I try to find over a 3M table, all the users who have the same username. I read something like this may do the trick.
User.find(:all, :group => [:username], :having => "count(*) > 1" )
However since I'm using Postgres this return me ActiveRecord::StatementInvalid: PG::Error: ERROR: column "users.id" must appear in the GROUP BY clause or be used in an aggregate function.
I'm trying something like this
User.select('users.id, users.username').having("count(*) > 1").group('users.username')
But still get the same error. Any idea what I'm doing wrong?
Update: I made it somehow work using User.select('users.*').group('users.id').having('count(users.username) > 1') but this query returns me this which looks like an empty array even if is founding 5 records.
GroupAggregate (cost=9781143.40..9843673.60 rows=3126510 width=1365)
Filter: (count(username) > 1)
-> Sort (cost=9781143.40..9788959.68 rows=3126510 width=1365)
Sort Key: id
-> Seq Scan on users (cost=0.00..146751.10 rows=3126510 width=1365)
(5 rows)
=> []
Any idea why this is happening and how to get those 5 rows?
I think the best you could get is to get usernames for duplicate records. That can be achieved with
User.select(:username).group(:username).having('COUNT(username) > 1')
"group by" in database collapses each group into one row in output. Most likely what you are intending will be produced by the following query:
User.where("name in (select name from users group by name having count(*)>1)").order(:name)
The inner query above finds all names that appear more than once. Then we find all rows with these names. Ordering by name will make your further processing easier. To speedup, add index to column name in users table.
There are alternate Postgres specific ways to solve this, however the above will work across all databases.
Related
I am trying to query my PostgreSQL database to get the latest (by created_at) and distinct (by user_id) Activity objects, where each user has multiple activities in the database. The activity object is structured as such:
Activity(id, user_id, created_at, ...)
I first tried to get the below query to work:
Activity.order('created_at DESC').select('DISTINCT ON (activities.user_id) activities.*')
however, kept getting the below error:
ActiveRecord::StatementInvalid: PG::InvalidColumnReference: ERROR: SELECT DISTINCT ON expressions must match initial ORDER BY expressions
According to this post: PG::Error: SELECT DISTINCT, ORDER BY expressions must appear in select list, it looks like The ORDER BY clause can only be applied after the DISTINCT has been applied. This does not help me, as I want to get the distinct activities by user_id, but also want the activities to be the most recently created activities. Thus, I need the activities to be sorted before getting the distinct activities.
I have come up with a solution that works, but first grouping the activities by user id, and then ordering the activities within the groups by created_at. However, this takes two queries to do.
I was wondering if what I want is possible in just one query?
This should work, try the following
Solution 1
Activity.select('DISTINCT ON (activities.user_id) activities.*').order('created_at DESC')
Solution 2
If not work Solution 1 then this is helpful if you create a scope for this
activity model
scope :latest, -> {
select("distinct on(user_id) activities.user_id,
activities.*").
order("user_id, created_at desc")
}
Now you can call this anywhere like below
Activity.latest
Hope it helps
So here's the lay of the land:
I have a Applicant model which has_many Lead records.
I need to group leads by applicant email, i.e. for each specific applicant email (there may be 2+ applicant records with the email) i need to get a combined list of leads.
I already have this working using an in-memory / N+1 solution
I want to do this in a single query, if possible. Right now I'm running one for each lead which is maxing out the CPU.
Here's my attempt right now:
Lead.
all.
select("leads.*, applicants.*").
joins(:applicant).
group("applicants.email").
having("count(*) > 1").
limit(1).
to_a
And the error:
Lead Load (1.2ms) SELECT leads.*, applicants.* FROM "leads" INNER
JOIN "applicants" ON "applicants"."id" = "leads"."applicant_id"
GROUP BY applicants.email HAVING count(*) > 1 LIMIT 1
ActiveRecord::StatementInvalid: PG::GroupingError: ERROR: column
"leads.id" must appear in the GROUP BY clause or be used in an
aggregate function
LINE 1: SELECT leads.*, applicants.* FROM "leads" INNER JOIN
"appli...
