I am building a Rails app with the following models:
# vote.rb
class Vote < ApplicationRecord
belongs_to :person
belongs_to :show
scope :fulfilled, -> { where(fulfilled: true) }
scope :unfulfilled, -> { where(fulfilled: false) }
end
# person.rb
class Person < ApplicationRecord
has_many :votes, dependent: :destroy
def self.order_by_votes(show = nil)
count = 'nullif(votes.fulfilled, true)'
count = "case when votes.show_id = #{show.id} AND NOT votes.fulfilled then 1 else null end" if show
people = left_joins(:votes).group(:id).uniq!(:group)
people = people.select("people.*, COUNT(#{count}) AS people.vote_count")
people.order('people.vote_count DESC')
end
end
The idea behind order_by_votes is to sort People by the number of unfulfilled votes, either counting all votes, or counting only votes associated with a given Show.
This seem to work fine when I test against SQLite. But when I switch to Postgres I get this error:
Error:
PeopleControllerIndexTest#test_should_get_previously_on_show:
ActiveRecord::StatementInvalid: PG::UndefinedColumn: ERROR: column people.vote_count does not exist
LINE 1: ...s"."show_id" = $1 GROUP BY "people"."id" ORDER BY people.vot...
^
If I dump the SQL using #people.to_sql, this is what I get:
SELECT people.*, COUNT(nullif(votes.fulfilled, true)) AS people.vote_count FROM "people" LEFT OUTER JOIN "votes" ON "votes"."person_id" = "people"."id" GROUP BY "people"."id" ORDER BY people.vote_count DESC
Why is this failing on Postgres but working on SQLite? And what should I be doing instead to make it work on Postgres?
(PS: I named the field people.vote_count, with a dot, so I can access it in my view without having to do another SQL query to actually view the vote count for each person in the view (not sure if this works) but I get the same error even if I name the field simply vote_count.)
(PS2: I recently added the .uniq!(:group) because of some deprecation warning for Rails 6.2, but I couldn't find any documentation for it so I am not sure I am doing it right, still the error is there without that part.)
Are you sure you're not getting a syntax error from PostgreSQL somewhere? If you do something like this:
select count(*) as t.vote_count from t ... order by t.vote_count
I get a syntax error before PostgreSQL gets to complain about there being no t.vote_count column.
No matter, the solution is to not try to put your vote_count in the people table:
people = people.select("people.*, COUNT(#{count}) AS vote_count")
...
people.order(vote_count: :desc)
You don't need it there, you'll still be able to reference the vote_count just like any "normal" column in people. Anything in the select list will appear as an accessor in the resultant model instances whether they're columns or not, they won't show up in the #inspect output (since that's generated based on the table's columns) but you call the accessor methods nonetheless.
Historically there have been quite a few AR problems (and bugs) in getting the right count by just using count on a scope, and I am not sure they are actually all gone.
That depends on the scope (AR version, relations, group, sort, uniq, etc). A defaut count call that a gem has to generically use on a scope is not a one-fit-all solution. For that known reason Pagy allows you to pass the right count to its pagy method as explained in the Pagy documentation.
Your scope might become complex and the default pagy collection.count(:all) may not get the actual count. In that case you can get the right count with some custom statement, and pass it to pagy.
#pagy, #records = pagy(collection, count: your_count)
Notice: pagy will efficiently skip its internal count query and will just use the passed :count variable.
So... just get your own calculated count and pass it to pagy, and it will not even try to use the default.
EDIT: I forgot to mention: you may want to try the pagy arel extra that:
adds specialized pagination for collections from sql databases with GROUP BY clauses, by computing the total number of results with COUNT(*) OVER ().
Thanks to all the comments and answers I have finally found a solution which I think is the best way to solve this.
First of, the issue occurred when I called pagy which tried to count my scope by appending .count(:all). This is what caused the errors. The solution was to not create a "field" in select() and use it in .order().
So here is the proper code:
def self.order_by_votes(show = nil)
count = if show
"case when votes.show_id = #{show.id} AND NOT votes.fulfilled then 1 else null end"
else
'nullif(votes.fulfilled, true)'
end
left_joins(:votes).group(:id)
.uniq!(:group)
.select("people.*, COUNT(#{count}) as vote_count")
.order(Arel.sql("COUNT(#{count}) DESC"))
end
This sorts the number of people on the number of unfulfilled votes for them, with the ability to count only votes for a given show, and it works with pagy(), and pagy_arel() which in my case is a much better fit, so the results can be properly paginated.
