Partitionable

A set of partitionable associations for the CakePHP ORM, allowing for basic limiting per group.

Requirements

CakePHP ORM 4.1+
A DBMS supported by CakePHP with window function support (MySQL 8, MariaDB 10.2, Postgres 9.4, SQL Sever 2017, Sqlite 3.25).

Installation

Use Composer to add the library to your project:

composer require icings/partitionable

I don't get it, what is this good for exactly?

What exactly are these associations good for, what is a "limit per group" you may ask.

Basically the associations provided in this library allow applying limits for hasMany and belongsToMany type of associations, so that it's possible to for example receive a maximum of n number of comments per article for an Articles hasMany Comments association.

Usage

Make sure to first check the Known Issues/Limitations section!

Then add the \Icings\Partitionable\ORM\AssociationsTrait trait to your table class, use its partitionableHasMany() and partitionableBelongsToMany() methods to add hasMany, respectively belongsToMany associations, configure a limit and a sort order, and you're done with the minimal setup and you can contain the partitionable associations just like any other associations.

Note that configuring a sort order is mandatory, as it is not possible to reliably partition the results without an explicit sort order, omitting it will result in an error!

Has Many

// ...
use Icings\Partitionable\ORM\AssociationsTrait;

class ArticlesTable extends \Cake\ORM\Table
{
    use AssociationsTrait;

    public function initialize(array $config): void
    {
        // ...

        $this
            ->partitionableHasMany('TopComments')
            ->setClassName('Comments')
            ->setLimit(3)
            ->setSort([
                'TopComments.votes' => 'DESC',
                'TopComments.id' => 'ASC',
            ]);
    }
}

$articlesQuery = $this->Articles
    ->find()
    ->contain('TopComments');

That would query the 3 highest voted comments for each article, eg the result would look something like:

[
    'title' => 'Some Article',
    'top_comments' => [
        [
            'votes' => 10,
            'body' => 'Some Comment',
        ],
        [
            'votes' => 9,
            'body' => 'Some Other Comment',
        ],
        [
            'votes' => 8,
            'body' => 'And Yet Another Comment',
        ],
    ],
]

Belongs To Many

// ...
use Icings\Partitionable\ORM\AssociationsTrait;

class StudentsTable extends \Cake\ORM\Table
{
    use AssociationsTrait;

    public function initialize(array $config): void
    {
        // ...

        $this
            ->partitionableBelongsToMany('TopGraduatedCourses')
            ->setClassName('Courses')
            ->setThrough('CourseMemberships')
            ->setLimit(3)
            ->setSort([
                'CourseMemberships.grade' => 'ASC',
                'CourseMemberships.id' => 'ASC',
            ])
            ->setConditions([
                'CourseMemberships.grade IS NOT' => null,
            ]);
    }
}

$studentsQuery = $this->Students
    ->find()
    ->contain('TopGraduatedCourses');

That would query the 3 highest graduated courses for each student, eg the result would look something like:

[
    'name' => 'Some Student',
    'top_graduated_courses' => [
        [
            'name' => 'Some Course',
            '_joinData' => [
                'grade' => 1,
            ],
        ],
        [
            'body' => 'Some Other Course',
            '_joinData' => [
                'grade' => 2,
            ],
        ],
        [
            'body' => 'And Yet Another Course',
            '_joinData' => [
                'grade' => 3,
            ],
        ],
    ],
]

Using options to configure the associations

Additionally to the chained method call syntax, options as known from the built-in associations are supported too, specifically the following options are supported for both partitionableHasMany() as well as partitionableBelongsToMany():

limit (int|null)
singleResult (bool)
filterStrategy (string)

$this
    ->partitionableHasMany('TopComments', [
        'className' => 'Comments',
        'limit' => 1,
        'singleResult' => false,
        'filterStrategy' => \Icings\Partitionable\ORM\Association\PartitionableHasMany::FILTER_IN_SUBQUERY_TABLE,
        'sort' => [
          'TopComments.votes' => 'DESC',
          'TopComments.id' => 'ASC',
        ],
    ]);

Changing settings on the fly

The limit and the sort order can be applied/changed on the fly in the containment's query builder:

$articlesQuery = $this->Articles
    ->find()
    ->contain('TopComments', function (\Cake\ORM\Query $query) {
        return $query
            ->limit(10)
            ->order([
                'TopComments.votes' => 'DESC',
                'TopComments.id' => 'ASC',
            ]);
    });

and via Model.beforeFind:

$this->Articles->TopComments
    ->getEventManager()
    ->on('Model.beforeFind', function ($event, \Cake\ORM\Query $query) {
        return $query
            ->limit(10)
            ->order([
                'TopComments.votes' => 'DESC',
                'TopComments.id' => 'ASC',
            ]);
    });

Limiting to a single result

When setting the limit to 1, the associations will automatically switch to using singular property names (if no property name has been set yet), and non-nested results.

