可能的重复：
i = true and false in Ruby is true?
What is the difference between Perl's ( or, and ) and ( ||, && ) short-circuit operators?
Ruby: difference between || and 'or'||是否与rails中的or相同？
案例A：
@year = params[:year] || Time.now.year
Events.all(:conditions => ['year = ?', @year])
将在
script/console中生成以下sql： SELECT * FROM `events` WHERE (year = 2000)
Case B：
@year = params[:year] or Time.now.year
Events.all(:conditions => ['year = ?', @year])
将在
script/console中生成以下sql： SELECT * FROM `events` WHERE (year = NULL)
最佳答案
与或行为不同的原因是因为运算符的优先级。
和&&的优先级都高于赋值运算符，赋值运算符（=）的优先级高于和/或
所以你的表达式将被计算如下：@year = params[:year] || Time.now.year
评估为@year = ( params[:year] || Time.now.year )
和@year = params[:year] or Time.now.year
评估为( @year = params[:year] ) or Time.now.year
如果对优先权规则有疑问，请用括号把你的意思说清楚。