This is a postgres specific issue. "the selected fields must appear in the GROUP BY clause".
must appear in the GROUP BY clause or be used in an aggregate function
You can try this
Lead.joins(:applicant)
.select('leads.*, applicants.email')
.group_by('applicants.email, leads.id, ...')
You will need to list all the fields in leads table in the group by clause (or all the fields that you are selecting).
I would just get all the records and do the grouping in memory. If you have a lot of records, I would paginate them or batch them.
group_by_email = Hash.new { |h, k| h[k] = [] }
Applicant.eager_load(:leads).each_batch(10_000) do |batch|
batch.each do |applicant|
group_by_email[:applicant.email] << applicant.leads
end
end
You need to use a .where rather than using Lead.all. The reason it is maxing out the CPU is you are trying to load every lead into memory at once. That said I guess I am still missing what you actually want back from the query so it would be tough for me to help you write the query. Can you give more info about your associations and the expected result of the query?
I am getting this error in the pg production mode, but its working fine in sqlite3 development mode.
ActiveRecord::StatementInvalid in ManagementController#index
PG::Error: ERROR: column "estates.id" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: SELECT "estates".* FROM "estates" WHERE "estates"."Mgmt" = ...
^
: SELECT "estates".* FROM "estates" WHERE "estates"."Mgmt" = 'Mazzey' GROUP BY user_id
#myestate = Estate.where(:Mgmt => current_user.Company).group(:user_id).all
If user_id is the PRIMARY KEY then you need to upgrade PostgreSQL; newer versions will correctly handle grouping by the primary key.
If user_id is neither unique nor the primary key for the 'estates' relation in question, then this query doesn't make much sense, since PostgreSQL has no way to know which value to return for each column of estates where multiple rows share the same user_id. You must use an aggregate function that expresses what you want, like min, max, avg, string_agg, array_agg, etc or add the column(s) of interest to the GROUP BY.
Alternately you can rephrase the query to use DISTINCT ON and an ORDER BY if you really do want to pick a somewhat arbitrary row, though I really doubt it's possible to express that via ActiveRecord.
Some databases - including SQLite and MySQL - will just pick an arbitrary row. This is considered incorrect and unsafe by the PostgreSQL team, so PostgreSQL follows the SQL standard and considers such queries to be errors.
If you have:
col1 col2
fred 42
bob 9
fred 44
fred 99
and you do:
SELECT col1, col2 FROM mytable GROUP BY col1;
then it's obvious that you should get the row:
bob 9
but what about the result for fred? There is no single correct answer to pick, so the database will refuse to execute such unsafe queries. If you wanted the greatest col2 for any col1 you'd use the max aggregate:
SELECT col1, max(col2) AS max_col2 FROM mytable GROUP BY col1;
I recently moved from MySQL to PostgreSQL and encountered the same issue. Just for reference, the best approach I've found is to use DISTINCT ON as suggested in this SO answer:
Elegant PostgreSQL Group by for Ruby on Rails / ActiveRecord
This will let you get one record for each unique value in your chosen column that matches the other query conditions:
MyModel.where(:some_col => value).select("DISTINCT ON (unique_col) *")
I prefer DISTINCT ON because I can still get all the other column values in the row. DISTINCT alone will only return the value of that specific column.
After often receiving the error myself I realised that Rails (I am using rails 4) automatically adds an 'order by id' at the end of your grouping query. This often results in the error above. So make sure you append your own .order(:group_by_column) at the end of your Rails query. Hence you will have something like this:
#problems = Problem.select('problems.username, sum(problems.weight) as weight_sum').group('problems.username').order('problems.username')
#myestate1 = Estate.where(:Mgmt => current_user.Company)
#myestate = #myestate1.select("DISTINCT(user_id)")
this is what I did.
In SQLite (development) I don't have any errors, but in production with Postgres I get the following error. I don't really understand the error.
PG::Error: ERROR: column "commits.updated_at" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: ...mmits"."user_id" = 1 GROUP BY mission_id ORDER BY updated_at...
