Trade model saves even when TradesDetail model is absent.

Trade model.php

<?php
App::uses('AppModel', 'Model');
/**
 * Trade Model
 *
 * @property Users $Users
 * @property Shifts $Shifts
 *
 * Status legend:
 * 	0 - Unprocessed
 *  1 - In progress
 *  2 - Complete
 *  3 - Cancelled
 *
 * User_status legend:
 * 	0 - Unprocessed
 *  1 - Email sent
 *  2 - Accepted
 *  3 - Rejected
 *
 */
class Trade extends AppModel {

	/**
	 * 	Search plugin initialization
	 */
	public $actsAs = array('Search.Searchable', 'Containable');
	public $components = array('sendTradeRequest');
	public $findMethods = array('tradeIndex' => true);
	public $order = array('Trade.updated' => 'DESC');
 	public $filterArgs = array(
/* 			array('name' => 'month', 'type' => 'value', 'field' => 'MONTH(Shift.date)'),
			array('name' => 'year', 'type' => 'value', 'field' => 'YEAR(Shift.date)'),
			array('name' => 'location', 'type' => 'value', 'field' => 'ShiftsType.location_id'),
 */	);

/**
 * Validation rules
 *
 * @var array
 */

 	//TODO: Add a validation rule so that a shift must actually belong to a user for him/her to
 	// trade it away - form security stuff

	public $validate = array(
		'user_id' => array(
			'numeric' => array(
				'rule' => array('numeric'),
				'message' => 'Please select a proper user (numeric)',
				//'allowEmpty' => false,
				'required' => true,
				//'last' => false, // Stop validation after this rule
				//'on' => 'create', // Limit validation to 'create' or 'update' operations
			),
			'notBlank' => array(
				'rule' => array('notBlank'),
				'message' => 'Please select a proper user (notBlank)',
				//'allowEmpty' => false,
				'required' => true,
				//'last' => false, // Stop validation after this rule
				//'on' => 'create', // Limit validation to 'create' or 'update' operations
			),
		),
		'shift_id' => array(
			'numeric' => array(
				'rule' => array('numeric'),
				'message' => 'Please select a proper shift (numeric)',
				//'allowEmpty' => false,
				'required' => true,
				//'last' => false, // Stop validation after this rule
				//'on' => 'create', // Limit validation to 'create' or 'update' operations
			),
			'notBlank' => array(
				'rule' => array('notBlank'),
				'message' => 'Please select a proper shift (notBlank)',
				//'allowEmpty' => false,
				'required' => true,
				//'last' => false, // Stop validation after this rule
				//'on' => 'create', // Limit validation to 'create' or 'update' operations
			),
			'checkDuplicate' => array(
				'rule' => 'checkDuplicate',
				'message' => 'This shift is already in the process of being traded! Please cancel the pre-existing trade before trying to trade this shift again'
			),
		),
	);

	//The Associations below have been created with all possible keys, those that are not needed can be removed

/**
 * belongsTo associations
 *
 * @var array
 */
	public $belongsTo = array(
		'User' => array(
			'className' => 'User',
			'foreignKey' => 'user_id',
			'conditions' => '',
			'fields' => '',
			'order' => ''
		),
		'Shift' => array(
			'className' => 'Shift',
			'foreignKey' => 'shift_id',
			'conditions' => '',
			'fields' => '',
			'order' => ''
		));
	public $hasMany = array(
		'TradesDetail' => array(
			'className' => 'TradesDetail',
			'foreignKey' => 'trade_id',
			'conditions' => '',
			'fields' => '',
			'order' => ''
		));

	public function getUnprocessedTrades($conditions = array()) {
		return $this->find('all', array(
				'recursive' => 3,
				'contain' => array(
						'Shift' => array(
								'fields' => array(
										'id',
										'date'),
								'ShiftsType' => array(
										'fields' => array(
												'times'),
										'Location' => array(
												'location')
										)
								),
						'TradesDetail' => array(
								'User' => array(
										'fields' => array(
												'id',
												'name',
												'email')
										)
								),
						'User' => array(
								'fields' => array(
										'id',
										'name',
										'email'),
								),
						),
				'conditions' => array_merge(
						array('status' => 0),
						$conditions),
		));
	}

	/*
	 * Check duplicate trade and return true if duplicate found
	*
	*/
	public function checkDuplicate($check) {
		$trade = $this->find('first', array(
				'fields' => array(
						'Trade.user_id',
						'Trade.shift_id',
						'Trade.user_status',
						'Trade.status'),
				'conditions' => array(
						'Trade.user_status <' => 2,
						'Trade.status !=' => 2,
						'Trade.user_id' => $this->data['Trade']['user_id'],
						'Trade.shift_id' => $this->data['Trade' ]['shift_id']),
		));
		return ($trade ? false : true);
	}
}

TradesController.php section

	public function index() {
		debug($this->request->data);
		if ($this->request->isPost() && !empty($this->request->data)) {
			if ($this->Trade->saveAll($this->request->data, array('deep' => true))) {
					$this->Flash->success(__('The trade has been saved'));
			} else {
				$this->Flash->alert(__('The trade could not be saved. Please fix the errors below and try again.'));
			}
		}

		$this->set('usersId', (isset($this->request->query['id']) ? $this->request->query['id'] : $this->_usersId()));
		$this->set('groupList', $this->User->getGroupsForUser($this->_usersId(), array(), true, true));
		$this->render();
	}

TradesDetail model.php

<?php
App::uses('AppModel', 'Model');
/**
 * TradesDetail Model
 *
 * @property User $User
 * @property Trade $Trade
 * @property User $User
 * Status:	0 - Unprocessed
 * 			1 - Request sent; awaiting reply
 * 			2 - Accepted
 * 			3 - Rejected
 */
class TradesDetail extends AppModel {
	public $actsAs = array('Search.Searchable', 'Containable');
	public $order = array('updated' => 'DESC');

/**
 * Display field
 *
 * @var string
 */
	public $displayField = 'trade_id';
/**
 * Validation rules
 *
 * @var array
 */
	public $validate = array(
 		'trade_id' => array(
			'numeric' => array(
				'rule' => array('numeric'),
				'message' => 'The linked trade must be known',
				'allowEmpty' => false,
				'required' => false,
				//'last' => false, // Stop validation after this rule
				//'on' => 'create', // Limit validation to 'create' or 'update' operations
			),
		),
		'user_id' => array(
			'numeric' => array(
				'rule' => array('numeric'),
				'message' => 'A user being traded to is required',
				'allowEmpty' => false,
				'required' => true,
				//'last' => false, // Stop validation after this rule
				//'on' => 'create', // Limit validation to 'create' or 'update' operations
			),
		),
		'status' => array(
			'numeric' => array(
				'rule' => array('numeric'),
				'message' => 'The format in which the status was submitted is incorrect',
				//'allowEmpty' => false,
				//'required' => false,
				//'last' => false, // Stop validation after this rule
				//'on' => 'create', // Limit validation to 'create' or 'update' operations
			),
		),
	);

	//The Associations below have been created with all possible keys, those that are not needed can be removed

/**
 * belongsTo associations
 *
 * @var array
 */
	public $belongsTo = array(
		'Trade' => array(
			'className' => 'Trade',
			'foreignKey' => 'trade_id',
			'conditions' => '',
			'fields' => '',
			'order' => ''
		),
		'User' => array(
			'className' => 'User',
			'foreignKey' => 'user_id',
			'conditions' => '',
			'fields' => '',
			'order' => ''
		)
	);
}