I'm having a really rough time figuring out how to do this query and others like it in arel from active record.
select users.id,
users.name,
maps.count as map_count,
from users
left join (select user_id, count(map_id) as count from maps_users group by user_id) maps on users.id = maps.user_id
On the surface, it looks just like Nik's example here (http://magicscalingsprinkles.wordpress.com/2010/01/28/why-i-wrote-arel/):
photo_counts = photos.
group(photos[:user_id]).
project(photos[:user_id], photos[:id].count)
users.join(photo_counts).on(users[:id].eq(photo_counts[:user_id]))
But I can't get it to work in rails using active record. I think the equivalent should be something like this, but it errors out :(
maps = Map.arel_table
map_counts = Map.group(maps[:owner_id]).
select(maps[:owner_id]).
select(maps[:id].count.as("map_count"))
users = User.joins(map_counts).on(User.arel_table[:id].eq(map_counts[:map_count]))
Any ideas on how to do it?
Well first replace the select with project. In relational algebra SELECT (restriction) is the WHERE clause.
Secondly you can do subselections.
sub_restriction = b.
where( b[:domain].eq(1) ).
project( b[:domain] )
restriction = a.
where( a[:domain].in sub_restriction )
"sub selections" DONE! :-)
Yeah, that article really made me want to learn Arel magic, too.
All the "do something intelligent with Arel" questions on Stackoverflow are getting answered with SQL. From articles and research, then, I can say that Arel is Not ActiveRecord. Despite the dynamic formulation of queries, Active doesn't have the power to map the results of a fully formed Arel projection.
You get the ability to specify operators with
https://github.com/activerecord-hackery/squeel
but no subselects.
Updated: OMG, I answered this question 5 years ago. No kidding the link was dead :)
Related
I need an active record relation that gives me the latest record of a region, city, bed combination. I have the sql query written as below, but I need to figure out if there is away to use a different approach to have it return an active record relation and not an array. Any suggestions?
Current query:
#current_ltm_market_stats = LtmStatsByBedCount.find_by_sql(" SELECT *
FROM ltm_stats_by_bed_counts lstats
WHERE lstats.city_id = '#{#city_id}'
#{#region_id_condition}
AND (lstats.beds,lstats.city_id,lstats.region_id,lstats.reporting_date)
IN (SELECT lstats.beds,
lstats.city_id,
lstats.region_id,
max(lstats.reporting_date)
FROM ltm_stats_by_bed_counts lstats
WHERE lstats.city_id = '#{#city_id}'
#{#region_id_condition}
GROUP BY city_id, region_id, beds)
ORDER BY lstats.year DESC,lstats.month DESC")
I had tried this before which did result in a relation but it runs really slowly and the result is not exactly the same. Are there any better rails ways to do this?
#all_ltm_market_stats = LtmStatsByBedCount.where(city_id: #market.city_id, region_id: #market.region_id)
#current_ltm_market_stats = #latest_year_ltm_market_stats.where(month: #latest_year_ltm_market_stats.all_ltm_market_stats.select('Max(year)'))
Information in the question is incomplete, so i might have to update my answer when additional details are added, But here is the initial draft with available information:
#current_ltm_market_stats = LtmStatsByBedCount.
where(city_id: #city_id).
where(#region_id_condition).
where("(beds, city_id, region_id, reporting_date) IN (
SELECT lstats.beds,
lstats.city_id,
lstats.region_id,
max(lstats.reporting_date)
FROM ltm_stats_by_bed_counts lstats
WHERE lstats.city_id = '#{#city_id}'
#{#region_id_condition}
GROUP BY city_id, region_id, beds)").
order(year: :desc, month: :desc)
Note that you might have to adjust your #region_id_condition a bit for this to work.
Theoretically it is equivalent of your SQL version(which means it will generate same sql excluding table alias) and returns the AR relation object. Which is the only requirement in the question. Obviously SQL might be improved with additional information as well.
Additionally, you will want to have carefully crafted indexes on this table if you are going to use this query on larger datasets frequently.