^
: SELECT COUNT(*) AS count_all, mission_id AS mission_id FROM "commits" WHERE "commits"."user_id" = 1 GROUP BY mission_id ORDER BY updated_at DESC
My controller method:
def show
#user = User.find(params[:id])
#commits = #user.commits.order("updated_at DESC").page(params[:page]).per(25)
#missions_commits = #commits.group("mission_id").count.length
end
UPDATE:
So i digged further into this PostgreSQL specific annoyance and I am surprised that this exception is not mentioned in the Ruby on Rails Guide.
I am using psql (PostgreSQL) 9.1.11
So from what I understand, I need to specify which column that should be used whenever you use the GROUP_BY clause. I thought using SELECT would help, which can be annoying if you need to SELECT a lot of columns.
Interesting discussion here
Anyways, when I look at the error, everytime the cursor is pointed to updated_at. In the SQL query, rails will always ORDER BY updated_at. So I have tried this horrible query:
#commits.group("mission_id, date(updated_at)")
.select("date(updated_at), count(mission_id)")
.having("count(mission_id) > 0")
.order("count(mission_id)").length
which gives me the following SQL
SELECT date(updated_at), count(mission_id)
FROM "commits"
WHERE "commits"."user_id" = 1
GROUP BY mission_id, date(updated_at)
HAVING count(mission_id) > 0
ORDER BY updated_at DESC, count(mission_id)
LIMIT 25 OFFSET 0
the error is the same.
Note that no matter what it will ORDER BY updated_at, even if I wanted to order by something else.
Also I don't want to group the records by updated_at just by mission_id.
This PostgreSQL error is just misleading and has little explanation to solving it. I have tried many formulas from the stackoverflow sidebar, nothing works and always the same error.
UPDATE 2:
So I got it to work, but it needs to group the updated_at because of the automatic ORDER BY updated_at. How do I count only by mission_id?
#missions_commits = #commits.group("mission_id, updated_at").count("mission_id").size
I guest you want to show general number of distinct Missions related with Commits, anyway it won't be number on page.
Try this:
#commits = #user.commits.order("updated_at DESC").page(params[:page]).per(25)
#missions_commits = #user.commits.distinct.count(:mission_id)
However if you want to get the number of distinct Missions on page I suppose it should be:
#missions_commits = #commits.collect(&:mission_id).uniq.count
Update
In Rails 3, distinct did not exist, but pure SQL counting should be used this way:
#missions_commits = #user.commits.count(:mission_id, distinct: true)
See the docs for PostgreSQL GROUP BY here:
http://www.postgresql.org/docs/9.3/interactive/sql-select.html#SQL-GROUPBY
Basically, unlike Sqlite (and MySQL) postgres requires that any columns selected or ordered on must appear in an aggregate function or the group by clause.
If you think it through, you'll see that this actually makes sense. Sqlite/MySQL cheat under the hood and silently drop those fields (not sure that's technically what happens).
Or thinking about it another way if you are grouping by a field, what's the point of ordering it? How would that even make sense unless you also had an aggregate function on the ordered field?
I have a relationship between two models, Registers and Competitions. I have a very complicated dynamic query that is being built and if the conditions are right I need to limit Registration records to only those where it's Competition parent meets a certain criteria. In order to do this without select from the Competition table I was thinking of something along the lines of...
Register.where("competition_id in ?", Competition.where("...").collect {|i| i.id})
Which produces this SQL:
SELECT "registers".* FROM "registers" WHERE (competition_id in 1,2,3,4...)
I don't think PostgreSQL liked the fact that the in parameters aren't surrounded by parenthesis. How can I compare the Register foreign key to a list of competition ids?
you can make it a bit shorter and skip the collect (this worked for me in 3.2.3).
Register.where(competition_id: Competition.where("..."))
this will result in the following sql:
SELECT "registers".* FROM "registers" WHERE "registers"."competition_id" IN (SELECT "competitions"."id" FROM "competitions" WHERE "...")
Try this instead:
competitions = Competition.where("...").collect {|i| i.id}
Register.where(:competition_id => competitions)