Rails 4 How to access model attributes in instance method

Question

I'm running into a seemingly simple problem, but I can't figure out what the problem is. I'm building an API using Rails 4.

I have a model called Constant

class Constant < ActiveRecord::Base

  def self.calcuate_something(a, b, c, d, e)
    self.number_one + self.number_two + (self.number_three*a) + (self.number_four*b) + (self.number_five*c) + (self.number_six*d) + (self.number_seven*e)
  end

end

This model has attributes number_one, number_two, number_three, number_four, number_five, number_six, and number_seven.

In my controller, I'm doing this:

@constant = Constant.find_by_id(params[:id])
number = @constant.calculate_something(1, 2, 3, 4, 5)

And then I get an error that says NoMethodError (undefined method 'number_one' for #<Class:0x007fd3d4466068>):. I'm not trying to access a method called number_one, I'm trying to find the field in the model.

I've verified that there is a column called number_one for all of my entries and that every single entry has a value. Is there something wrong with my syntax or setup??

Thanks in advance!

Keith · Accepted Answer · 2014-08-19 21:40:53Z

2

The issue is with your definition of self.calculate_something(a, b, c, d, e). Using the keyword self is actually creating a class method instead of an instance method, and therefore doesn't have access to those attributes.

Try refactoring it to def calculate_something(a, b, c, d, e)

Update from comments:

The second issues is @constant is being assigned to the ActiveRecord_Relation generated by Constant.find_by_id(params[:id]) instead of the record instance since rails doesn't automatically evaluate queries until necessary. This can be fixed by changing find_by_id to find, or force the query to execute by Constant.find_by_id(params[:id]).first. Personally, I like changing to find as you know the specific record you want.

edited Aug 19, 2014 at 21:40

answered Aug 19, 2014 at 21:24

Keith

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2 Comments

srisonti Over a year ago

But then when I do that, I get a NoMethodError for calculate_something. More specifically, it looks like this NoMethodError (undefined method 'number_one' for #<Constant::ActiveRecord_Relation:0x007fd3d43fec88>):. Why is this happening??

Keith Over a year ago

ahh, doing find_by_id is returning a result set, which is the ActiveRecord_Relation part. Change find_by_id to find, or change the statement to Constant.find_by_id(params[:id]).first

Mohamed Yakout · Accepted Answer · 2014-08-19 21:37:14Z

1

Read about Class and Instance Methods in Ruby, you want to define method for object as instance method without self.:

def calculate_something(a, b, c, d, e)

But, if you want to define class method, so you need to use self.

Don't use find_by_id, you need to use find as the following:

@constant = Constant.find(params[:id])

answered Aug 19, 2014 at 21:37

Mohamed Yakout

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