fix bugs in sales and purchases API.

server/src/models/Bill.js

@@ -1,5 +1,6 @@
 import { Model, mixin } from 'objection';
 import moment from 'moment';
+import { difference } from 'lodash';
 import TenantModel from '@/models/TenantModel';
 import CachableQueryBuilder from '@/lib/Cachable/CachableQueryBuilder';
 import CachableModel from '@/lib/Cachable/CachableModel';
@@ -29,8 +30,25 @@ export default class Bill extends mixin(TenantModel, [CachableModel]) {
 
   /**
    * Due amount of the given.
+   * @return {number}
    */
   get dueAmount() {
-    return Math.max(this.balance - this.paymentAmount, 0);
+    return this.amount - this.paymentAmount;
+  }
+
+  /**
+   * Retrieve the not found bills ids as array.
+   * @param {Array} billsIds 
+   * @return {Array}
+   */
+  static async getNotFoundBills(billsIds) {
+    const storedBills = await this.tenant().query().whereIn('id', billsIds);
+    const storedBillsIds = storedBills.map((t) => t.id);
+
+    const notFoundBillsIds = difference(
+      billsIds,
+      storedBillsIds,
+    );
+    return notFoundBillsIds;
   }
 }