created model for expenses and userBalance

src/models/expenseModel.js

@@ -0,0 +1,20 @@
+const mongoose = require("mongoose");
+
+const expenseSchema = new mongoose.Schema({
+  groupID: String,
+  payer: String,
+  description: String,
+  amount: Number,
+  participants: [String],
+  exclude_member: [String],
+  created_at: { type: Date, default: Date.now() },
+});
+
+const userBalance = new mongoose.Schema({
+  userID: String,
+  groupID: String,
+  balance: Number,
+});
+
+module.exports.Expense = mongoose.model("Expense", expenseSchema);
+module.exports.UserBalance = mongoose.model("UserBalance", userBalance);