Ruby-on-rails 检查 Rails 中的控制器是否存在记录

Check if record exists from controller in Rails

提问by MrYoshiji

In my app a User can create a Business. When they trigger the indexaction in my BusinessesControllerI want to check if a Business is related to the current_user.id:

在我的应用程序中，用户可以创建业务。当他们触发index我的操作时，我BusinessesController想检查一个业务是否与以下内容相关current_user.id：

• If yes: display the business.
• If no: redirect to the newaction.

• 如果是：显示业务。
• 如果否：重定向到new操作。

I was trying to use this:

我试图使用这个：

if Business.where(:user_id => current_user.id) == nil
  # no business found
end

But it always returns true even when the business doesn't exist...

但即使业务不存在，它也总是返回 true ......

How can I test if a record exists in my database?

如何测试我的数据库中是否存在记录？

回答by MrYoshiji

Why your code does not work?

为什么你的代码不起作用？

The wheremethod returns an ActiveRecord::Relationobject (acts like an array which contains the results of the where), it can be empty but it will never be nil.

该where方法返回一个ActiveRecord::Relation对象（就像一个包含 的结果的数组where），它可以是空的，但永远不会是nil。

Business.where(id: -1) 
 #=> returns an empty ActiveRecord::Relation ( similar to an array )
Business.where(id: -1).nil? # ( similar to == nil? )
 #=> returns false
Business.where(id: -1).empty? # test if the array is empty ( similar to .blank? )
 #=> returns true

---

---

How to test if at least one record exists?

如何测试是否至少存在一条记录？

Option 1:Using .exists?

选项 1：使用.exists?

if Business.exists?(user_id: current_user.id)
  # same as Business.where(user_id: current_user.id).exists?
  # ...
else
  # ...
end

---

---

Option 2:Using .present?(or .blank?, the opposite of .present?)

选项 2：使用.present?（或.blank?，相反.present?）

if Business.where(:user_id => current_user.id).present?
  # less efficiant than using .exists? (see generated SQL for .exists? vs .present?)
else
  # ...
end

---

---

Option 3:Variable assignment in the if statement

选项 3：if 语句中的变量赋值

if business = Business.where(:user_id => current_user.id).first
  business.do_some_stuff
else
  # do something else
end

This option can be considered a code smell by some linters (Rubocop for example).

某些 linters（例如 Rubocop）可以将此选项视为代码异味。

Option 3b:Variable assignment

选项 3b：变量分配

business = Business.where(user_id: current_user.id).first
if business
  # ...
else
  # ...
end

You can also use .find_by_user_id(current_user.id)instead of .where(...).first

您也可以使用.find_by_user_id(current_user.id)代替.where(...).first

---

---

Best option:

最佳选择：

• If you don't use the Businessobject(s): Option 1
• If you need to use the Businessobject(s): Option 3

• 如果您不使用Business对象：选项 1
• 如果您需要使用Business对象：选项 3

回答by Hamed

In this case I like to use the exists?method provided by ActiveRecord:

在这种情况下，我喜欢使用exists?ActiveRecord 提供的方法：

Business.exists? user_id: current_user.id

回答by user2427993

with 'exists?':

与“存在？”：

Business.exists? user_id: current_user.id #=> 1 or nil

with 'any?':

与“任何？”：

Business.where(:user_id => current_user.id).any? #=> true or false

---

---

If you use something with .where, be sure to avoid trouble with scopes and better use .unscoped

如果你使用.where 的东西，一定要避免作用域的问题，最好使用 .unscoped

Business.unscoped.where(:user_id => current_user.id).any?

回答by Puhlze

ActiveRecord#where will return an ActiveRecord::Relation object (which will never be nil). Try using .empty? on the relation to test if it will return any records.

ActiveRecord#where 将返回一个 ActiveRecord::Relation 对象（永远不会为零）。尝试使用 .empty？在关系上测试它是否会返回任何记录。

回答by marimaf

When you call Business.where(:user_id => current_user.id)you will get an array. This Array may have no objects or one or many objects in it, but it won't be null. Thus the check == nil will never be true.

当你打电话时，Business.where(:user_id => current_user.id)你会得到一个数组。这个数组可能没有对象，也可能有一个或多个对象，但它不会为空。因此 check == nil 永远不会是真的。

You can try the following:

您可以尝试以下操作：

if Business.where(:user_id => current_user.id).count == 0

So you check the number of elements in the array and compare them to zero.

因此，您检查数组中的元素数量并将它们与零进行比较。

or you can try:

或者你可以尝试：

if Business.find_by_user_id(current_user.id).nil?

this will return one or nil.

这将返回一或零。

回答by Nino van Hooff

business = Business.where(:user_id => current_user.id).first
if business.nil?
# no business found
else
# business.ceo = "me"
end

回答by user3633260

I would do it this way if you needed an instance variable of the object to work with:

如果您需要使用对象的实例变量，我会这样做：

if @business = Business.where(:user_id => current_user.id).first
  #Do stuff
else
  #Do stuff
end