当从db获取用户时，我应该如何检查null?

If I attempt to fetch a record from the db like:

如果我试图从db中获取一条记录，比如:

@user = User.find_by_user_name("blah")

How do I check if the value was null? i.e. the record was found?

如何检查该值是否为空?也就是说找到了记录?

And should I use and or && as they seem to be different?

我是否应该使用或者&&因为它们看起来是不同的?

if @user and @user.age > 0
  ....
end

5 个解决方案

#1

if @user
  # not nil
else
  # nil
end

should be suffice to check if the value was not nil.

应该足以检查值是否为nil。

And the difference between && and 'and' is that && has a higher precedence over 'and'. See here

与“And”的区别在于&&的优先级高于“And”。在这里看到的

#2

This will raise an exception if the value is null:

如果值为空，则会引发异常:

@user = User.find_by_user_name!("blah")

#3

If there's no record, @user will be nil, so an if check will work.

如果没有记录，@user将为nil，因此，如果检查将工作。

The difference between && and and is precedence. && has higher precedence.

与&&之间的区别是优先级。& &具有更高的优先级。

#4

@user will be nil if it hasn't been found, as suggested. You can check for nil explicitly like so, of course:

@user如果没有找到，将为nil。你可以像这样明确地检查nil，当然

@user.nil?
@user === nil

nil will falsify an if statement, like false. See NilClass and FalseClass for differences.

nil将伪造if语句，比如false。差异见NilClass和FalseClass。

#5

As mentioned by others, the finder will return nil if the record was not found, thus you have a few options, including the one you mentioned in your question and the other answers here.

正如其他人所提到的，如果没有找到记录，finder将返回nil，因此您有一些选项，包括您在问题中提到的那个选项和这里的其他答案。

Since @user = User.find_by_user_name("blah") evaluates to nil if the user isn't found, I personally prefer the following Ruby idiom:

由于@user = User.find_by_user_name(“blah”)在未找到用户时将计算为nil，所以我个人更喜欢下面的Ruby习惯用法:

if @user = User.find_by_user_name("blah")
  # do stuff with @user
else
  # not found
end

#1

if @user
  # not nil
else
  # nil
end