Rails accessing object from string containing object name

Question

I am attempting to access an object based on a string containing the object's name. The object is scoped (I am using cancan to ensure that users can only access records they are allowed to access). I have included some code below with where I have got to:

operation_type:string, one of %w(add sub mult div)
left_operand_object:string
left_operand:string
right_operand_object:string
right_operand:string


def result
    r = [access object using left_operand_object]
    k = [access object using right_operand_object]

    if operation_type == 'add'
        r.instance_variable_get('@'+left_operand) + k.instance_variable_get('@'+left_operand)
    elsif operation_type == 'sub'
        r.instance_variable_get('@'+left_operand) - k.instance_variable_get('@'+left_operand)
    ...
end

So, two questions:

How can I complete the r = [access object using left_operand_object] line, to allow me to access the scoped object based on its name from the string left_operand_object?
Will instance_variable_get work if left_operand is something like customers.count or would it only work without the count method?

Any help greatly appreciated!

Nils Landt · Accepted Answer · 2012-07-02 18:24:01Z

1

Did I understand correctly, left_operand_object would be something like "Customer"?

If so, you can use left_operand_object.constantize to get your Customer class.
Probably safer to run left_operand_object.classify.constantize, so "row_entry" would become RowEntry.

I'm not perfectly clear what your second question is about. If you want to call the count method, you could do r.send(left_operand.to_sym) (assuming `left_operand = "count").

If left_operand is something like "sales.count", this would not work. I don't know if there's an idiomatic way to do it, but left_operand.split(".").inject(r) { |a, b| a.send b } does the trick for this particular case.

edited Jul 2, 2012 at 18:24

answered Jul 1, 2012 at 22:31

Nils Landt

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2 Comments

Harry Over a year ago

Yes, you understand correctly - left_operand_object would be something like "customer". Thanks for the info, I'll look into using constantize - I assume this will assign the object to the r variable, as expected?

Harry Over a year ago

I think the second question was a little unclear - I am asking if I can use the count method inside instance_variable_get to return the count as an integer, but I think your answer has given me the direction I need to go - left_operand would be something like "sales.count" (assuming I was trying to use customer.sales.count).

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