add changes

backend/models/User.js

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+const bcrypt = require('bcrypt');
+const { v4: uuidv4 } = require('uuid');
+
+module.exports = (sequelize, DataTypes) => {
+  const User = sequelize.define('User', {
+    id: {
+      type: DataTypes.CHAR(36),
+      primaryKey: true,
+      defaultValue: () => uuidv4(),
+      allowNull: false,
+      comment: 'UUID primary key'
+    },
+    username: {
+      type: DataTypes.STRING(50),
+      allowNull: false,
+      unique: {
+        msg: 'Username already exists'
+      },
+      validate: {
+        notEmpty: {
+          msg: 'Username cannot be empty'
+        },
+        len: {
+          args: [3, 50],
+          msg: 'Username must be between 3 and 50 characters'
+        },
+        isAlphanumeric: {
+          msg: 'Username must contain only letters and numbers'
+        }
+      },
+      comment: 'Unique username'
+    },
+    email: {
+      type: DataTypes.STRING(100),
+      allowNull: false,
+      unique: {
+        msg: 'Email already exists'
+      },
+      validate: {
+        notEmpty: {
+          msg: 'Email cannot be empty'
+        },
+        isEmail: {
+          msg: 'Must be a valid email address'
+        }
+      },
+      comment: 'User email address'
+    },
+    password: {
+      type: DataTypes.STRING(255),
+      allowNull: false,
+      validate: {
+        notEmpty: {
+          msg: 'Password cannot be empty'
+        },
+        len: {
+          args: [6, 255],
+          msg: 'Password must be at least 6 characters'
+        }
+      },
+      comment: 'Hashed password'
+    },
+    role: {
+      type: DataTypes.ENUM('admin', 'user'),
+      allowNull: false,
+      defaultValue: 'user',
+      validate: {
+        isIn: {
+          args: [['admin', 'user']],
+          msg: 'Role must be either admin or user'
+        }
+      },
+      comment: 'User role'
+    },
+    profileImage: {
+      type: DataTypes.STRING(255),
+      allowNull: true,
+      field: 'profile_image',
+      comment: 'Profile image URL'
+    },
+    isActive: {
+      type: DataTypes.BOOLEAN,
+      allowNull: false,
+      defaultValue: true,
+      field: 'is_active',
+      comment: 'Account active status'
+    },
+    
+    // Statistics
+    totalQuizzes: {
+      type: DataTypes.INTEGER,
+      allowNull: false,
+      defaultValue: 0,
+      field: 'total_quizzes',
+      validate: {
+        min: 0
+      },
+      comment: 'Total number of quizzes taken'
+    },
+    quizzesPassed: {
+      type: DataTypes.INTEGER,
+      allowNull: false,
+      defaultValue: 0,
+      field: 'quizzes_passed',
+      validate: {
+        min: 0
+      },
+      comment: 'Number of quizzes passed'
+    },
+    totalQuestionsAnswered: {
+      type: DataTypes.INTEGER,
+      allowNull: false,
+      defaultValue: 0,
+      field: 'total_questions_answered',
+      validate: {
+        min: 0
+      },
+      comment: 'Total questions answered'
+    },
+    correctAnswers: {
+      type: DataTypes.INTEGER,
+      allowNull: false,
+      defaultValue: 0,
+      field: 'correct_answers',
+      validate: {
+        min: 0
+      },
+      comment: 'Number of correct answers'
+    },
+    currentStreak: {
+      type: DataTypes.INTEGER,
+      allowNull: false,
+      defaultValue: 0,
+      field: 'current_streak',
+      validate: {
+        min: 0
+      },
+      comment: 'Current daily streak'
+    },
+    longestStreak: {
+      type: DataTypes.INTEGER,
+      allowNull: false,
+      defaultValue: 0,
+      field: 'longest_streak',
+      validate: {
+        min: 0
+      },
+      comment: 'Longest daily streak achieved'
+    },
+    
+    // Timestamps
+    lastLogin: {
+      type: DataTypes.DATE,
+      allowNull: true,
+      field: 'last_login',
+      comment: 'Last login timestamp'
+    },
+    lastQuizDate: {
+      type: DataTypes.DATE,
+      allowNull: true,
+      field: 'last_quiz_date',
+      comment: 'Date of last quiz taken'
+    }
+  }, {
+    sequelize,
+    modelName: 'User',
+    tableName: 'users',
+    timestamps: true,
+    underscored: true,
+    indexes: [
+      {
+        unique: true,
+        fields: ['email']
+      },
+      {
+        unique: true,
