How to do left join in Doctrine?

If you have an association on a property pointing to the user (let's say CreditEntityUserCreditHistory#user, picked from your example), then the syntax is quite simple:

public function getHistory($users) {
    $qb = $this->entityManager->createQueryBuilder();
    $qb
        ->select('a', 'u')
        ->from('CreditEntityUserCreditHistory', 'a')
        ->leftJoin('a.user', 'u')
        ->where('u = :user')
        ->setParameter('user', $users)
        ->orderBy('a.created_at', 'DESC');

    return $qb->getQuery()->getResult();
}

Since you are applying a condition on the joined result here, using a LEFT JOIN or simply JOIN is the same.

If no association is available, then the query looks like following

public function getHistory($users) {
    $qb = $this->entityManager->createQueryBuilder();
    $qb
        ->select('a', 'u')
        ->from('CreditEntityUserCreditHistory', 'a')
        ->leftJoin(
            'UserEntityUser',
            'u',
            DoctrineORMQueryExprJoin::WITH,
            'a.user = u.id'
        )
        ->where('u = :user')
        ->setParameter('user', $users)
        ->orderBy('a.created_at', 'DESC');

    return $qb->getQuery()->getResult();
}

This will produce a resultset that looks like following:

array(
    array(
        0 => UserCreditHistory instance,
        1 => Userinstance,
    ),
    array(
        0 => UserCreditHistory instance,
        1 => Userinstance,
    ),
    // ...
)

By chance did I stumple upon this answer, which was for allowing a Fieldset to be empty but validate it if at least a single input was filled in.

By extending my own AbstractFormInputFilter and AbstractFieldsetInputFilter classes from an AbstractInputFilter class, which incorporates the answer, I'm now able to supply FielsetInputFilters, such as the AddressFieldsetInputFilter, with an additional ->setRequired(false). Which is then validated in the AbstractInputFilter, if it actually is empty.

The linked answer gives this code:

<?php
namespace ApplicationInputFilter;

use ZendInputFilter as ZFI;

class InputFilter extends ZFIInputFilter
{
    private $required = true;

    /**
     * @return boolean
     */
    public function isRequired()
    {
        return $this->required;
    }

    /**
     * @param boolean $required
     *
     * @return $this
     */
    public function setRequired($required)
    {
        $this->required = (bool) $required;
        return $this;
    }

    /**
     * @return bool
     */
    public function isValid()
    {
        if (!$this->isRequired() && empty(array_filter($this->getRawValues()))) {
            return true;
        }

        return parent::isValid();
    }
}

As I mentioned I used this code to extend my own AbstractInputFilter, allowing small changes in *FieldsetInputFilterFactory classes.

AddressFieldsetInputFilterFactory.php

class AddressFieldsetInputFilterFactory extends AbstractFieldsetInputFilterFactory
{
    /**
     * @param ServiceLocatorInterface|ControllerManager $serviceLocator
     * @return InputFilter
     */
    public function createService(ServiceLocatorInterface $serviceLocator)
    {
        parent::setupRequirements($serviceLocator, Address::class);

        /** @var CoordinatesFieldsetInputFilter $coordinatesFieldsetInputFilter */
        $coordinatesFieldsetInputFilter = $this->getServiceManager()->get('InputFilterManager')
            ->get(CoordinatesFieldsetInputFilter::class);
        $coordinatesFieldsetInputFilter->setRequired(false); // <-- Added option

        return new AddressFieldsetInputFilter(
            $coordinatesFieldsetInputFilter,
            $this->getEntityManager(),
            $this->getTranslator()
        );
    }
}

Might not be a good idea for everybody's projects, but it solves my problem of not always wanting to validate a Fieldset and it definitely solves the original issue of not creating an Entity with just an ID, as shown in the screenshot in the question.

With dplyr, we can use left_join

library(dplyr)
left_join(df2, df1, by = c("e"="a"))