Rails 3或arel链/合并SQL条件有条件

Question

是否有一种方法在Rails 3中有条件地合并记录条件而不做字符串或数组合并?

例如:

    conditions = #?    
    if !params[:x].blank?
      # add a condition
    end
    if user.role?(:admin)
      # add a condition
    end
    if params[:y]
     # add a condition
    end
    etc
    result = Record.where(xx).group(:id).order(some_var) 
    # xx would merge all then group and order

Answer 1

Easy:

# oh, the joy of lazy evaluation...
result = Record.where(true) # I'd like to do Record.all, but that fetches the records eagerly!
result = result.where("...") if params[:x].present?
result = result.where("...") if user.role?(:admin)
result = result.where("...") if params[:y].present?

顺便说一下，不要在irb中尝试这样做:"read-eval-print"的"print"部分将强制计算记录集。

EDIT:我刚刚发现你可以用Record.scoped代替Record.where(true)。但是，这在Rails 4中不起作用。