Run mysql query from a controller in ruby on rails

You can execute raw SQL through ActiveRecord::Base.connection but I rarely recommend it and certainly not in this case, but for edification purposes

def set_dcs
  sql= <<SQL 
      select employees.nombre, employees.apellido_paterno, employees.apellido_materno 
      from employees, activities, field_tests
      where activities.field_test_id=field_tests.id and employees.id=activities.dcs_id and field_tests.id=id
SQL 
  ActiveRecord::Base.connection.exec_query(sql)
end

I was not sure what the trailing id is in reference to and it will raise a SQL error due to it's ambiguity. I am going to assume it is a parameter and the primary search condition where as the rest are table joins.

That being said since you are using rails these can be true associations and joins resulting in far more readable controller code e.g. Model Definitions

class Employee < ActiveRecord::Base
   has_many :activities, foreign_key: :dcs_id
   has_many :field_tests, through: :activities
end 
class FieldTest < ActiveRecord::Base
   has_many :activities
end
class Activity < ActiveRecord::Base
   belongs_to :employee, foreign_key: :dcs_id
   belongs_to :field_test
end

Then the controller is simply

 Employee.
  select("employees.nombre, employees.apellido_paterno, employees.apellido_materno").
  joins(:field_tests).where(field_tests: {id: SOME_ID}) 

the resulting SQL will be similar to

 SELECT 
   employees.nombre, 
   employees.apellido_paterno, 
   employees.apellido_materno
 FROM 
   employees
   INNER JOIN activities ON employees.id = activities.dcs_id 
   INNER JOIN field_tests ON activities.field_test_id = field_tests.id
 WHERE 
   field_tests.id = SOME_ID

And this will return a collection of Employee Objects rather than an ActiveRecord::Result (returned from ActiveRecord::Base.connection.exec_query) which is more similar to an Array than anything else.