I'm doing a simple blog application - There are posts, which have many tags through a posts_tags table (my models are below). What I have implemented is if a user clicks a tag, it will show just the posts with that tag. What I want is for the user to them be able to select another tag, and it will filter to only the posts that have both of those tags, then a third, then a fourth, etc. I'm having difficulty making the active record query - especially dynamically. The closest I've gotten is listed below - however its in pure SQL and I would like to at least have it in ActiveRecord Rubyland syntax even with the complexity it contains.
Also, the "having count 2" does not work, its saying that "count" does not exist and even if I assign it a name. However, it is outputting in my table (the idea behind count is that if it contains a number that is as much as how many tags we are searching for, then theoretically/ideally it has all the tags)
My current test SQL query
select posts_tags.post_id,count(*) from posts_tags where tag_id=1 or tag_id=3 group by post_id ### having count=2
The output from the test SQL (I know it doesnt contain much but just with some simple seed data).
post_id | count
---------+-------
1 | 2
2 | 1
My Models:
/post.rb
class Post < ActiveRecord::Base
has_many :posts_tags
has_many :tags, :through => :posts_tags
end
/tag.rb
class Tag < ActiveRecord::Base
has_many :posts_tags
has_many :posts, :through => :posts_tags
end
/poststag.rb
class PostsTag < ActiveRecord::Base
belongs_to :tag
belongs_to :post
end
Give a try to:
Post.joins(:tags).where(tags: {id: [1, 3]}).select("posts.id, count(*)").group("posts.id").having("count(*) > 2")
I think "count = 2" is not correct. It should be "count(*) = 2". Your query then will be
select post_id,count(post_id)
from posts_tags
where tag_id = 1 or tag_id = 3
group by post_id
having count(post_id) = 2
In general you want to stay away from writing raw sql when using rails. Active Record has great helper methods to make your sql more readable and maintainable.
If you only have a few tags you can create scopes for each of them (http://guides.rubyonrails.org/active_record_querying.html#scopes)
Since people are clicking on tags one at a time you could just query for each tag and then use the & operator on the arrays. Because you have already requested the exact same set of data from the database the query results should be cached meaning you are only hitting the db for the newest query.
Related
I have something like this:
duplicates = ['a','b','c','d']
if duplicates.length > 4
Photo.includes(:tags).where('tags.name IN (?)',duplicates)
.references(:tags).limit(15).each do |f|
returned_array.push(f.id)
end
end
duplicates is an array of tags that were duplicated with other Photo tags
What I want is to get Photo which includes all tags from duplicates array, but right now I get every Photo that include at least one tag from array.
THANKS FOR ANSWERS:
I try them and somethings starts to work but wasn't too clear for me and take some time to execute.
Today I make it creating arrays, compare them, take duplicates which exist in array more than X times and finally have uniq array of photos ids.
If you want to find photos that have all the given tags you just need to apply a GROUP and use HAVING to set a condition on the group:
class Photo
def self.with_tags(*names)
t = Tag.arel_table
joins(:tags)
.where(tags: { name: names })
.group(:id)
.having(t[:id].count.eq(tags.length)) # COUNT(tags.id) = ?
end
end
This is somewhat like a WHERE clause but it applies to the group. Using .gteq (>=) instead of .eq will give you records that can have all the tags in the list but may have more.
A better way to solve this is to use a better domain model that doesn't allow duplicates in the first place:
class Photo < ApplicationRecord
has_many :taggings
has_many :tags, through: :taggings
end
class Tag < ApplicationRecord
has_many :taggings
has_many :photos, through: :taggings
validates :name,
uniqueness: true,
presenece: true
end
class Tagging < ApplicationRecord
belongs_to :photo
belongs_to :tag
validates :tag_id,
uniqueness: { scope: :photo_id }
end
By adding unique indexes on tags.name and a compound index on taggings.tag_id and taggings.photo_id duplicates cannot be created.
