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Rails: named_scope, lambda and blocks

Posted by: admin November 30, 2017 Leave a comment

Questions:

I thought the following two were equivalent:

named_scope :admin, lambda { |company_id| {:conditions => ['company_id = ?', company_id]} }

named_scope :admin, lambda do |company_id| 
  {:conditions => ['company_id = ?', company_id]}
end

but Ruby is complaining:

ArgumentError: tried to create Proc object without a block

Any ideas?

Answers:

it’s a parser problem. try this

named_scope :admin, (lambda do |company_id| 
  {:conditions => ['company_id = ?', company_id]}
end)

Questions:
Answers:

I think the problem may be related to the difference in precedence between {...} and do...end

There’s some SO discussion here

I think assigning a lambda to a variable (which would be a Proc) could be done with a do
... end
:

my_proc = lambda do 
  puts "did it"
end
my_proc.call #=> did it

Questions:
Answers:

It’s something related to precedence as I can tell

1.upto 3 do # No parentheses, block delimited with do/end
  |x| puts x 
end

1.upto 3 {|x| puts x } # Syntax Error: trying to pass a block to 3!

Questions:
Answers:

If you’re on ruby 1.9 or later 1, you can use the lambda literal (arrow syntax), which has high enough precedence to prevent the method call from “stealing” the block from the lambda.

named_scope :admin, ->(company_id) do 
  {:conditions => ['company_id = ?', company_id]}
end

1 The first stable Ruby 1.9.1 release was 2009-01-30.