Proper way to write a value for an attribute whose name is in a variable?

Question

Lots of searching here and around suggested that you could use a second argument to send() to write a value to an attribute, but in Rails 4 you are told you have the wrong number of parameters:

> prj = Project.where(:id => 123).first
> fieldname = "project_start_date"
> prj.send(fieldname, Date.today)
ArgumentError : wrong number of arguments (1 for 0)

That approach was thought to be synonymous with

> prj.write_attribute(fieldname, Date.today)

But that errors with

NoMethodError : private method `write_attribute'

which is odd since the docs say this is a part of the instance public methods.

The ActiveRecord docs suggest using the class update method:

# Updates one record
Person.update(15, user_name: 'Samuel', group: 'expert')

# Updates multiple records
people = { 1 => { "first_name" => "David" }, 2 => { "first_name" => "Jeremy" } }
Person.update(people.keys, people.values)
So what's a Rails 4 guy supposed to do?

In my case that would translate to:

Project.update(123, project_start_date: '2013/09/04') #not using variables for testing sake

and that yields me a nice:

 ActiveRecord::StatementInvalid: PG::SyntaxError: ERROR:  zero-length delimited identifier at or near """"
LINE 1: ...dual".* FROM "project"  WHERE "project"."" = $1 LI...

So what's a Rails 4 user supposed to use, other than writing out the actual SQL statements?

Peter Brown · Accepted Answer · 2013-09-04 10:20:31Z

2

You're close in your first example. The reason send isn't working is because you're essentially trying to do:

prj.project_start_date(Date.today)

which doesn't make sense because the project_start_date method does not take an argument. You need to change it to the setter

prj.project_start_date = Date.today

which would translate to:

prj.send("#{fieldname}=", Date.today)

answered Sep 4, 2013 at 10:20

Peter Brown

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1 Comment

tobinjim Over a year ago

Thank you for uncrossing my eyeballs on this one!

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