Here is what I'm trying to do
class Question
has_many :votes
end
class Vote
belongs_to :question
end
I want to find all questions ordered by the number of votes they have. I want to express this in Arel (in Rails 3) without using any counter caches.
Is there any way of doing this ?
Thanks.
Try next one:
Question.joins(:votes).select("questions.id, *other question coulmns*, count(votes.id) as vote_count").order("vote_count DESC").group("questions.id")
Try this:
Question.select("questions.*, a.vote_count AS vote_count").
joins("LEFT OUTER JOIN (
SELECT b.question_id, COUNT(b.id) AS vote_count
FROM votes b
GROUP BY b.question_id
) a ON a.question_id = questions.id")
Solution is DB agnostic. Make sure you add an index on the question_id column in the votes table( you should add the index even if you don't use this solution).
Related
In my rails application, at some point, I query my model simply. I want to query customers order information like how many orders were given by this customer within three months.
Just now, I query the model in that way:
#customer = Customer.all
customer.rb
class Customer < ApplicationRecord
audited
has_many :orders
end
And customer may have orders.
order.rb
class Order < ApplicationRecord
audited
belongs_to :customer
end
What I would like to do is to query customers model and to inject aggregate function result to every customer records.
EDİT
I tried to simulate every solution but couln't achieve.
I have the following query in mysql.
How do I need to code in ruby with activerecord to create that query ?
SELECT
(SELECT
COUNT(*)
FROM
orders o
WHERE
o.customer_id = c.id
AND startTime BETWEEN '2017.12.04' AND '2018.01.04') AS count_last_month,
(SELECT
COUNT(*)
FROM
orders o
WHERE
o.customer_id = c.id
AND startTime BETWEEN '2017.10.04' AND '2018.01.04') AS count_last_three_month,
c.*
FROM
customers c;
How can I achieve that?
Thanks.
Customer.
joins(:orders).
group('customers.id').
where('orders.created_at > DATE_SUB(NOW(), INTERVAL 3 MONTH)')
select('sum(orders.id), customers.*')
As my understanding of you question. I have this solution for you question. Please have a look and try it once. In below query, 'includes' used to solve N+1 problem.
Customer.includes(:orders).where('created_at BETWEEN ? AND ?', Time.now.beginning_of_day, Time.now.beginning_of_day-3.months).group_by{|c|c.orders.count}
If you are looking for particular customer's order count then you can try this one.
#customer.orders.where('created_at BETWEEN ? AND ?', Time.now.beginning_of_day, Time.now.beginning_of_day-3.months).group_by{|c|c.orders.count}
here are my models:
class Article
has_many :votes
end
class Vote
belongs_to :article
belongs_to :user
end
Now I am trying to order the articles, by the count of votes in the past 24 hours. Any suggestions for how to do this?
I have tried this:
Article.left_joins(:votes).group("articles.id").order("count(votes.id) DESC")
However, this is ordering by all the votes, not the votes in last 24h. Any suggestions?
One more thing is, I still need to get the articles with no votes. So I am not sure how to use the where clause here...
You need to add the date when the vote was created for an article prior to the count of votes in your order.
Try this:
Article.left_joins(:votes)
.group("articles.id")
.order("DATE(votes.created_at) DESC, count(votes.id) DESC")
Then if you only want to get the articles that has been upvoted for the past 24hr, you can chain this to your query:
.where("votes.created_at >= ?", 1.day.ago)
Finally I got it work. It turns out left_joins is not necessary. My current solution is using select clause in the order() function:
Article.order("(select count(*) from votes where votes.article_id = articles.id and votes.created_at >= NOW() - '1 day'::INTERVAL ) desc")
Maybe not elegant, but works well.
When I use this request
#questions = Question.joins("left join answers as a on a.user_id = #{current_user.id} and a.question_id = questions.id").select('questions.*, a.*')
Question.id is null. Does anyone know why? Maybe it needs an alias or something like that.
My schema:
class Answer:
belong_to: User
belongs_to: Question
end
class User:
has_many: answers
end
class Question:
has_one: answer
end
The problem is that the answers table probably also has a column named id, which is shadowing the question's id. Although in the result set one column is called questions.id and the other a.id, active record just looks at the last part ('id')
There's no good way around this that I am aware of other than aliasing problematic columns from the answers table, which unfortunately means explicitly naming all of them (although you can of course automate this to an extent)
This is probably because of the ID columns of two tables conflicting with one another.
Update:
Simply changing the order of select columns will help. So instead of .select("questions.*", "answers.*"), try .select("answers.*", "questions.*").