I have tried converting this plain sql query to rails active record but I am unable to do so.
select vote_shares.election_year as vs_election_name,
vote_shares.party as vs_party,
(sum(vote_shares.party_seats)/totals.total)*100 AS vs
from pcdemographics INNER JOIN vote_shares on vote_shares.pc_id = pcdemographics.pc_id,
(
SELECT vote_shares.election_name, sum(vote_shares.party_seats) as total
FROM `pcdemographics`
INNER JOIN vote_shares on vote_shares.pc_id = pcdemographics.pc_id
GROUP BY `election_name`
) AS totals
where vote_shares.election_name=totals.election_name
group by vote_shares.party,vote_shares.election_name;
This is what I have tried
#vssubquery = Pcdemographic.select('vote_shares.election_name, sum(vote_shares.party_seats) as total').joins('INNER JOIN vote_shares on vote_shares.pc_id = pcdemographics.pc_id')
Pcdemographic.select("vote_shares.election_year as vs_election_year,
vote_shares.party as vs_party,
(sum(vote_shares.party_seats)/'#{totals.total}')*100 AS vs").from(#vssubquery,:totals)
.joins("INNER JOIN vote_shares on vote_shares.pc_id = pcdemographics.pc_id and vote_shares.election_name='#{totals.election_name}'")
My answer might not be what you hoped for but I recommend not using AR, use Sequel (http://sequel.jeremyevans.net/) instead. It uses the concept of Datasets which I don't think has any equivalent in AR.
Disclaimer: Nobody asked me to advertise for it. I used both AR and Sequel and I found that Sequel is much better to perform complex queries and avoid the N+1 problem.
Did you try find_by_sql method?
How do I form the following query using active record?
SELECT c.*
FROM `course_enrollments` ce JOIN courses c ON ce.course_id = c.id
WHERE ce.created_at
BETWEEN '2000-01-01' and '2012-01-01' [AND ANOTHER POSSIBLE CONDITION]
GROUP BY c.id
I want to be able to do something like: (I know the below is not correct, but I just want to show a general example)
courses = Course.joins(:course_enrollments).where('course_enrollments.created_at' => params[:start_date]..params[:end_date]).group('courses.id')
if some_condition
courses = courses.where(:some_field => 1)
end
The following should get you on the way
Course.joins(:course_enrolements).
where("course_enrolements.created_at between '2000-01-01' and '2012-01-01'").
group("courses.id").
where(MORE CONDITIONS)
use .to_sql to analyze output
Take a look at this Railscast. There are quite a number of ways to do the same elegantly esp. to address your [AND ANOTHER POSSIBLE CONDITION] concern. Also take a look at Squeel gem if its not suggested in the Railscast.
Can anyone explain this?
Project.includes([:user, :company])
This executes 3 queries, one to fetch projects, one to fetch users for those projects and one to fetch companies.
Project.select("name").includes([:user, :company])
This executes 3 queries, and completely ignores the select bit.
Project.select("user.name").includes([:user, :company])
This executes 1 query with proper left joins. And still completely ignores the select.
It would seem to me that rails ignores select with includes. Ok fine, but why when I put a related model in select does it switch from issuing 3 queries to issuing 1 query?
Note that the 1 query is what I want, I just can't imagine this is the right way to get it nor why it works, but I'm not sure how else to get the results in one query (.joins seems to only use INNER JOIN which I do not in fact want, and when I manually specifcy the join conditions to .joins the search gem we're using freaks out as it tries to re-add joins with the same name).
I had the same problem with select and includes.
For eager loading of associated models I used native Rails scope 'preload' http://apidock.com/rails/ActiveRecord/QueryMethods/preload
It provides eager load without skipping of 'select' at scopes chain.
I found it here https://github.com/rails/rails/pull/2303#issuecomment-3889821
Hope this tip will be helpful for someone as it was helpful for me.
Allright so here's what I came up with...
.joins("LEFT JOIN companies companies2 ON companies2.id = projects.company_id LEFT JOIN project_types project_types2 ON project_types2.id = projects.project_type_id LEFT JOIN users users2 ON users2.id = projects.user_id") \
.select("six, fields, I, want")
Works, pain in the butt but it gets me just the data I need in one query. The only lousy part is I have to give everything a model2 alias since we're using meta_search, which seems to not be able to figure out that a table is already joined when you specify your own join conditions.
