How to implement a 'has many through' relationship in Doctrine?

In Doctrine2, the "has many through" relationship is implemented by explicitly creating an intermediate entity (join entity) that links two other entities with Many-to-One relationships.

1. Three classes of entities are created: Source, Target, and Intermediate (JoinEntity).

2. The Intermediate entity (for example, UserGroup for "User has many Group through UserGroup") contains two fields, which are Many-to-One relationships to the Source (User) and Target (Group) entities.

3. The Source and Target entities have a One-to-Many relationship with the Intermediate entity.

4. For convenience in accessing Target entities through the Source (for example, to get all Group for a User), a method can be added to the Source entity that filters the Intermediate entities.

Example structure:

// User.php
<?php

use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\Common\Collections\Collection;
use Doctrine\ORM\Mapping as ORM;

#[ORM\Entity]
class User
{
    #[ORM\Id]
    #[ORM\GeneratedValue]
    #[ORM\Column]
    private ?int $id = null;

    #[ORM\OneToMany(targetEntity: UserGroup::class, mappedBy: 'user')]
    private Collection $userGroups; // Relationship to intermediate entity

    public function __construct()
    {
        $this->userGroups = new ArrayCollection();
    }

    public function getId(): ?int
    {
        return $this->id;
    }

    /**
     * Get groups of the user through the intermediate entity.
     * This is the equivalent of "has many through".
     * Note: You might want to fetch this collection efficiently depending on your needs.
     */
    public function getGroups(): Collection
    {
        return $this->userGroups->map(fn(UserGroup $userGroup) => $userGroup->getGroup());
    }

    public function getUserGroups(): Collection
    {
        return $this->userGroups;
    }

    public function addUserGroup(UserGroup $userGroup): static
    {
        if (!$this->userGroups->contains($userGroup)) {
            $this->userGroups->add($userGroup);
            $userGroup->setUser($this);
        }

        return $this;
    }

    public function removeUserGroup(UserGroup $userGroup): static
    {
        if ($this->userGroups->removeElement($userGroup)) {
            // set the owning side to null (unless already changed)
            if ($userGroup->getUser() === $this) {
                $userGroup->setUser(null);
            }
        }

        return $this;
    }
}

// Group.php
<?php

use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\Common\Collections\Collection;
use Doctrine\ORM\Mapping as ORM;

#[ORM\Entity]
class Group
{
    #[ORM\Id]
    #[ORM\GeneratedValue]
    #[ORM\Column]
    private ?int $id = null;

    #[ORM\OneToMany(targetEntity: UserGroup::class, mappedBy: 'group')]
    private Collection $userGroups; // Relationship to intermediate entity

    public function __construct()
    {
        $this->userGroups = new ArrayCollection();
    }

    public function getId(): ?int
    {
        return $this->id;
    }

    public function getUserGroups(): Collection
    {
        return $this->userGroups;
    }

    public function addUserGroup(UserGroup $userGroup): static
    {
        if (!$this->userGroups->contains($userGroup)) {
            $this->userGroups->add($userGroup);
            $userGroup->setGroup($this);
        }

        return $this;
    }

    public function removeUserGroup(UserGroup $userGroup): static
    {
        if ($this->userGroups->removeElement($userGroup)) {
            // set the owning side to null (unless already changed)
            if ($userGroup->getGroup() === $this) {
                $userGroup->setGroup(null);
            }
        }

        return $this;
    }
}

// UserGroup.php
<?php

use Doctrine\ORM\Mapping as ORM;

#[ORM\Entity]
#[ORM\Table(name: 'user_user_group')] // Example table name
class UserGroup
{
    #[ORM\Id]
    #[ORM\GeneratedValue]
    #[ORM\Column]
    private ?int $id = null;

    #[ORM\ManyToOne(targetEntity: User::class, inversedBy: 'userGroups')]
    #[ORM\JoinColumn(nullable: false)]
    private ?User $user = null; // Many-to-One to User

    #[ORM\ManyToOne(targetEntity: Group::class, inversedBy: 'userGroups')]
    #[ORM\JoinColumn(nullable: false)]
    private ?Group $group = null; // Many-to-One to Group

    public function getId(): ?int
    {
        return $this->id;
    }

    public function getUser(): ?User
    {
        return $this->user;
    }

    public function setUser(?User $user): static
    {
        $this->user = $user;

        return $this;
    }

    public function getGroup(): ?Group
    {
        return $this->group;
    }

    public function setGroup(?Group $group): static
    {
        $this->group = $group;

        return $this;
    }
}

This approach, although it requires creating an additional entity, provides full control over the relationship data and makes it easy to add additional fields to the intermediate table (for example, the date a user joined a group).