Rails - Validate unique month in year

Question

Using Rails 6.

I have a model, MonthlyConsumption , which is a measure of a monthly energy consumption. It contains the following fields:

• month as an integer from 1-12; This is later converted into a month using some key-value pairs eg 1: 'January'
• year : an integer, eg 2020
• amount : an integer, the amount of power consumption in kW
• city_id : A reference to a City model

I want to validate that there is only 1 possible entry in one month of a given year, per city.

For example:

• 500kW in January 2020 in City X & 600kW in January 2021 in City X should both be valid
• 500kW in January 2020 in City X & 600kW in January 2020 in City Y should both be valid
• 500kW in January 2020 in City X & 600kW in January 2020 in City X should be invalid

What would be the best way of accomplishing this validation?

Answer 1

就像是：

validates :month, uniqueness: { scope: [:city_id, :year] }