models.js

var Sequelize = require('sequelize');
var DataTypes = Sequelize.DataTypes;
var sequelize = new Sequelize(
   // ignore configs
  );
  
// Represents user account information
var User = sequelize.define("user", {
  username : { type : DataTypes.STRING },
  password : { type : DataTypes.STRING },
  balance : { type : DataTypes.INTEGER }
});

// Create user friend relations
User.belongsToMany(User, {
  through : "user_friends",
  foreignKey: "friend_id",
  as : "Friends"
});

// Represent a request to borrow or lend money, may be denied
var LendingContract = sequelize.define("lending_contract", {
   // The amount of money to be lent or borrowed
  amount : { type : DataTypes.DECIMAL(10, 2), allowNull : false },
  // Monthly interest rate of this contract, 
  monthly_interest_rate : { type : DataTypes.FLOAT, allowNull : false, defaultValue: 0 },
  // The amount of days within which the lender is supposed to be repaid
  duration_days : { type : DataTypes.INTEGER },
  // Represents who initiated this contract
  author : {
    type: DataTypes.INTEGER,
    references : {
      model : User,
      key : "id"
    },
    allowNull : false
  },
  // Represents who will accept or deny this contract
  target : {
    type: DataTypes.INTEGER,
    references : {
      model : User,
      key : "id"
    },
    allowNull : false
  },
  // Represents who the lender is
  lender : {
    type: DataTypes.INTEGER,
    references : {
      model : User,
      key : "id"
    },
    allowNull : false
  },
  // Represents who the lendee is, lender & lendee column can be replaced by a simple boolean column "is_lending".
  // But in that case, both author & target can be lender or lendee, that can be very confusing when writing business logic.
  // This approach is clearer to read and easier to program.
  lendee : {
    type: DataTypes.INTEGER,
    references : {
      model : User,
      key : "id"
    },
    allowNull : false
  },
  // The state of this contract
  state : {
    type : DataTypes.ENUM,
    values : ["PENDING", "ACCEPTED", "DENIED", "REPAID"]
  }
});
  
// This records the actual bank transaction from user to user for the purpose of auditing.
// If contract_id is null, this is a simple transaction without being required to be repaid.
var Transaction = sequelize.define("transaction", {
  amount : { type : DataTypes.DECIMAL(10, 2), allowNull : false },
  from_user_id : {
    type: DataTypes.INTEGER,
    references : {
      model : User,
      key : "id"
    },
    allowNull : false
  },
  to_user_id : {
    type: DataTypes.INTEGER,
    references : {
      model : User,
      key : "id"
    },
    allowNull : false
  },
  contract_id : {
    type: DataTypes.INTEGER,
    references : {
      model : LendingContract,
      key : "id"
    }
  }
});