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Edit File: BelongsTo.php

<?php

namespace Illuminate\Database\Eloquent\Relations;

use BackedEnum;
use Illuminate\Database\Eloquent\Builder;
use Illuminate\Database\Eloquent\Collection;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\Concerns\ComparesRelatedModels;
use Illuminate\Database\Eloquent\Relations\Concerns\InteractsWithDictionary;
use Illuminate\Database\Eloquent\Relations\Concerns\SupportsDefaultModels;

/**
 * @template TRelatedModel of \Illuminate\Database\Eloquent\Model
 * @template TDeclaringModel of \Illuminate\Database\Eloquent\Model
 *
 * @extends \Illuminate\Database\Eloquent\Relations\Relation<TRelatedModel, TDeclaringModel, ?TRelatedModel>
 */
class BelongsTo extends Relation
{
    use ComparesRelatedModels,
        InteractsWithDictionary,
        SupportsDefaultModels;

/**
     * The child model instance of the relation.
     *
     * @var TDeclaringModel
     */
    protected $child;

/**
     * The foreign key of the parent model.
     *
     * @var string
     */
    protected $foreignKey;

/**
     * The associated key on the parent model.
     *
     * @var string
     */
    protected $ownerKey;

/**
     * The name of the relationship.
     *
     * @var string
     */
    protected $relationName;

/**
     * Create a new belongs to relationship instance.
     *
     * @param  \Illuminate\Database\Eloquent\Builder<TRelatedModel>  $query
     * @param  TDeclaringModel  $child
     * @param  string  $foreignKey
     * @param  string  $ownerKey
     * @param  string  $relationName
     * @return void
     */
    public function __construct(Builder $query, Model $child, $foreignKey, $ownerKey, $relationName)
    {
        $this->ownerKey = $ownerKey;
        $this->relationName = $relationName;
        $this->foreignKey = $foreignKey;

// In the underlying base relationship class, this variable is referred to as
        // the "parent" since most relationships are not inversed. But, since this
        // one is we will create a "child" variable for much better readability.
        $this->child = $child;

parent::__construct($query, $child);
    }

/** @inheritDoc */
    public function getResults()
    {
        if (is_null($this->getForeignKeyFrom($this->child))) {
            return $this->getDefaultFor($this->parent);
        }

return $this->query->first() ?: $this->getDefaultFor($this->parent);
    }

/**
     * Set the base constraints on the relation query.
     *
     * @return void
     */
    public function addConstraints()
    {
        if (static::$constraints) {
            // For belongs to relationships, which are essentially the inverse of has one
            // or has many relationships, we need to actually query on the primary key
            // of the related models matching on the foreign key that's on a parent.
            $table = $this->related->getTable();

$this->query->where($table.'.'.$this->ownerKey, '=', $this->getForeignKeyFrom($this->child));
        }
    }

/** @inheritDoc */
    public function addEagerConstraints(array $models)
    {
        // We'll grab the primary key name of the related models since it could be set to
        // a non-standard name and not "id". We will then construct the constraint for
        // our eagerly loading query so it returns the proper models from execution.
        $key = $this->related->getTable().'.'.$this->ownerKey;

$whereIn = $this->whereInMethod($this->related, $this->ownerKey);

$this->whereInEager($whereIn, $key, $this->getEagerModelKeys($models));
    }

/**
     * Gather the keys from an array of related models.
     *
     * @param  array<int, TDeclaringModel>  $models
     * @return array
     */
    protected function getEagerModelKeys(array $models)
    {
        $keys = [];

// First we need to gather all of the keys from the parent models so we know what
        // to query for via the eager loading query. We will add them to an array then
        // execute a "where in" statement to gather up all of those related records.
        foreach ($models as $model) {
            if (! is_null($value = $this->getForeignKeyFrom($model))) {
                $keys[] = $value;
            }
        }

sort($keys);

return array_values(array_unique($keys));
    }

/** @inheritDoc */
    public function initRelation(array $models, $relation)
    {
        foreach ($models as $model) {
            $model->setRelation($relation, $this->getDefaultFor($model));
        }

return $models;
    }

/** @inheritDoc */
    public function match(array $models, Collection $results, $relation)
    {
        // First we will get to build a dictionary of the child models by their primary
        // key of the relationship, then we can easily match the children back onto
        // the parents using that dictionary and the primary key of the children.
        $dictionary = [];

foreach ($results as $result) {
            $attribute = $this->getDictionaryKey($this->getRelatedKeyFrom($result));

