protected function getOneByCompoundKey($key)
	{
		$sanitizedKey = [];

		// We can can potentially accept an associaitve or standard array.
		if($this->isAssociativeArray($key))
		{
			if(array_key_exists('entity_type', $key) && array_key_exists('entity_id', $key))
			{
				$sanitizedKey['entity_type'] = $key['entity_type'];
				$sanitizedKey['entity_id']   = $key['entity_id'];
			}
			else
			{
				return new Payload(null, 'insufficient_entity_key');
			}
		}
		else
		{
			$sanitizedKey['entity_type'] = $key[0];
			$sanitizedKey['entity_id']   = $key[1];
		}

		return $this->aggregate[$this->formatEntityType($sanitizedKey['entity_type'])]->first(
			['filter' => ['id' => $sanitizedKey['entity_id']]]
		);
	}