cats'
},
{
mytext: 'dogs cats pigs'
}
],
err => {
// Query the first page.
MongoPaging.search(
db.collection('myobjects'),
'dogs',
{
fields: {
mytext: 1
},
limit: 2
},
(err, result) => {
console.log(result);

// Query next page.
MongoPaging.search(
db.collection('myobjects'),
'dogs',
{
limit: 2,
next: result.next // This queries the next page
},
(err, result) => {
console.log(result);
}
);
}
);
}
);
}
);
});
`

Output:

`sh
page 1 { results:
[ { _id: 581668318c11596af22a62de, mytext: 'dogs', score: 1 },
{ _id: 581668318c11596af22a62df, mytext: 'dogs cats', score: 0.75 } ],
next: 'WzAuNzUseyIkb2lkIjoiNTgxNjY4MzE4YzExNTk2YWYyMmE2MmRmIn1d' }
page 2 { results:
[ { _id: 581668318c11596af22a62e0, score: 0.6666666666666666 } ] }
`

$3

A popular use of this module is with Express to implement a basic API. As a convenience for this use-case, this library exposes a findWithReq function that
takes the request object from your Express middleware and returns results:

So this code using find():

`js
router.get('/myobjects', (req, res, next) => {
MongoPaging.find(
db.collection('myobjects'),
{
query: {
userId: req.user._id
},
paginatedField: 'created',
fields: {
// Also need to read req.query.fields to use to filter these fields
_id: 1,
created: 1
},
limit: req.query.limit, // Also need to cap this to 25
next: req.query.next,
previous: req.query.previous
},
function(err, result) {
if (err) {
next(err);
} else {
res.json(result);
}
}
);
});
`

Is more elegant with findWithReq():

`js
router.get('/myobjects', (req, res, next) => {
MongoPaging.findWithReq(
req,
db.collection('myobjects'),
{
query: {
userId: req.user._id
},
paginatedField: 'created',
fields: {
_id: 1,
created: 1
},
limit: 25 // Upper limit
},
function(err, result) {
if (err) {
next(err);
} else {
res.json(result);
}
}
);
});
`

findWithReq() also handles basic security such as making sure the limit and fields requested on the URL are within the allowed values you specify in
params.

$3

If the limit parameter isn't passed, then this library will default to returning 50 results. This can be overridden by setting
mongoPaging.config.DEFAULT_LIMIT = ;. Regardless of the limit passed in, a maximum of 300 documents will be returned. This can be
overridden by setting mongoPaging.config.MAX_LIMIT = ;.

$3

mongo-cursor-pagination uses _id as a secondary sorting field when providing a paginatedField property. It is recommended that you have an index for
optimal performance. Example:

`
MongoPaging.find(db.people, {
query: {
name: 'John'
},
paginatedField: 'city'
limit: 25,
});
`

For the above query to be optimal, you should have an index like:

`
db.people.ensureIndex({
name: 1,
city: 1,
_id: 1
});
`

Running tests

To run tests, you first must start a Mongo server on port 27017 and then run npm test.

Changelog

* 5.0.0 Now 50 results are returned by default, and up to 300 results can be returned if the limit parameter is used. These can be overridden by setting
mongoPaging.config.DEFAULT_LIMIT and mongoPaging.config.MAX_LIMIT respectively.

* 4.1.1 Fixed bug that would overwrite $or in queries passed in.

* 4.1.0 Adds sortAscending option to sort by the paginatedField ascending. Defaults to false (existing behavior).

* 4.0.0 Breaking API change: next and previous attributes are now always returned with every response (in case the client wants to poll for new changes).
New attributes hasPrevious and hasNext should now be used know if there are more results in the previous or next page. Before the change, next and
previously could not be replied upon to know if there were more pages.

* 3.1.1 Don't use let for backwards compatibility.

* 3.1.0 findInReq() now accepts dot notation for fields. So you can pass ?fields=users.userId to only turn the userId property for users in the
response.

* 3.0.1 Fixed bug where the _id field was always returned when a paginatedField was used.

* 3.0.0 Breaking API change: find() no longer accepts a string for limit. Added findWithReq.

* 2.0.0 Changed API to so you now set global config on the config object instead of the root export itself (e.g.
require('mongo-cursor-pagination').config.MAX_LIMIT = 100). The default MAX_LIMIT is now a more reasonable 25 instead of 100. Added search(). Fixed edge
case where pages will be incorrect if paginatedField has duplicate values.

* 1.1.0 Add MAX_LIMIT global setting to clamp

* 1.0.0 Initial release

Future ideas

* Add support to search()` to query previous pages.