Ruby-on-rails 在Rails中通过它的belongs_to关系查询记录
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Query records through its belongs_to relation in Rails
提问by Will
I have an Activities model, and they belong_to a Location
我有一个活动模型，它们属于一个位置
How do i select all the activities whose location.country = Australia? (for example)
如何选择 location.country = Australia 的所有活动？（例如）
Can I do this within a scope?
我可以在一个范围内做到这一点吗？
回答by Nycen
With the latest rails versions you can do:
使用最新的 rails 版本，您可以执行以下操作：
Activity.joins(:location).where(locations: { country: "Australia" })
Beware:
谨防：
- it is location (singular) in
joins(:location)because it's a belongs_to relationship - it is locations (plural) in
where(…)because it's the table name
- 它是位置（单数）
joins(:location)因为它是一个belongs_to关系 - 它是位置（复数），
where(…)因为它是表名
The latter means that if you had the following:
后者意味着，如果你有以下几点：
belongs_to :location, class_name: "PublicLocation"
the query would be:
查询将是：
Activity.joins(:location).where(public_locations: { country: "Australia" })
回答by Andrew
The kind of query you're talking about is a join. You can try queries like this in the console like:
您正在谈论的查询类型是连接。您可以在控制台中尝试这样的查询，例如：
Activity.joins(:locations).where('locations.country = "Australia"')
This means that SQL is going to take all the activities and locations associated with then, find the locations where country=Australia, and then return you the activities that are associated with those locations.
这意味着 SQL 将获取与 then 关联的所有活动和位置，找到 country=Australia 的位置，然后返回与这些位置关联的活动。
To make this into a more reusable scope, define it on your model with a variable for country:
为了使其成为更可重用的范围，请在您的模型上使用国家变量定义它：
scope :in_country, lambda {|country| joins(:locations).where('locations.country = ?',country)}
You can learn more about this in the API docs.
您可以在API 文档 中了解更多相关信息。
回答by Dave S.
Yes, a scope can be used. Something like this ought to work on the Activities model:
是的，可以使用范围。像这样的事情应该适用于活动模型：
scope :down_under,
joins(:locations).
where("locations.country = 'Australia')
