So I'm getting a bunch of Volunteers records, with some filtering and sorting, which is fine. But I'd like to also get a count of the number of Children each volunteer is helping (using volunteer_id on children table), as a sub-query in the select clause to avoid having to perform a separate query for each record. As a bonus it would be good to be able to sort by this count too!
I'd like to end up with a generated query like this and be able to access the 'kids' column:
SELECT id, name, (SELECT COUNT(*) FROM children WHERE volunteer_id = volunteers.id) AS kids FROM volunteers
Is there any way of doing this with Arel? I've had a bit of a scout around and haven't found anything yet.
Alternatively, is it possible to join to the children table and get: count(children.id) ?
Thanks for any help :)
The proper way of doing this with SQL is with a GROUP BY clause:
SELECT v.id, v.name, COUNT(*) AS kids
FROM volunteers v
LEFT OUTER JOIN children c ON v.id = c.volunteer_id
GROUP BY v.id, v.name
There is a method .group() in AR for using GROUP BY queries.
Related
There are three models that matter here: Objective, Student, and Seminar. All are associated with has_and_belongs_to_many.
There is an ObjectiveStudent join model that includes columns "ready" and "points_all_time". There is an ObjectiveSeminar join model that includes column "priority".
I need to collect all of the objectives that are associated with a given student and also with a given seminar.
They need to also be marked with a "priority" above zero in the seminar. So I think I need this line:
obj_sems = ObjectiveSeminar.where(:seminar => given_seminar).where("priority > ?", 0)
Finally, they need to also be objectives where the student is ready, but has not scored above 7. So I think I need this line:
obj_studs = ObjectiveStudent.where(:user => given_student, :ready => true).where("points_all_time <= ?", 7)
Is there a way to gather all the objectives whose join table records appear in both of the above queries? Note that neither of the lists return objectives; they return objective_seminars, and objective_students, respectively. My end goal is to collect the objectives that meet all of the above criteria.
Or am I approaching this all wrong?
Bonus question: I would also love to sort the objectives by their priority in the given seminar. But I'm afraid that would add too much to the database load. What are your thoughts on this?
Thank you in advance for any insight.
In order to get Objectives you'll need to start your query from that.
In order to query with an AND condition the associated tables, you'll need inner joins with these tables.
Finally you'll need a distinct operator to only fetch each objective once.
The extended version of what (I think) you need is:
Objective.joins(objective_seminars: :seminar, objective_student: :student).
where(seminars: seminar_search_params, strudents: student_search_params).
where('objective_seminars.priority > 0').
where('objective_students.ready = 1 AND points_all_time <= 7').
order('objective_seminars.priority ASC').
distinct
Now for the database load it all depends on your indexes and the size of your tables.
The above query will translate to the following SQL (or something similar).
SELECT DISTINCT objectives.* FROM objectives
INNER JOIN objective_students ON objective_students.objective_id = objectives.id
INNER JOIN students ON students.id = objective_students.student_id
INNER JOIN objective_seminars ON objective_seminars.objective_id = objectives.id
INNER JOIN seminars ON seminars.id = objective_seminars.seminar_id
WHERE seminars_query AND
students_query AND
objective_seminars.priority > 0 AND
objective_students.ready = 1 AND points_all_time <= 7 AND
objective_seminars.priority ASC
So you'll need to add or extend your indexes so that all 5 tables queries can have an index helping out. The actual index implementation is up to you and depends on your application's specific (read - write load, tables size, cardinality etc)
I have a working SQL query for Postgres v10.
SELECT *
FROM
(
SELECT DISTINCT ON (title) products.title, products.*
FROM "products"
) subquery
WHERE subquery.active = TRUE AND subquery.product_type_id = 1
ORDER BY created_at DESC
With the goal of the query to do a distinct based on the title column, then filter and order them. (I used the subquery in the first place, as it seemed there was no way to combine DISTINCT ON with ORDER BY without a subquery.
I am trying to express said query in ActiveRecord.