+        fields: ['username']
+      },
+      {
+        fields: ['role']
+      },
+      {
+        fields: ['is_active']
+      },
+      {
+        fields: ['created_at']
+      }
+    ]
+  });
+
+  // Instance methods
+  User.prototype.comparePassword = async function(candidatePassword) {
+    try {
+      return await bcrypt.compare(candidatePassword, this.password);
+    } catch (error) {
+      throw new Error('Password comparison failed');
+    }
+  };
+
+  User.prototype.toJSON = function() {
+    const values = { ...this.get() };
+    delete values.password; // Never expose password in JSON
+    return values;
+  };
+
+  User.prototype.updateStreak = function() {
+    const today = new Date();
+    today.setHours(0, 0, 0, 0);
+    
+    if (this.lastQuizDate) {
+      const lastQuiz = new Date(this.lastQuizDate);
+      lastQuiz.setHours(0, 0, 0, 0);
+      
+      const daysDiff = Math.floor((today - lastQuiz) / (1000 * 60 * 60 * 24));
+      
+      if (daysDiff === 1) {
+        // Consecutive day - increment streak
+        this.currentStreak += 1;
+        if (this.currentStreak > this.longestStreak) {
+          this.longestStreak = this.currentStreak;
+        }
+      } else if (daysDiff > 1) {
+        // Streak broken - reset
+        this.currentStreak = 1;
+      }
+      // If daysDiff === 0, same day - no change to streak
+    } else {
+      // First quiz
+      this.currentStreak = 1;
+      this.longestStreak = 1;
+    }
+    
+    this.lastQuizDate = new Date();
+  };
+
+  User.prototype.calculateAccuracy = function() {
+    if (this.totalQuestionsAnswered === 0) return 0;
+    return ((this.correctAnswers / this.totalQuestionsAnswered) * 100).toFixed(2);
+  };
+
+  User.prototype.getPassRate = function() {
+    if (this.totalQuizzes === 0) return 0;
+    return ((this.quizzesPassed / this.totalQuizzes) * 100).toFixed(2);
+  };
+
+  User.prototype.toSafeJSON = function() {
+    const values = { ...this.get() };
+    delete values.password;
+    return values;
+  };
+
+  // Class methods
+  User.findByEmail = async function(email) {
+    return await this.findOne({ where: { email, isActive: true } });
+  };
+
+  User.findByUsername = async function(username) {
+    return await this.findOne({ where: { username, isActive: true } });
+  };
+
+  // Hooks
+  User.beforeCreate(async (user) => {
+    // Hash password before creating user
+    if (user.password) {
+      const salt = await bcrypt.genSalt(10);
+      user.password = await bcrypt.hash(user.password, salt);
+    }
+    
+    // Ensure UUID is set
+    if (!user.id) {
+      user.id = uuidv4();
+    }
+  });
+
+  User.beforeUpdate(async (user) => {
+    // Hash password if it was changed
+    if (user.changed('password')) {
+      const salt = await bcrypt.genSalt(10);
+      user.password = await bcrypt.hash(user.password, salt);
+    }
+  });
+
+  User.beforeBulkCreate(async (users) => {
+    for (const user of users) {
+      if (user.password) {
+        const salt = await bcrypt.genSalt(10);
+        user.password = await bcrypt.hash(user.password, salt);
+      }
+      if (!user.id) {
+        user.id = uuidv4();
+      }
+    }
+  });
+
+  // Define associations
+  User.associate = function(models) {
+    // User has many quiz sessions (when QuizSession model exists)
+    if (models.QuizSession) {
+      User.hasMany(models.QuizSession, {
+        foreignKey: 'userId',
+        as: 'quizSessions'
+      });
+    }
+
+    // User has many bookmarks (when Question model exists)
+    if (models.Question) {
+      User.belongsToMany(models.Question, {
+        through: 'user_bookmarks',
+        foreignKey: 'userId',
+        otherKey: 'questionId',
+        as: 'bookmarkedQuestions'
+      });
+
+      // User has created questions (if admin)
+      User.hasMany(models.Question, {
+        foreignKey: 'createdBy',
+        as: 'createdQuestions'
+      });
+    }
+
+    // User has many achievements (when Achievement model exists)
+    if (models.Achievement) {
+      User.belongsToMany(models.Achievement, {
+        through: 'user_achievements',
+        foreignKey: 'userId',
+        otherKey: 'achievementId',
+        as: 'achievements'
+      });
+    }
+  };
+
+  return User;
+};