Implement completion endpoint for allowance (#55)

Closes #19

Reviewed-on: #55

backend/db.go

@@ -175,6 +175,39 @@ func (db *Db) DeleteAllowance(userId int, allowanceId int) error {
 	return nil
 }
 
+func (db *Db) CompleteAllowance(userId int, allowanceId int) error {
+	tx, err := db.db.Begin()
+	if err != nil {
+		return err
+	}
+	defer tx.MustRollback()
+
+	// Get the cost of the allowance
+	var cost int
+	err = tx.Query("select balance from allowances where id = ? and user_id = ?").
+		Bind(allowanceId, userId).ScanSingle(&cost)
+	if err != nil {
+		return err
+	}
+
+	// Delete the allowance
+	err = tx.Query("delete from allowances where id = ? and user_id = ?").
+		Bind(allowanceId, userId).Exec()
+	if err != nil {
+		return err
+	}
+
+	// Add a history entry
+	err = tx.Query("insert into history (user_id, timestamp, amount) values (?, ?, ?)").
+		Bind(userId, time.Now().Unix(), -cost).
+		Exec()
+	if err != nil {
+		return err
+	}
+
+	return tx.Commit()
+}
+
 func (db *Db) UpdateUserAllowance(userId int, allowance *UpdateAllowanceRequest) error {
 	tx, err := db.db.Begin()
 	if err != nil {
@@ -356,24 +389,26 @@ func (db *Db) CompleteTask(taskId int) error {
 
 		remainingReward := reward
 
-		// Distribute the reward to the allowances
-		for allowanceRow := range tx.Query("select id, weight from allowances where user_id = ? and weight > 0").Bind(userId).Range(&err) {
-			var allowanceId int
-			var allowanceWeight float64
-			err = allowanceRow.Scan(&allowanceId, &allowanceWeight)
-			if err != nil {
-				return err
-			}
+		if sumOfWeights > 0 {
+			// Distribute the reward to the allowances
+			for allowanceRow := range tx.Query("select id, weight from allowances where user_id = ? and weight > 0").Bind(userId).Range(&err) {
+				var allowanceId int
+				var allowanceWeight float64
+				err = allowanceRow.Scan(&allowanceId, &allowanceWeight)
+				if err != nil {
+					return err
+				}
 
-			// Calculate the amount to add to the allowance
-			amount := int((allowanceWeight / sumOfWeights) * float64(remainingReward))
-			sumOfWeights -= allowanceWeight
-			err = tx.Query("update allowances set balance = balance + ? where id = ? and user_id = ?").
-				Bind(amount, allowanceId, userId).Exec()
-			if err != nil {
-				return err
+				// Calculate the amount to add to the allowance
+				amount := int((allowanceWeight / sumOfWeights) * float64(remainingReward))
+				sumOfWeights -= allowanceWeight
+				err = tx.Query("update allowances set balance = balance + ? where id = ? and user_id = ?").
+					Bind(amount, allowanceId, userId).Exec()
+				if err != nil {
+					return err
+				}
+				remainingReward -= amount
 			}
-			remainingReward -= amount
 		}
 
 		// Add the remaining reward to the user