Ruby on Rails error when validates_uniqueness_of using collection_select - ruby-on-rails

Question

First, sorry for my bad English. I'm still learning.
I have 3 tables in my DB:
Problem
has_many :registers
has_many :solutions, through : :registers
Solution
has_many :problems
has_many :problems, through : :registers
Register
belongs_to: problem
belongs_to :solution
The system is working well. I am able to insert new data in all of the 3 tables.
In the views for the table/model Register, to select problems and solutions, I make use of collection_select, like this:
= form_for #register do |f|
.field
= f.label :problem_id
= collection_select( :register, :problem_id, #problems, :id, :name, {}, { :multiple => false })
.field
= f.label :solution_id
= collection_select( :register, :solution_id, #courses, :id, :name, {}, { :multiple => false })
.field
= f.label :entry_at
= f.datetime_select :entry_at
.actions = f.submit
The problem only appears when I try to add this validation to Register:
validates_uniqueness_of :student_id , scope: :course_id
Then I get:
> undefined method `map' for nil:NilClass
> = collection_select( :register, :problem_id, #problems, :id, :name, {}, { :multiple => false })
And I dont know why.
So, I tried to do the validation by the controller:
def create
#register = Register.new(register_params)
problem_id = #register.problem_id
solution_id = #register.solution_id
if Register.exists?(['problem_id LIKE ? AND solution_id LIKE ?', problem_id, solution_id ])
render 'new'
else
#register.save
respond_with(#register)
end
end
But the error remains.
I believe that the cause is the collection_select, but I don't know how to solve it.
Saying one more time, I am able to persist date in all the 3 DB tables. But when I try to avoid duplication, the error appears.

Answer 1

This is how I solve this problem:
def create
#register = register.new(register_params)
#if #register.save
# respond_with(#register)
#else
# #register = register.all
# render :new
#end
problem_id = #register.problem_id
solution_id = #register.solution_id
if register.exists?(['problem_id LIKE ? AND solution_id LIKE ?', problem_id, solution_id ])
#register = register.new
#solutions = Solution.all
#problems = Problem.all
flash[:error] = "Problem alread in the register for this solution"
render 'new'
else
#register.save
respond_with(#register)
end
end