=================== MySQL ====================
CREATE TABLE IF NOT EXISTS `classes` (
  `class_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `created_date` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `term_id` int(11) unsigned NOT NULL,
  `is_core` enum('Y','N') NOT NULL DEFAULT 'N',
  `course_id` int(11) unsigned NOT NULL,
  PRIMARY KEY (`class_id`),
  UNIQUE KEY `term_course` (`term_id`,`course_id`),
  KEY `class_courseID_FK1` (`course_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=1663 ;


ALTER TABLE `classes`
  ADD CONSTRAINT `classe_courseID_FK1` FOREIGN KEY (`course_id`) REFERENCES `courses` (`course_id`) ON DELETE CASCADE ON UPDATE NO ACTION,
  ADD CONSTRAINT `classe_termID_FK1` FOREIGN KEY (`term_id`) REFERENCES `terms` (`term_id`) ON DELETE CASCADE ON UPDATE NO ACTION;

==================== Model ====================
<?php

namespace app\models;

use Yii;

/**
 * This is the model class for table "classes".
 *
 * @property integer $class_id
 * @property string $created_date
 * @property integer $term_id
 * @property string $is_core
 * @property integer $course_id
 *
 * @property Categories[] $categories
 * @property Courses $course
 * @property Terms $term
 * @property Comments[] $comments
 * @property IncidentReferrals[] $incidentReferrals
 * @property Student2class[] $student2classes
 * @property Users[] $users
 */
class Classes extends \yii\db\ActiveRecord
{
    /**
     * @inheritdoc
     */
    public static function tableName()
    {
        return 'classes';
    }

    /**
     * @inheritdoc
     */
    public function rules()
    {
        return [
            [['created_date'], 'safe'],
            [['term_id', 'course_id'], 'required'],
            [['term_id', 'course_id'], 'integer'],
            [['is_core'], 'string'],
            [['term_id', 'course_id'], 'unique', 'targetAttribute' => ['term_id', 'course_id'], 'message' => 'The combination of Term ID and Course ID has already been taken.']
        ];
    }

    /**
     * @inheritdoc
     */
    public function attributeLabels()
    {
        return [
            'class_id' => 'Class ID',
            'created_date' => 'Created Date',
            'term_id' => 'Term ID',
            'is_core' => 'Is Core',
            'course_id' => 'Course ID',
        ];
    }

    /**
     * @return \yii\db\ActiveQuery
     */
    public function getCategories()
    {
        return $this->hasMany(Categories::className(), ['class_id' => 'class_id']);
    }

    /**
     * @return \yii\db\ActiveQuery
     */
    public function getCourse()
    {
        return $this->hasOne(Courses::className(), ['course_id' => 'course_id']);
    }

    /**
     * @return \yii\db\ActiveQuery
     */
    public function getTerm()
    {
        return $this->hasOne(Terms::className(), ['term_id' => 'term_id']);
    }

    /**
     * @return \yii\db\ActiveQuery
     */
    public function getComments()
    {
        return $this->hasMany(Comments::className(), ['class_id' => 'class_id']);
    }

    /**
     * @return \yii\db\ActiveQuery
     */
    public function getIncidentReferrals()
    {
        return $this->hasMany(IncidentReferrals::className(), ['class_id' => 'class_id']);
    }

    /**
     * @return \yii\db\ActiveQuery
     */
    public function getStudent2classes()
    {
        return $this->hasMany(Student2class::className(), ['class_id' => 'class_id']);
    }

    /**
     * @return \yii\db\ActiveQuery
     */
    public function getUsers()
    {
        return $this->hasMany(Users::className(), ['user_id' => 'user_id'])->viaTable('student2class', ['class_id' => 'class_id']);
    }
}