postgresql Rails 连接查询
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Rails joins query
提问by 1dolinski
I have three models
我有三个模型
Tag=>:id,:nameTagging=>:id,:tag_id,:post_idPost=>:id,:summary
Tag=>:id,:nameTagging=>:id,:tag_id,:post_idPost=>:id,:summary
I know the id of the tag. I would like to query for all of the posts that have a specific tag_id, through the Taggingsmodel.
我知道标签的 ID。我想tag_id通过Taggings模型查询所有具有特定,的帖子。
Something like
就像是
@post = Post.joins(:taggings).where(:tag_id => 17)
but this doesn't work because it is looking for the tag_id in the Postmodel and not the Taggingmodel.
但这不起作用，因为它正在寻找Post模型中的 tag_id而不是Tagging模型。
I'm not sure how to do this.
我不知道该怎么做。
回答by Raphael Abreu
I don't like to use string in ActiveRecord queries, so, I prefer this sintax:
我不喜欢在 ActiveRecord 查询中使用字符串，所以，我更喜欢这个语法：
@post = Post.joins(:taggings).where(taggings: {tag_id: 17})
回答by Alexandru Emil Lupu
First of all :
首先 ：
class Post < ActiveRecord::Base
has_many :taggings
has_many :tags, :through => :taggings
end
class Taggins < ActiveRecord::Base
belongs_to :post
belongs_to :tag
end
class Tag < ActiveRecord::Base
has_many :taggings
has_many :posts, :through => :taggings
end
If you have the tag object you can do
如果你有标签对象，你可以做
@posts = @tag.posts
or
或者
class Post < ....
....
def self.find_by_tag_id(tag_id)
Post.joins(:taggings).where('taggings.tag_id = ?', tag_id)
end
end
回答by Tom Harrison
Using the .where format you can pass a string like .where("taggings.tag_id = ?", 17) to qualify the joined taggings table.
使用 .where 格式，您可以传递像 .where("taggings.tag_id = ?", 17) 这样的字符串来限定连接的标记表。
回答by 1dolinski
As @tharrison mentioned. A solution is:
正如@tharrison 提到的。一个解决办法是：
@post = Post.joins(:taggings).where("taggings.tag_id = ?", 17)