For example, limiting this association to 1:

$this
    ->partitionableHasMany('TopComments')
    ->setClassName('Comments')
    ->setLimit(1)
    ->setSort([
        'TopComments.votes' => 'DESC',
        'TopComments.id' => 'ASC',
    ]);

would return a result like this:

[
    'title' => 'Some Article',
    'top_comment' => [
        'votes' => 10,
        'body' => 'Some Comment',
    ],
]

while a limit of greater or equal to 2, would return a result like this:

[
    'title' => 'Some Article',
    'top_comments' => [
        [
            'votes' => 10,
            'body' => 'Some Comment',
        ],
        [
            'votes' => 5,
            'body' => 'Some Other Comment',
        ],
    ],
]

This behavior can be disabled using the association's disableSingleResult() method, and likewise enabled using enableSingleResult(). Calling the latter will also cause the limit to be set to 1. Furthermore, setting the limit to greater or equal to 2, will automatically disable the single result mode.

With the single result mode disabled:

$this
    ->partitionableHasMany('TopComments')
    ->setClassName('Comments')
    ->setLimit(1)
    ->disableSingleResult()
    ->setSort([
        'TopComments.votes' => 'DESC',
        'TopComments.id' => 'ASC',
    ]);

a limit of 1 would return a result like this:

[
    'title' => 'Some Article',
    'top_comments' => [
        [
            'votes' => 10,
            'body' => 'Some Comment',
        ],
    ],
]

Filter Strategies

The associations currently provide a few different filter strategies that affect how the query that obtains the associated data is being filtered.

Not all queries are equal, while one strategy may work fine for one query, it might cause problems for another.

The strategy can be set using the association's setFilterStrategy() method:

use Icings\Partitionable\ORM\Association\PartitionableHasMany;

// ...

$this
    ->partitionableHasMany('TopComments')
    ->setClassName('Comments')
    ->setFilterStrategy(PartitionableHasMany::FILTER_IN_SUBQUERY_TABLE)
    ->setLimit(3)
    ->setSort([
        'TopComments.votes' => 'DESC',
        'TopComments.id' => 'ASC',
    ]);

Please refer to the API docs for SQL examples of how the different strategies work:

The currently available strategies are:

\Icings\Partitionable\ORM\Association\PartitionableAssociationInterface::FILTER_IN_SUBQUERY_CTE
\Icings\Partitionable\ORM\Association\PartitionableAssociationInterface::FILTER_IN_SUBQUERY_JOIN
\Icings\Partitionable\ORM\Association\PartitionableAssociationInterface::FILTER_IN_SUBQUERY_TABLE (default)
\Icings\Partitionable\ORM\Association\PartitionableAssociationInterface::FILTER_INNER_JOIN_CTE
\Icings\Partitionable\ORM\Association\PartitionableAssociationInterface::FILTER_INNER_JOIN_SUBQUERY

Known Issues/Limitations

These associations are not meant for save or delete operations, only for read operations!
MySQL 5 is not supported as it doesn't support the required window functions used for row numbering. While it's possible to emulate the required row numbering, these constructs are rather fragile and there's way too many situations in which they will break, respectively silently produce wrong results.
MariaDB, when running in ONLY_FULL_GROUP_MY mode, erroneously requires a GROUP BY clause to be present when using window functions like the one used for row numbering (https://jira.mariadb.org/browse/MDEV-17785). There isn't much that can be done about it until the bug is fixed, other than disabling ONLY_FULL_GROUP_BY, or adding grouping to the association's query accordingly.
SQL Server does not support common table expressions in subqueries, hence the FILTER_IN_SUBQUERY_CTE strategy cannot be used with it. In fact, it's also not possible to use custom common table expressions in the association's query with any other strategy, as it would result in the expression to be used in a subquery too, or nested in another common table expression, which also isn't supported.

cnizzardini/partitionable