@instance_variable not available inside a ruby block?

问题

With the following code:

def index
  @q = ""
  @q = params[:search][:q] if params[:search]
  q = @q
  @search = Sunspot.search(User) do
    keywords q
  end
  @users = @search.results
end

If @q is used instead of q, the search always returns results for an empty query (""). Why is this? Is the @q variable unavailable to the do...end block?

回答1:

It depends on how the block is being called. If it is called using the yield keyword or the Proc#call method, then you'll be able to use your instance variables in the block. If it's called using Object#instance_eval or Module#class_eval then the context of the block will be changed and you won't be able to access your instance variables.

@x = "Outside the class"

class Test
  def initialize
    @x = "Inside the class"
  end

  def a(&block)
    block.call
  end

  def b(&block)
    self.instance_eval(&block)
  end
end

Test.new.a { @x } #=> "Outside the class"
Test.new.b { @x } #=> "Inside the class"

In your case, it looks like Sunspot.search is calling your block in a different context using instance_eval, because the block needs easy access to that keywords method.

回答2:

As Jeremy says, Sunspot executes its search DSL in a new scope.

In order to use an instance variable in the Sunspot.search block, you'll need to pass it an argument. Something like this should work (not tested):

  @q = params[:search][:q] if params[:search]
  @search = Sunspot.search(User) do |query|
    query.keywords @q
  end
  @users = @search.results

See here for a better explanation: http://groups.google.com/group/ruby-sunspot/msg/d0444189de3e2725

来源：https://stackoverflow.com/questions/5150483/instance-variable-not-available-inside-a-ruby-block

标签

ruby

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instance

block