How to make use of ruby procs to dynamically execute a database query

Question

I have a model called Bucket which has this instance method Bucket#populate_students_academicwise(mentor)

def populate_students_academicwise(mentor)
  student_ids = Mark.find(:all,
    :joins => self.student_current_klass,
    :conditions => ["marks.subject_id = ? AND st.klass_id = ? AND marks.klass_id = ? AND u.account_enabled IS TRUE AND sub.active IS TRUE AND k.active IS TRUE", mentor.subject_id, mentor.klass_id, mentor.klass_id],
    :group => "marks.student_id",
    :having => ["ROUND(AVG(marks_obtained)/AVG(marks_total)*100) BETWEEN ?  AND  ?", min_range, max_range]).collect(&:student_id)

  self.assign_students(student_ids)
end

Now, this query returns a set of students whose academic performance is between a range of values

ROUND(AVG(marks_obtained)/AVG(marks_total)*100) BETWEEN ?  AND  ?)

I call this method from a bucket instance like this bucket.populate_students_academicwise(mentor)

I'd like to negate that query, meaning, return the set of students whose academic performance is NOT between a range of values. All I can think of is to create another method that runs negative of the above query. So I have another method Bucket#negate_populate_students_academicwise(mentor)

def negate_populate_students_academicwise(mentor)
    Mark.find(:all,
      :joins => self.student_current_klass,
      :conditions => ["marks.subject_id = ? AND st.klass_id = ? AND marks.klass_id = ? AND u.account_enabled IS TRUE AND sub.active IS TRUE AND k.active IS TRUE", mentor.subject_id, mentor.klass_id, mentor.klass_id],
      :group => "marks.student_id",
      :having => ["ROUND(AVG(marks_obtained)/AVG(marks_total)*100) NOT BETWEEN ?  AND  ?", min_range, max_range]).collect(&:student_id)
end

Now, this query returns a set of students whose academic performance is NOT between a range of values ROUND(AVG(marks_obtained)/AVG(marks_total)*100) NOT BETWEEN ? AND ? ROUND(AVG(marks_obtained)/AVG(marks_total)*100) NOT BETWEEN ? AND ?

How can I call the first method Bucket#populate_students_academicwise(mentor) with a negate method appended to it bucket.populate_students_academicwise(mentor).negate that would call a proc to negate the query?

Answer 1

So you want to basically have both functionalities and still have them in a DRY way?

I would say you should use a default parameter.

bucket.populate_students_academicwise(mentor) # dont negate
bucket.populate_students_academicwise(mentor,false) # negate

Within you method

def populate_students_academicwise(mentor,in_range = true)
  student_ids = Mark.find(:all,
                   :joins => self.student_current_klass,
                   :conditions => ["marks.subject_id = ? AND st.klass_id = ? AND marks.klass_id = ? AND u.account_enabled IS TRUE AND sub.active IS TRUE AND k.active IS TRUE", mentor.subject_id, mentor.klass_id, mentor.klass_id],
                   :group => "marks.student_id",
                   :having => ["ROUND(AVG(marks_obtained)/AVG(marks_total)*100) #{'NOT' if !in_range} BETWEEN ?  AND  ?", min_range, max_range]).collect(&:student_id)
  self.assign_students(student_ids)
end

With that little bit of query manipulation you get your NOT (or no NOT) depending on the parameter and dont need to worry about proc here.