Rails: named_scope, lambda and blocks

问题:

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?

回答1:

it's a parser problem. try this

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

回答2:

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 

回答3:

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! 

回答4:

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 

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

文章来源: Rails: named_scope, lambda and blocks