How to set a property in ruby datamapper that is a function of another property of the same object?

Question

I want to add two new properties to a ruby datamapper model, one that is a date cast of a timestamp property and another that is a value from another object connected through a unique key.

So for the first case I have property :date, DateTime

and I want to add another property :date_date, Date that by default will be equal to date.to_date

Answer 1

You should see the docs: https://datamapper.org/docs/properties.html

Specifically the sections on "Available Types" and "Setting default values".

You can do it like this:

property :date_date, Date, default: -> do |obj, prop|
  obj.date.to_date
end

You can alternatively set it through a callback (https://datamapper.org/docs/callbacks.html), for example:

property :date_date, Date

before_save :set_date_date

def set_date_date
  self.date_date = date.to_date
end

Note it works basically the same way in Rails' ActiveRecord as well.