Mam takie tabele:

[SQL] pobierz, plaintext 
CREATE TABLE `request_hr` (
	`c_id` INT(11) NOT NULL,
	`request_id` INT(10) UNSIGNED NOT NULL,
	PRIMARY KEY (`c_id`, `request_id`),
	INDEX `request_hr_request_id_foreign` (`request_id`),
	CONSTRAINT `request_hr_request_id_foreign` FOREIGN KEY (`request_id`) REFERENCES `employment_requests` (`id`),
	CONSTRAINT `request_hr_user_id_foreign` FOREIGN KEY (`c_id`) REFERENCES `users` (`c_id`)
)
COLLATE='utf8_unicode_ci'
ENGINE=InnoDB
;
CREATE TABLE `employment_requests` (
	`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
	`type` ENUM('onboarding','offboarding','ChangeOfPosition') NOT NULL DEFAULT 'onboarding' COLLATE 'utf8_unicode_ci',
	`status` INT(11) NOT NULL,
	`first_name` VARCHAR(255) NOT NULL COLLATE 'utf8_unicode_ci',
	`last_name` VARCHAR(255) NOT NULL COLLATE 'utf8_unicode_ci',
	`c_id` INT(10) NULL DEFAULT NULL,
	`job_title` TEXT NOT NULL COLLATE 'utf8_unicode_ci',
	`team` TEXT NOT NULL COLLATE 'utf8_unicode_ci',
	`localisation` VARCHAR(255) NOT NULL COLLATE 'utf8_unicode_ci',
	`room` VARCHAR(255) NOT NULL COLLATE 'utf8_unicode_ci',
	`manager_id` INT(10) UNSIGNED NULL DEFAULT NULL,
	`created_by` INT(10) UNSIGNED NOT NULL,
	`created_at` TIMESTAMP NULL DEFAULT NULL,
	`updated_at` TIMESTAMP NULL DEFAULT NULL,
	`deleted_at` TIMESTAMP NULL DEFAULT NULL,
	`start` DATE NULL DEFAULT NULL,
	`stop` DATE NULL DEFAULT NULL,
	`contract_type` VARCHAR(255) NOT NULL COLLATE 'utf8_unicode_ci',
	`vat` TINYINT(1) NOT NULL,
	`company1` VARCHAR(255) NULL DEFAULT NULL COLLATE 'utf8_general_ci',
	`per_hour1` TINYINT(1) NULL DEFAULT NULL,
	`pretax1` TEXT NULL COLLATE 'utf8_general_ci',
	`net1` TEXT NULL COLLATE 'utf8_general_ci',
	`beginning1` DATE NULL DEFAULT NULL,
	`end1` DATE NULL DEFAULT NULL,
	`company2` VARCHAR(255) NULL DEFAULT NULL COLLATE 'utf8_unicode_ci',
	`per_hour2` TINYINT(1) NULL DEFAULT NULL,
	`pretax2` TEXT NULL COLLATE 'utf8_unicode_ci',
	`net2` TEXT NULL COLLATE 'utf8_unicode_ci',
	`beginning2` DATETIME NULL DEFAULT NULL,
	`end2` DATETIME NULL DEFAULT NULL,
	`is_student` TINYINT(1) NOT NULL DEFAULT '0',
	`language` VARCHAR(255) NOT NULL COLLATE 'utf8_unicode_ci',
	`range` LONGTEXT NOT NULL COLLATE 'utf8_unicode_ci',
	`business_unit_id` INT(11) NOT NULL,
	`offboarding_conversation_date` DATE NULL DEFAULT NULL,
	`documents_mutual` TINYINT(1) NOT NULL,
	`documents_period` TINYINT(1) NOT NULL,
	`documents_additional_info` TEXT NULL COLLATE 'utf8_unicode_ci',
	`emails_to_inform` TEXT NULL COLLATE 'utf8_unicode_ci',
	`autoresponder_text` TEXT NULL COLLATE 'utf8_unicode_ci',
	`additional_info` TEXT NULL COLLATE 'utf8_unicode_ci',
	`note_taken_access` TEXT NULL COLLATE 'utf8_unicode_ci',
	PRIMARY KEY (`id`),
	INDEX `employment_requests_manager_id_foreign` (`manager_id`),
	INDEX `employment_requests_created_by_foreign` (`created_by`),
	CONSTRAINT `employment_requests_created_by_foreign` FOREIGN KEY (`created_by`) REFERENCES `users` (`id`),
	CONSTRAINT `employment_requests_manager_id_foreign` FOREIGN KEY (`manager_id`) REFERENCES `users` (`id`)
)
COLLATE='utf8_unicode_ci'
ENGINE=InnoDB
AUTO_INCREMENT=1
;
CREATE TABLE `users` (
	`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
	`name` VARCHAR(255) NOT NULL COLLATE 'utf8_unicode_ci',
	`email` VARCHAR(255) NOT NULL COLLATE 'utf8_unicode_ci',
	`username` VARCHAR(255) NULL DEFAULT NULL COLLATE 'utf8_unicode_ci',
	`notes` VARCHAR(1024) NOT NULL COLLATE 'utf8_unicode_ci',
	`uprawnienia` TEXT NULL COLLATE 'utf8_unicode_ci',
	`password` VARCHAR(60) NOT NULL COLLATE 'utf8_unicode_ci',
	`remember_token` VARCHAR(100) NULL DEFAULT NULL COLLATE 'utf8_unicode_ci',
	`created_at` TIMESTAMP NOT NULL DEFAULT '0000-00-00 00:00:00',
	`updated_at` TIMESTAMP NOT NULL DEFAULT '0000-00-00 00:00:00',
	`deleted_at` TIMESTAMP NULL DEFAULT NULL,
	`c_id` INT(11) NOT NULL,
	`budget_mgr_id` INT(11) NOT NULL,
	PRIMARY KEY (`id`),
	INDEX `users_c_id_IDX` (`c_id`) USING BTREE,
	INDEX `users_id_IDX` (`id`) USING BTREE,
	INDEX `budget_mgr_id` (`budget_mgr_id`),
	FULLTEXT INDEX `users_email_IDX` (`email`),
	FULLTEXT INDEX `users_username_IDX` (`username`)
)
COLLATE='utf8_unicode_ci'
ENGINE=InnoDB
AUTO_INCREMENT=1
;
 