The issue as I see it is that you're only doing one join, which means that you have to specify that tags.name is within the list of duplicates.
You could solve this in two places:
In the database query
In you application code
For your example the query is something like "find all records in the photos table which also have a relation to a specific set of records in the tags table". So we need to join the photos table to the tags table, and also specify that the only tags we join are those within the duplicate list.
We can use a inner join for this
select photos.* from photos
inner join tags as d1 on d1.name = 'a' and d1.photo_id = photos.id
inner join tags as d2 on d2.name = 'b' and d2.photo_id = photos.id
inner join tags as d3 on d3.name = 'c' and d3.photo_id = photos.id
inner join tags as d4 on d4.name = 'd' and d4.photo_id = photos.id
In ActiveRecord it seems we can't specify aliases for joins, but we can chain queries, so we can do something like this:
query = Photo
duplicate.each_with_index do |tag, index|
join_name = "d#{index}"
query = query.joins("inner join tags as #{join_name} on #{join_name}.name = '#{tag}' and #{join_name}.photo_id = photos.id")
end
Ugly, but gets the job done. I'm sure there would be a better way using arel instead - but it demonstrates how to construct a SQL query to find all photos that have a relation to all of the duplicate tags.
The other method is to extent what you have and filter in the application. As you already have the photos that has at least one of the tags, you could just select those which have all the tags.
Photo
.includes(:tags)
.joins(:tags)
.where('tags.name IN (?)',duplicates)
.select do |photo|
(duplicates - photo.tags.map(&:name)).empty?
end
(duplicates - photo.tags.map(&:name)).empty? takes the duplicates array and removes all occurrences of any item that is also in the photo tags. If this returns an empty array then we know that the tags in the photo had all the duplicate tags as well.
This could have performance issues if the duplicates array is large, since it could potentially return all photos from the database.
I have following models in my rails app:
class Student < ApplicationRecord
has_many :tickets, dependent: :destroy
has_and_belongs_to_many :articles, dependent: :destroy
class Article < ApplicationRecord
has_and_belongs_to_many :students, dependent: :destroy
class Ticket < ApplicationRecord
belongs_to :student, touch: true
I need to extract all Students who has less than articles and I need to extract all Students who's last ticket title is 'Something'.
Everything I tried so far takes a lot of time. I tried mapping and looping through all Students. But I guess what I need is a joined request. I am looking for the most efficient way to do it, as database I am working with is quite large.
go with #MCI's answer for your first question. But a filter/select/find_all or whatever (although I havn't heared about filter method in ruby) through students record takes n queries where n is the number of student records (called an n+1 query).
studs = Student.find_by_sql(%{select tmp.id from (
select student_id as id from tickets where name='Something' order by tickets.created_at desc
) tmp group by tmp.id})
You asked
"I need to extract all Students who has less than articles". I'll presume you meant "I need to extract all Students who have less than X articles". In that case, you want group and having https://guides.rubyonrails.org/active_record_querying.html#group.
For example, Article.group(:student_id).having('count(articles.id) > X').pluck(:student_id).
To address your second question, you can use eager loading https://guides.rubyonrails.org/active_record_querying.html#eager-loading-associations to speed up your code.
result = students.filter do |student|
students.tickets.last.name == 'Something'
end
Here association is HABTM so below query should work
x = 10
Student.joins(:articles).group("articles_students.student_id").having("count(articles.id) < ?",x)
I have a user model which has many subscriptions. I need to make two selections:
active users defined as user with a subscription in the last month
inactive users the ones that don't meet the (1) criteria
My subscription model has a simple scope .latest which is defined as ordered("created_at DESC").first.
To make selection (1) I use:
User.joins(:subscriptions).where("subscriptions.created_at > ?", 1.month.ago).distinct
This works, no problem there. However, I can't seem to define a working query for selection (2). Currently I use selection (1) and 'subtract' that from User.all to get the remaining users. This feels a bit like a hack.