Rails has always ignored the select argument(s) when using include or includes. If you want to use your select argument then use joins instead.
You might be having a problem with the query gem you're talking about but you can also include sql fragments using the joins method.
Project.select("name").joins(['some sql fragement for users', 'left join companies c on c.id = projects.company_id'])
I don't know your schema so i'd have to guess at the exact relationships but this should get you started.
I might be totally missing something here but select and include are not a part of ActiveRecord. The usual way to do what you're trying to do is like this:
Project.find(:all, :select => "users.name", :include => [:user, :company], :joins => "LEFT JOIN users on projects.user_id = users.id")
Take a look at the api documentation for more examples. Occasionally I've had to go manual and use find_by_sql:
Project.find_by_sql("select users.name from projects left join users on projects.user_id = users.id")
Hopefully this will point you in the right direction.
I wanted that functionality myself,so please use it.
Include this method in your class
#ACCEPTS args in string format "ASSOCIATION_NAME:COLUMN_NAME-COLUMN_NAME"
def self.includes_with_select(*m)
association_arr = []
m.each do |part|
parts = part.split(':')
association = parts[0].to_sym
select_columns = parts[1].split('-')
association_macro = (self.reflect_on_association(association).macro)
association_arr << association.to_sym
class_name = self.reflect_on_association(association).class_name
self.send(association_macro, association, -> {select *select_columns}, class_name: "#{class_name.to_sym}")
end
self.includes(*association_arr)
end
And you will be able to call like: Contract.includes_with_select('user:id-name-status', 'confirmation:confirmed-id'), and it will select those specified columns.
The preload solution doesn't seem to do the same JOINs as eager_load and includes, so to get the best of all worlds I also wrote my own, and released it as a part of a data-related gem I maintain, The Brick.
By overriding ActiveRecord::Associations::JoinDependency.apply_column_aliases() like this then when you add a .select(...) then it can act as a filter to choose which column aliases get built out.
With gem 'brick' loaded, in order to enable this selective behaviour, add the special column name :_brick_eager_load as the first entry in your .select(...), which turns on the filtering of columns while the aliases are being built out. Here's an example:
Employee.includes(orders: :order_details)
.references(orders: :order_details)
.select(:_brick_eager_load,
'employees.first_name', 'orders.order_date', 'order_details.product_id')
Because foreign keys are essential to have everything be properly associated, they are automatically added, so you do not need to include them in your select list.
Hope it can save you both query time and some RAM!
In rails I have 2 tables:
bans(ban_id, admin_id)
ban_reasons(ban_reason_id, ban_id, reason_id)
I want to find all the bans for a certain admin where there is no record in the ban_reasons table. How can I do this in Rails without looping through all the ban records and filtering out all the ones with ban.ban_reasons.nil? I want to do this (hopefully) using a single SQL statement.
I just need to do: (But I want to do it the "rails" way)
SELECT bans.* FROM bans WHERE admin_id=1234 AND
ban_id NOT IN (SELECT ban_id FROM ban_reasons)
Your solution works great (only one request) but it's almost plain SQL:
bans = Ban.where("bans.id NOT IN (SELECT ban_id from ban_reason)")
You may also try the following, and let rails do part of the job:
bans = Ban.where("bans.id NOT IN (?)", BanReason.select(:ban_id).map(&:ban_id).uniq)
ActiveRecord only gets you to a point, everything after should be done by raw SQL. The good thing about AR is that it makes it pretty easy to do that kind of stuff.
However, since Rails 3, you can do almost everything with the AREL API, although raw SQL may or may not look more readable.
I'd go with raw SQL and here is another query you could try if yours doesn't perform well:
SELECT b.*
FROM bans b
LEFT JOIN ban_reason br on b.ban_id = br.ban_id
WHERE br.ban_reason_id IS NULL
Using Where Exists gem (which I'm author of):
Ban.where(admin_id: 123).where_not_exists(:ban_reasons)