I have the following relationship in Rails: Campaign -has-many- Promise(s)
And I have the following Ruby code to return verbose list of campaigns with promises count:
def campaigns
results = Campaign
.where(user_id: current_user.id)
.left_outer_joins(:promises)
.select('campaigns.*', 'COUNT(DISTINCT promises.id) AS promises_count')
.group('campaigns.id')
.ransack(params[:q])
.result(distinct: true)
render json: {
results: results.page(params[:page]).per(params[:per_page]),
total_results: results.count(:id)
}
end
Everything works fine, unless I try to sort by promises_count. Ransack (or something else?) generates the following SQL for Postgres:
SELECT DISTINCT
campaigns.*,
COUNT(DISTINCT promises.id) AS promises_count
FROM "campaigns"
LEFT OUTER JOIN "promises"
ON "promises"."campaign_id" = "campaigns"."id"
LEFT OUTER JOIN "promises" "promises_campaigns"
ON "promises_campaigns"."campaign_id" = "campaigns"."id"
WHERE "campaigns"."user_id" = 1 GROUP BY campaigns.id;
It works, but there is no ORDER BY for some reason. When I sort by other properties, it works fine. I think Ransack is missing something, and handles promises_count different way because it's generated property and not real one.
It's possible to sort in Postgres, for example, manual query with added ORDER BY works:
SELECT DISTINCT
campaigns.*,
COUNT(DISTINCT promises.id) AS promises_count
FROM "campaigns"
LEFT OUTER JOIN "promises"
ON "promises"."campaign_id" = "campaigns"."id"
LEFT OUTER JOIN "promises" "promises_campaigns"
ON "promises_campaigns"."campaign_id" = "campaigns"."id"
WHERE "campaigns"."user_id" = 1
GROUP BY campaigns.id
ORDER BY promises_count desc;
How do I make Ransack work? I tried different combinations of queries without too much luck.
Related
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')
I have the following code and output:
SELECT CustomerCategoryName, COUNT(a.CustomerID) AS CustomersInThisCategory
FROM Sales.Customers AS a
RIGHT JOIN Sales.CustomerCategories AS b on a.CustomerCategoryID = b.CustomerCategoryID
GROUP BY CustomerCategoryName
ORDER BY CustomersInThisCategory DESC
This generates the following output:
When I add the following COUNT aggregate funcation and Inner Join:
SELECT CustomerCategoryName, COUNT(a.CustomerID) AS CustomersInThisCategory, COUNT(c.OrderID) AS Orders
FROM Sales.Customers AS a
RIGHT JOIN Sales.CustomerCategories AS b on a.CustomerCategoryID = b.CustomerCategoryID
INNER JOIN Sales.Orders AS c ON a.CustomerID = c.CustomerID
GROUP BY CustomerCategoryName
ORDER BY CustomersInThisCategory DESC
The output changes to:
I am not sure as to why the CustomersInThisCategory column is changing to the same as the Orders column? I'm also not sure why the results in the first ouput with 0 values are being removed in the second query as I still have the Right join present.
Any feedback would be much appreciated.
For your first query, count ( distinct a.customerId) should give you the unique customer ids in a category.
Regarding your second question, right join is performed before inner join. So the inner join will splice the records, for which a match is not found.
Hope my answer helps.
When I do includes it left joins the table I want to filter on, but when I add pluck that join disappears. Is there any way to mix pluck and left join without manually typing the sql for 'left join'
Here's my case:
Select u.id
From users u
Left join profiles p on u.id=p.id
Left join admin_profiles a on u.id=a.uid
Where 2 in (p.prop, a.prop, u.prop)
Doing this is just loading all the values:
Users.includes(:AdminProfiles, :Profiles).where(...).map{ |a| a[:id] }
But when I do pluck instead of map, it doesn't left join the profile tables.
Your problem is that you're using includes which doesn't really do a join, instead it fires a second query after the first one to query for the associations, in your case you want them both to be actually joined, so for that replace includes(:something) with joins(:something) and every thing should work fine.
Replying to your comment, i'm gonna quote few parts from the rails guide about active record query interface
From the section Solution to N + 1 queries problem
clients = Client.includes(:address).limit(10)
clients.each do |client|
puts client.address.postcode
end
The above code will execute just 2 queries, as opposed to 11 queries in the previous case:
SELECT * FROM clients LIMIT 10
SELECT addresses.* FROM addresses WHERE (addresses.client_id IN (1,2,3,4,5,6,7,8,9,10))
as you can see, two queries, no joins at all.
From the section Specifying Conditions on Eager Loaded Associations link
Even though Active Record lets you specify conditions on the eager loaded associations just like joins, the recommended way is to use joins instead.
Then an example:
Article.includes(:comments).where(comments: { visible: true })
This would generate a query which contains a LEFT OUTER JOIN whereas the joins method would generate one using the INNER JOIN function instead.
SELECT "articles"."id" AS t0_r0, ... "comments"."updated_at" AS t1_r5 FROM "articles" LEFT OUTER JOIN "comments" ON "comments"."article_id" = "articles"."id" WHERE (comments.visible = 1)
If there was no where condition, this would generate the normal set of two queries.
What I want to do is to join table and sum 3 columns.
self.document_products.joins("JOIN products ON products.id = document_products.product_id").group("products.tax_id").select("sum(a), sum(b), sum(c)")
Gives me
#<ActiveRecord::Relation [#<DocumentProduct id: nil>]>
Something like that works:
self.document_products.joins("JOIN products ON products.id = document_products.product_id").group("products.tax_id").sum("a")
But I want to have 3 sums. I can`t do sum("a, b, c"). Where is the problem?
So, the code is building a SQL query using the ActiveRecord chained method syntax. It's possible to use .to_sql as the final part of most such chains (basically, as long as it's still an ActiveRecord object, rather than having been converted to an Array, for example) to see the SQL generated, or indeed inspecting the log, if it's on. Considering the common part of the chain:
self.document_products.joins("JOIN products ON products.id = document_products.product_id").group("products.tax_id")
This generates something like (might not be exact, because I'm guessing a little about your application):
SELECT "document_products".* FROM "document_products" JOIN products ON products.id = document_products.product_id WHERE "document_products"."document_id" = 1497 GROUP BY products.tax_id
The two final methods you list are very different; select selects which columns in the query to return, whereas sum is an aggregate function which expects a single value to be returned in each case. Considering the select, we get something like the following generated:
SELECT SUM(products.a), SUM(products.b), SUM(products.c) FROM "document_products" JOIN products ON products.id = document_products.product_id WHERE "document_products"."document_id" = 1497 GROUP BY products.tax_id
When this query is interpreted, the expected data cannot be found, leading to the problem described. Ensuring that the GROUP BY clause is included in the SELECT part, however, yields the necessary information. Try something like this:
self.document_products.joins("JOIN products ON products.id = document_products.product_id").group("products.tax_id").select("products.tax_id, sum(a), sum(b), sum(c)")
This generates SQL something like:
SELECT products.tax_id, SUM(products.a), SUM(products.b), SUM(products.c) FROM "document_products" JOIN products ON products.id = document_products.product_id WHERE "document_products"."document_id" = 1497 GROUP BY products.tax_id
This appears to return the necessary information, and is, I think, what you're looking for (or close to it).
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.