I have been doing
Product.select("*")
.from(Product.select("DISTINCT ON (product.title) product.title, meals.*"))
.where("subquery.active IS true")
.where("subquery.meal_type_id = ?", 1)
.order("created_at DESC")
and, that works! But, it's fairly messy with the string where clauses in there. Is there a better way to express this query with ActiveRecord/Arel, or am I just running into the limits of what ActiveRecord can express?
I think the resulting ActiveRecord call can be improved.
But I would start improving with original SQL query first.
Subquery
SELECT DISTINCT ON (title) products.title, products.* FROM products
(I think that instead of meals there should be products?) has duplicate products.title, which is not necessary there. Worse, it misses ORDER BY clause. As PostgreSQL documentation says:
Note that the “first row” of each set is unpredictable unless ORDER BY is used to ensure that the desired row appears first
I would rewrite sub-query as:
SELECT DISTINCT ON (title) * FROM products ORDER BY title ASC
which gives us a call:
Product.select('DISTINCT ON (title) *').order(title: :asc)
In main query where calls use Rails-generated alias for the subquery. I would not rely on Rails internal convention on aliasing subqueries, as it may change anytime. If you do not take this into account you could merge these conditions in one where call with hash-style argument syntax.
The final result:
Product.select('*')
.from(Product.select('DISTINCT ON (title) *').order(title: :asc))
.where(subquery: { active: true, meal_type_id: 1 })
.order('created_at DESC')
My requirement is to get distinct records and in order
User.joins('INNER JOIN report_posts ON posts.id = report_posts.post_id').select('DISTINCT ON (report_posts.post_id) posts.id as report_posts.id as reported_id, report_posts.reported_at').order('report_posts.reported_at desc')
I know this is not possible in postgresql, I already read this Postgresql DISTINCT ON with different ORDER BY
I want its solution that how can I do do this, its alternate way?
You need to include the DISTINCT column in your order:
.order('report_posts.post_id, report_posts.reported_at desc')
I want to expand this question.
order by foreign key in activerecord
I'm trying to order a set of records based on a value in a really large table.
When I use join, it brings all the "other" records data into the objects.. As join should..
#table users 30+ columns
#table bids 5 columns
record = Bid.find(:all,:joins=>:users, :order=>'users.ranking DESC' ).first
Now record holds 35 fields..
Is there a way to do this without the join?
Here's my thinking..
With the join I get this query
SELECT * FROM "bids"
left join users on runner_id = users.id
ORDER BY ranking LIMIT 1
Now I can add a select to the code so I don't get the full user table, but putting a select in a scope is dangerous IMHO.
When I write sql by hand.
SELECT * FROM bids
order by (select users.ranking from users where users.id = runner_id) DESC
limit 1
I believe this is a faster query, based on the "explain" it seems simpler.
More important than speed though is that the second method doesn't have the 30 extra fields.
If I build in a custom select inside the scope, it could explode other searches on the object if they too have custom selects (there can be only one)
What you would like to achieve in active record writing is something along
SELECT b.* from bids b inner join users u on u.id=b.user_id order by u.ranking desc
In active record i would write such as:
Bids.joins("inner join users u on bids.user_id=u.id").order("u.ranking desc")
I think it's the only to make a join without fetching all attributes from the user models.
Hi I'm trying to practice SQL, and this is more for console use not in my actual model. I have a Collection model that has_many: products. I want to do a search for all the collections that have more than 15 products.
This is what I have written (and is wrong):
Collection.find_by_sql("SELECT c.id FROM collections c WHERE count(product_id) > 15")
Can anyone help me out? Thanks
You can do this with a subquery:
SELECT *,
(SELECT COUNT(*) FROM services WHERE collection_id = c.id) services_count
FROM collections c WHERE services_count > 15
Can you try this?, I have not tested it.
Also, there is a way to do this without plain SQL, using Active Record, but I don't remember how to do it.
Maybe this sql query will help you
select collections.*, count(products.id) as count_products from collections
inner join products on products.collection_id = collections.id
having count_products > 15