$dictionary[$attribute] = $result;
        }

// Once we have the dictionary constructed, we can loop through all the parents
        // and match back onto their children using these keys of the dictionary and
        // the primary key of the children to map them onto the correct instances.
        foreach ($models as $model) {
            $attribute = $this->getDictionaryKey($this->getForeignKeyFrom($model));

if (isset($dictionary[$attribute])) {
                $model->setRelation($relation, $dictionary[$attribute]);
            }
        }

return $models;
    }

/**
     * Associate the model instance to the given parent.
     *
     * @param  TRelatedModel|int|string|null  $model
     * @return TDeclaringModel
     */
    public function associate($model)
    {
        $ownerKey = $model instanceof Model ? $model->getAttribute($this->ownerKey) : $model;

$this->child->setAttribute($this->foreignKey, $ownerKey);

if ($model instanceof Model) {
            $this->child->setRelation($this->relationName, $model);
        } else {
            $this->child->unsetRelation($this->relationName);
        }

return $this->child;
    }

/**
     * Dissociate previously associated model from the given parent.
     *
     * @return TDeclaringModel
     */
    public function dissociate()
    {
        $this->child->setAttribute($this->foreignKey, null);

return $this->child->setRelation($this->relationName, null);
    }

/**
     * Alias of "dissociate" method.
     *
     * @return TDeclaringModel
     */
    public function disassociate()
    {
        return $this->dissociate();
    }

/**
     * Touch all of the related models for the relationship.
     *
     * @return void
     */
    public function touch()
    {
        if (! is_null($this->getParentKey())) {
            parent::touch();
        }
    }

/** @inheritDoc */
    public function getRelationExistenceQuery(Builder $query, Builder $parentQuery, $columns = ['*'])
    {
        if ($parentQuery->getQuery()->from == $query->getQuery()->from) {
            return $this->getRelationExistenceQueryForSelfRelation($query, $parentQuery, $columns);
        }

return $query->select($columns)->whereColumn(
            $this->getQualifiedForeignKeyName(), '=', $query->qualifyColumn($this->ownerKey)
        );
    }

/**
     * Add the constraints for a relationship query on the same table.
     *
     * @param  \Illuminate\Database\Eloquent\Builder<TRelatedModel>  $query
     * @param  \Illuminate\Database\Eloquent\Builder<TDeclaringModel>  $parentQuery
     * @param  array|mixed  $columns
     * @return \Illuminate\Database\Eloquent\Builder<TRelatedModel>
     */
    public function getRelationExistenceQueryForSelfRelation(Builder $query, Builder $parentQuery, $columns = ['*'])
    {
        $query->select($columns)->from(
            $query->getModel()->getTable().' as '.$hash = $this->getRelationCountHash()
        );

$query->getModel()->setTable($hash);

return $query->whereColumn(
            $hash.'.'.$this->ownerKey, '=', $this->getQualifiedForeignKeyName()
        );
    }

/**
     * Determine if the related model has an auto-incrementing ID.
     *
     * @return bool
     */
    protected function relationHasIncrementingId()
    {
        return $this->related->getIncrementing() &&
            in_array($this->related->getKeyType(), ['int', 'integer']);
    }

/**
     * Make a new related instance for the given model.
     *
     * @param  TDeclaringModel  $parent
     * @return TRelatedModel
     */
    protected function newRelatedInstanceFor(Model $parent)
    {
        return $this->related->newInstance();
    }

/**
     * Get the child of the relationship.
     *
     * @return TDeclaringModel
     */
    public function getChild()
    {
        return $this->child;
    }

/**
     * Get the foreign key of the relationship.
     *
     * @return string
     */
    public function getForeignKeyName()
    {
        return $this->foreignKey;
    }

/**
     * Get the fully qualified foreign key of the relationship.
     *
     * @return string
     */
    public function getQualifiedForeignKeyName()
    {
        return $this->child->qualifyColumn($this->foreignKey);
    }

/**
     * Get the key value of the child's foreign key.
     *
     * @return mixed
     */
    public function getParentKey()
    {
        return $this->getForeignKeyFrom($this->child);
    }

/**
     * Get the associated key of the relationship.
     *
     * @return string
     */
    public function getOwnerKeyName()
    {
        return $this->ownerKey;
    }

/**
     * Get the fully qualified associated key of the relationship.
     *
     * @return string
     */
    public function getQualifiedOwnerKeyName()
    {
        return $this->related->qualifyColumn($this->ownerKey);
    }

/**
     * Get the value of the model's foreign key.
     *
     * @param  TRelatedModel  $model
     * @return int|string
     */
    protected function getRelatedKeyFrom(Model $model)
    {
        return $model->{$this->ownerKey};
    }

/**
     * Get the value of the model's foreign key.
     *
     * @param  TDeclaringModel  $model
     * @return mixed
     */
    protected function getForeignKeyFrom(Model $model)
    {
        $foreignKey = $model->{$this->foreignKey};

return $foreignKey instanceof BackedEnum ? $foreignKey->value : $foreignKey;
    }

/**
     * Get the name of the relationship.
     *
     * @return string
     */
    public function getRelationName()
    {
        return $this->relationName;
    }
}

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