[SQL] pobierz, plaintext 

i takie model:

[PHP] pobierz, plaintext 
<?php
 
class EmploymentRequest extends Model
{
    use SoftDeletes;
 
    const BY_MUTUAL_AGREEMENT = 'Za porozumieniem stron';
    const BY_PERIOD_OF_NOTICE_OBSERVED = 'Z zachowaniem okresu wypowiedzenia';
    const AUTORESPONDER_MIN_STRING_LENGTH = 5;
    const OFFBOARDING = 'offboarding';
    const ONBOARDING = 'onboarding';
    const CHANGE_OF_POSITION = 'changeOfPosition';
    const NEW = 0;
    const SUPPORTED = 1;
    const DONE = 2;
    const C_ID = 'c_id';
    const DELETED_AT = 'deleted_at';
    const FURNITURE = 'furniture';
    const SOFTWARE = 'software';
    const HARDWARE = 'hardware';
    const CHANGE_OF_POSITION_MAPPED = 'Zmiana roli';
    const EMPLOYMENT_TYPES = [
        self::FURNITURE,
        self::SOFTWARE,
        self::HARDWARE
    ];
    const BOARDING_TYPES = [
        self::ONBOARDING,
        self::OFFBOARDING,
        self::CHANGE_OF_POSITION
    ];
    const BOARDING_TYPES_MAPPING = [
        self::CHANGE_OF_POSITION => self::CHANGE_OF_POSITION_MAPPED
    ];
 
    public $timestamps = true;
 
