Error undefined local variable or method `created_at' - mysql

Question

I write query to find out how much the user has reduced calories in a week, but I have this error.
How to avoid mistakes?
def self.calories_burned(current_user)
week = ((created_at - current_user.first_program_started_at.utc.beginning_of_day) / 86400 / 7).ceil.to_i || 1
find_by_sql("
SELECT
count(*) as cnt,
WEEK(#{week}) as week_number
FROM
user_daily_updates
WHERE
user_id=#{current_user.id}
GROUP BY
week_number
")
end

Answer 1

When you write this:
def self.calories_burned(current_user)
etc...
end
it's a method that can only be called like this:
MyUserModel.calories_burned(some_user)
In this case you are running the code on the class before it is instantiated, this means that the model hasn't even attached itself to a connection to the database and because of that you will not be able to access attributes that pertain to your model.
On the other hand, if you write this:
def calories_burned
etc...
end
You don't need to pass the user to the method, you simply call it on the controller after instantiating your model, like this:
id = 123
current_user = MyUserModel.find(id)
current_user.calories_burned
where current_user.calories_burned will return the value you are looking for based on the current user.
After taking a closer look at your method
it should look more like this:
def calories_burned
week = ((created_at - first_program_started_at.utc.beginning_of_day) / 86400 / 7).ceil.to_i || 1
UserDailyUpdate.where(["user_id = ?", id]).where(["week = ?",week]]).count(:all, :group => 'week_number')
end
If I understood correctly what you were trying to do with your query, this should give you the same result. Now, I should mention I am assuming that when you created the table user_daily_updates, you also created a model UserDailyUpdate.
You should take a look at ActiveRecord's documentation for searching using conditions (all of section 2) in order to have a better understanding of what I just did.

Answer 2

Apneadiving is right, but I would also look to refactor your code:
def self.calories_burned(current_user)
week = ((##created_at## - current_user.first_program_started_at.utc.beginning_of_day) / 86400 / 7).ceil.to_i || 1
select("#{self.table_name}.*, count(*) as cnt, WEEK(#{week}) as week_number").where(user_id: current_user.id).group(:week_number)
end
You'll need to pull created_at from the db, as it won't be present unless you have an instance of a class already loaded