Added the donation classes

ref #1

src/models/DistanceDonation.ts

@@ -0,0 +1,40 @@
+import { IsInt, IsNotEmpty, IsObject, IsPositive } from "class-validator";
+import { Donation } from "./Donation";
+import { Runner } from "./Runner";
+
+/**
+ * Defines the DistanceDonation class.
+ * For distanceDonations a donor pledges to donate a certain amount for each kilometer ran by a runner.
+*/
+export class DistanceDonation extends Donation {
+  /**
+   * The donation's associated runner.
+   * Used as the source of the donation's distance.
+   */
+  @IsObject()
+  @IsNotEmpty()
+  runner: Runner;
+
+  /**
+   * The donation's amount donated per distance.
+   * The amount the donor set to be donated per kilometer that the runner ran.
+   */
+  @IsInt()
+  @IsPositive()
+  amountPerDistance: number;
+
+  /**
+   * The donation's amount in cents (or whatever your currency's smallest unit is.).
+   * Get's calculated from the runner's distance ran and the amount donated per kilometer.
+   */
+  public get amount(): number {
+    let calculatedAmount = 0;
+    try {
+      calculatedAmount = this.amountPerDistance * (this.runner.distance / 1000);
+    } catch (error) {
+      throw error;
+    }
+    return calculatedAmount;
+  }
+
+}