The selection I need is:
all users whose most recent subscription was created more than 1 month ago
It's the most recent part of the query that has me stuck.
Any help appreciated.
Quick & dirty way: use complex SQL like this
Assume you are using auto incremental ID
User.select("users.*, MAX(subscriptions.id) as last_subscription_id")
.joins(:subscriptions)
.group("subscriptions.user_id")
.having("last_subscription_id = (select id from subscriptions where user_id=users.id and created_at < '2017-10-01 09:23:28.182475' order by id desc limit 1)")
Recommended way
Add last_subscription_id to users table and setup a belongs_to relationship user belongs_to last_subscription then do the joins normally. You need to update last_subscription_id of an user when new subscription for this user is created too.
Example: User class looks like this (I include has_many :subscriptions to show that we have 2 relations now)
class User < ActiveRecord::Base
has_many :subscriptions
belongs_to :last_subscription, class_name: 'Subscription', foreign_key: :last_subscription_id
end
And query will be
User.joins(:last_subscription).where("subscriptions.created_at < ?", 1.month.ago)
for the most recent part you can do this.
User.joins("LEFT JOIN subscriptions ON subscriptions.user_id = users.id").order("subscriptions.created_at DESC").select("subscriptions.created_at AS max_date").group_by(&:id).select{|key, value| value[0].max_date < 1.month.ago}.values.flatten
Consider I have 3 tables: Users (has_many) -> Websites (has_many) -> Visits.
How would one find out the total number of Visits each user has without writing plain sql code?
I have an idea of which I'm not very proud since I let rails do the math instead of mysql:
count = 0
user.websites.each |website|
count += website.visits.count()
I'm new with rails and maybe i'm missing some docs. Is it possible to find out that count just from the query builder?
you can define that the User has many Visits through the Websites like this:
class User < ActiveRecord::Base
has_many :websites
has_many :visits, :through => :websites
end
now, if you do
some_user.visits.count
this sql is executed:
(0.4ms) SELECT COUNT(*) FROM "visits" INNER JOIN "websites" ON "visits"."website_id" = "websites"."id" WHERE "websites"."user_id" = 1
=> 8
That is, ActiveRecord creates the SQL query for you.
I am trying to do a QUERY in my controller to get a list of suppliers with a category ID.
I have my models set up like this.
class Supplier < ActiveRecord::Base
has_and_belongs_to_many :sub_categories
end
class Category < ActiveRecord::Base
has_many :sub_categories
end
class SubCategory < ActiveRecord::Base
belongs_to :category
has_and_belongs_to_many :suppliers
end
A supplier can have Many sub_categories that are under one single category. So i can grab the category of a supplier by doing this.
#supplier.sub_categories.first.category.name
This returns the category that the supplier comes under because they have to have at least 1 sub category which is then linked to a category.
What i am trying to do is by passing a category_id i wish to return all suppliers that come under that category.
I had it written like this but it doesnt seem to be working.
#category = Category.find(params[:category_id])
#suppliers = Supplier.where('sub_category.first.category.id = ?', #category.id)
i get the following sql error
Mysql2::Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '.id = 20)' at line 1: SELECT `suppliers`.* FROM `suppliers` WHERE (sub_category.first.category.id = 20)
Well, that's certainly an sql error. The stuff inside the where() call gets translated directly to SQL, and that's not sq;l. :)
You need to join tables together. I'm assuming there's a sub_category_suppliers table that completes the habtm association. (BTW, I much prefer to use has_many :through exclusively)
I think it would be something like this:
Supplier.joins(:sub_category_suppliers => :sub_categories).
where('sub_categories.category_id =?', #category.id).
group('suppliers.id')
As Caley Woods suggested, this should be placed in the Supplier model as a scope:
scope :by_category, lambda { |category_id|
joins(:sub_category_suppliers => :sub_categories).
where('sub_categories.category_id =?', category_id).
group('suppliers.id')
}
and then called as Supplier.by_category(#category.id)