Filter nested model attributes in RoR - ruby-on-rails

Question

I have two models: Users and PaymentMethods, the association between this models is:
class User < ApplicationRecord
has_many :payment_methods, dependent: :destroy
end
class PaymentMethod < ApplicationRecord
belongs_to :user, optional: true
end
I want to loop in each user and see in an attribute of PaymentMethod, named 'period_end_date'. so I do this:
#users = User.all
#users.each do |u|
u.payment_methods.last.period_end_date
end
I'm getting this error => NoMethodError: undefined method `payment_methods' for User::ActiveRecord_Relation
The error is shown because I have 2 test users, in the first user there is still no data in the attribute 'period_end_date' and association exist, but is empty, in the second user there is data in the attributes, if I say, u.payment_methods.last.period_end_date I get => Wed, 13 Jun 2018 (only in the second user)
I want to filter in my loop only the users who has data in PaymentMethod attributes for get rid of => NoMethodError: undefined method `payment_methods' for User::ActiveRecord_Relation
How I do this?
thanks

Answer 1

I want to filter in my loop only the users who has data in PaymentMethod attributes for get rid of => NoMethodError: undefined method `payment_methods' for User::ActiveRecord_Relation
The actual problem seems to be you have users without payment methods (see my comment on your question).
You have some options, depending on how you're going to use the results.
1) You can filter out users without payment methods when you query them from the database like this:
#users = User.joins :payment_methods
2) If #users must include users that without payment methods, you can skip them when looping like this:
#users.map do |user|
next unless user.payment_methods.any?
user.payment_methods.last.period_end_date
end
3) You can guard by checking for payment_methods before calling .last.
User.all.map do |user|
user.payment_methods.last.period_end_date if user.payment_methods.any?
end
4) You can add a period_end_date method to the user
class User < ApplicationRecord
def period_end_date
payment_methods.limit(1).pluck :period_end_date
end
end
5) push #4 into the association by extending it with a helper method
class User < ApplicationRecord
has_many :payment_methods, class_name: 'PaymentMethod' do
def last_period_end_date
last.period_end_date if any?
end
end
end
which you can call like this
User.all.map do |user|
user.payment_methods.last_period_end_date
end
If you're really only concerned about PaymentMethods without a period_end_date then try this:
6) You can still filter users when you query them from the database
#users = User.joins(:payment_methods).where.not(payment_methods: { period_end_date: nil })
7) This can be simplified a bit by pushing the where.not conditions into a scope of the PaymentMethod class:
class PaymentMethod < ApplicationRecord
scope :period_ends, -> { where.not period_end_date: nil }
end
and merging it
#users = User.joins(:payment_methods).merge PaymentMethod.period_ends
Notes
payment_methods.last doesn't specify an order, you should set one (either as part of this chain, when you specify the association, or with a default scope) otherwise the order is up to your database and may be indeterminate.
chain .includes(:payment_methods) to eager load the payment methods and avoid n+1 queries
it sounds like a nil period_end_date could be invalid data. Consider adding a validation / database constraint to prevent this from happening