    protected $fillable = [
        'first_name',
        'last_name',
        self::C_ID,
        'job_title',
        'start',
        'manager_id',
        'team',
        'room',
        'localisation',
        'created_by' ,
        'contract_type',
        'status',
        'vat',
        'company1',
        'pretax1',
        'net1',
        'per_hour1',
        'beginning1',
        'end1',
        'company2',
        'pretax2',
        'net2',
        'per_hour2',
        'beginning2',
        'end2',
        'is_student',
        'language',
        'range',
        'business_unit_id',
        'stop',
        'type',
        'offboarding_conversation_date',
        'documents_mutual',
        'documents_period',
        'documents_additional_info',
        'emails_to_inform',
        'autoresponder_text',
        'additional_info',
        'note_taken_access'
    ];
 
    public function supporters()
    {
        return $this->belongsToMany('App\User', 'request_supporter', 'request_id', 'user_id');
    }
 
    public function hrsTakenAccess(): BelongsToMany
    {
        return $this->belongsToMany(User::class, 'request_hr', 'request_id', 'c_id');
    }
}
 
[PHP] pobierz, plaintext 

Chciałbym z tabeli request_hr pobrać wszystkie c_id dla danego requestu tak więc mam relację belongsToMany w modelu EmploymentRequest która nazywa się hrsTakenAccess ale otrzymuję zapytanie z laravela co wygląda tak:

[SQL] pobierz, plaintext 
SELECT `users`.*, `request_hr`.`request_id` AS `pivot_request_id`, `request_hr`.`c_id` AS `pivot_c_id` FROM `users` INNER JOIN `request_hr` ON `users`.`id` = `request_hr`.`c_id` WHERE `request_hr`.`request_id` = ? AND `users`.`deleted_at` IS NULL
[SQL] pobierz, plaintext 

co jest złe bo zamiast `users`.`id` chce otrzymać `users`.`c_id` co robię nie tak ?

Jak jest złe, skoro masz , więc możesz sobie pobrać zarówno user->id jak i user->c_id

@Pilsener mysle ze chodzi raczej o to
.... ON `users`.`id`

A w jaki sposób to zapytanie powstaje?
I totalny bałagan w nazwach, jak ktoś ma się w tym połapać?
I skoro chcesz relację many-to-many User do EmploymentRequest to dlaczego nie po user.id i employment_request.id?
Relację mapuje się po kluczu głównym i na 99% w ORM nie można tego zmienić a już na pewno nie w archaicznym Eloquent.
I nie polecam używania relacji w Larwie, bo raz, że nie ma nad tym kontroli, dwa - powstaje dużo zbędnych zapytań do bazy a trzy - kiedy trzeba coś zmienić (np. dodać dodatkowy where) musimy to zmieniać w wielu miejscach.
Najlepiej jest stworzyć sobie serwisy typu "EntityRepository" i używać query builderów:
https://laravel.com/docs/5.2/queries#selects

Wyprostuję trochę post przedmówcy:

[PHP] pobierz, plaintext 
return $this->hasMany('App\Comment', 'foreign_key', 'local_key');
[PHP] pobierz, plaintext 

Stosują po prostu metodę z dodatkowym zapytaniem zamiast joinem. Wydajnościowo wychodzi bardzo podobnie (nie mam już testów aby przywołać). Co do where to można nawet tak zrobić:

[PHP] pobierz, plaintext 
return $this->hasMany('App\Comment', 'foreign_key', 'local_key')->where(...);
[PHP] pobierz, plaintext 

Jeśli chodzi o problem autora to:

[PHP] pobierz, plaintext 
belongsToMany(string $related, string $table = null, string $foreignPivotKey = null, string $relatedPivotKey = null, string $parentKey = null, string $relatedKey = null, string $relation = null) 
[PHP] pobierz, plaintext 

Jak widać jak narazie określiłeś tylko klucze dla tabeli "pivot" (tzn. "pośredniej"). Widzę że w modelu User masz id, zapewne domyślnie ustawione jako primaryKey, jako że w relacji nie wyspecyfikowałeś dalszych kluczy to bierze właśnie ten klucz (id).