Old answer:
Try the following:
# Assuming the following:
# Question.table_name == "questions"
# Answer.table_name == "answers"
question_columns = Question.column_names # or %w{questions.*}
answer_columns = Answer.column_names.map { |c| "answer_#{c}" }
columns = question_columns + answer_columns
#questions =
Question.
joins("LEFT OUTER JOIN answers ON (
answers.question_id = questions.id
AND
answers.user_id = #{current_user.id}
)").
select(*columns)
#questions.first.id #=> should return some integer
#questions.first.answer_id #=> may or may not return something
However, unless you absolutely need a LEFT JOIN along with those select-columns, it would be much cleaner to accomplish your task using the following:
class Answer
belongs_to :question
end
class User
has_many :answers
has_many :questions, through: :answers
end
current_user.questions
I came across about the problem excluding data, if the attribute x of one of the associated data has the value 'a'.
Example:
class Order < ActiveRecord::Base
has_many :items
end
class Item < ActiveRecord::Base
belongs_to :order
validate_presence_of :status
end
The query should return all Orders that don't have an Item with status = 'paid' (status != 'paid').
Because of the 1:n association an Order can have many Items. And one of the Itmes can have the status = 'paid'. These Orders must be excluded from the result of my query even if the order has other items with status different from 'paid'.
How would I solve this problem:
paid_items = Items.where(status: 'paid').pluck(:order_id)
orders_wo_paid = Order.where('id NOT IN (?)', paid_items)
Is there an ActiveRecord solution, that solves this problem in one query.
Or are there other ways to solve this question?
I 'm not looking for ruby solution such as:
Order.select do |order|
!order.items.pluck(:status).include?('paid')
end
thx for ideas and inspirations.
You can do:
Order.where('orders.id NOT IN (?)', Item.where(status: 'paid').select(:order_id))
If you're using Rails 4.x then:
Order.where.not(id: Item.where(status: 'paid').select(:order_id))
The query you are interested in is the following, but creating with activerecord will be hard/no very readable:
SELECT
orders.*
FROM
orders
LEFT JOIN
order_items ON orders.id = order_items.order_id
GROUP BY
order_items.order_id
HAVING
COUNT(DISTINCT order_items.id) = COUNT(DISTINCT order_items.status <> 'paid')
Sorry for the sql indentation, I have no idea which are the conventions for it.
A way (not the best one at all) to it with rails (unfortunately writing sql for the most important parts) would be the following:
Order.group(:order_id).joins("LEFT JOIN order_items ON orders.id = order_items.order_id")
.having("COUNT(DISTINCT order_items.id) = COUNT(DISTINCT order_items.status <> 'paid')")
Of course you can play with AREL to get rid of the hard coded sql, but in my opinion it will not be easier to read.
You can have an example of creating lefts joins in this gist: https://gist.github.com/mildmojo/3724189
Support I have two models for items and categories, in a many-to-many relation
class Item < ActiveRecord::Base
has_and_belongs_to_many :categories
class Category < ActiveRecord::Base
has_and_belongs_to_many :items
Now I want to filter out categories which contain at least one items, what will be the best way to do this?
I would like to echo #Delba's answer and expand on it because it's correct - what #huan son is suggesting with the count column is completely unnecessary, if you have your indexes set up correctly.
I would add that you probably want to use .uniq, as it's a many-to-many you only want DISTINCT categories to come back:
Category.joins(:items).uniq
Using the joins query will let you more easily work conditions into your count of items too, giving much more flexibility. For example you might not want to count items where enabled = false:
Category.joins(:items).where(:items => { :enabled => true }).uniq
This would generate the following SQL, using inner joins which are EXTREMELY fast:
SELECT `categories`.* FROM `categories` INNER JOIN `categories_items` ON `categories_items`.`category_id` = `categories`.`id` INNER JOIN `items` ON `items`.`id` = `categories_items`.`item_id` WHERE `items`.`enabled` = 1
Good luck,
Stu
Category.joins(:items)
More details here: http://guides.rubyonrails.org/active_record_querying.html#joining-tables
please notice, what the other guys answererd is NOT performant!
the most performant solution:
better to work with a counter_cache and save the items_count in the model!
scope :with_items, where("items_count > 0")
has_and_belongs_to_many :categories, :after_add=>:update_count, :after_remove=>:update_count
def update_count(category)
category.items_count = category.items.count
category.save
end
for normal "belongs_to" relation you just write
belongs_to :parent, :counter_cache=>true
and in the parent_model you have an field items_count (items is the pluralized has_many class name)
http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html
in a has_and_belongs_to_many relation you have to write it as your own as above
scope :has_item, where("#{table_name}.id IN (SELECT categories_items.category_id FROM categories_items")
This will return all categories which have an entry in the join table because, ostensibly, a category shouldn't have an entry there if it does not have an item. You could add a AND categories_items.item_id IS NOT NULL to the subselect condition just to be sure.
In case you're not aware, table_name is a method which returns the table name of ActiveRecord class calling it. In this case it would be "categories".