feat: add domain models

src/app/models/Purchase.ts

@@ -0,0 +1,24 @@
+import { Establishment } from './Establishment';
+import { Product } from './Product';
+
+export class Purchase {
+  #price: number;
+  constructor(
+    public establishment: Establishment,
+    public product: Product,
+    price = 0,
+    public quantity = 1,
+    public date = Date.now(),
+    public id?: number
+  ) {
+    this.#price = price * 100;
+  }
+
+  get price() {
+    return this.#price / 100;
+  }
+
+  set price(price: number) {
+    this.#price = price * 100;
+  }
+}