Related
I'm working with a database I have no control over, and cannot make alterations to. This database has a table called warehouse_items. Each warehouse item is uniquely identified by a primary key indicating the item id.
Unfortunately, that primary key attribute is named WAREHOUSE_ITEM.ID
(Note the obnoxious period between "item" and "id")
When I try to run a basic query, such as:
WarehouseItem.find('wh3453')
I get an Undefined Table error.
Fortunately, when looking at what Rails is attempting to do, the problem becomes obvious:
: SELECT "warehouse_items".* FROM "warehouse_items" WHERE "WAREHOUSE_ITEM"."ID" = $1 LIMIT $2
Because of the period in the attribute name, Rails is treating "WAREHOUSE_ITEM.ID" as a table/attribute combination, rather than an attribute name with a period in it.
When I run the following PSQL query by hand, I get exactly what I need:
SELECT "warehouse_items".* FROM "warehouse_items" WHERE "warehouse_items"."WAREHOUSE_ITEM.ID" = 'wh3453'
Why is Rails screwing this up, and how can I fix it?
EDIT:
Also worth noting: I've tried using self.primary_key to override the primary key to no avail.
I've tried both a string and a symbol, as in:
self.primary_key="WAREHOUSE_ITEM.ID"
and
self.primary_key=:"WAREHOUSE_ITEM.ID"
Neither one has worked...
Thanks for all the help, everyone!
A suggestion in the comments to use find_by_sql does work! However, I stumbled onto a different solution that works even better.
First, I aliased the annoying attribute name to something simple: id
alias_attribute :id, :"WAREHOUSE_ITEM.ID"
Notice that it's still a symbol, which is important for the next step.
I then overwrite the primary_key method with a custom function:
def self.primary_key
return "id"
end
Now, when I do WarehouseItem.find('wh3453'), Rails defaults to checking id, which is aliased to the correct symbol and it works as intended!!!
I have User model where it has some attributes like is_admin, is_verified, and also has association with other model such as Badges and Activities.
I have table design in HTML where in default, the user will be ordered by is_verified, then is_admin, then number of badges, then number of activities respectively. But I don't know how to create one.
I have tried sample code like this:
users = User.all.limit(10)
users.order(is_verified: :true).order(is_admin: :true).order(users.map{|user| user.badges.count}).order(users.map{|user| user.activites.count})
But this will not work since order only accept :asc, :desc, :ASC, :DESC, "asc", "desc", "ASC", "DESC"]
Do you have any new methods to do this, I'm new to query? Thank you very much for your help.
I have sample design like this:
order accepts multiple args, so you could use something like the following:
users.order(is_verified: :desc, is_admin: :desc)
.order('badges_count DESC', 'activities_count DESC')
For the boolean columns, they're typically stored in the db as 1 for true, 0 for false, hence the desc direction. If you're not defaulting these columns to one or the other (presumably false), you might want to adjust to (i.e.) order("is_verified DESC NULLS LAST", ...).
This would require a bit of a refactor to run efficiently, relying on counter_caches for badges and activities. If you're going to be querying this regularly, I'd highly advise adding this in. There's a good guide here.
All that will be required is:
new columns for activities_count and badges_count
counter_cache: true set on the belongs_to side of the association
the counters reseting using this method, i.e. User.reset_counters(:badges, :activities)
Otherwise, as is, you could use:
User.left_joins(:badges, :activities)
.order(is_verified: :desc, is_admin: :desc)
.group("badges.id", "activities.id")
.order('COUNT(badges.id) DESC', 'COUNT(activities.id) DESC')
If you're curious as to it's output, you can call to_sql to find how it's actually calling the data from the db.
Have a go at that and let me know how you get on - happy to help if you have any questions!
You could try ordering with database itself, which is more fast than perform sorting on loaded records array. By default if you order any boolean field in descending order, all records having boolean value as true will come first. The final query may look like,
User.order(is_verified: :desc).order(is_admin: :desc).
left_joins(:badges).
group(:id).
order("count(badges.id) desc")
left_joins(:activities)
group(:id).
order("count(activities.id) desc")
Left joins (Left Outer Joins) include documents having zero or more associated records. You can find the details in the documentation page.
Hope it helps !
In my controller method for the the index view I have the following line.
#students_instance = Student.includes(:memo_tests => {:memo_target => :memo_level})
So for each Student I eager-load all necessary info.
Later on in a .map block, I call the .where() method on one of the relations as shown below.
#all_students = #students_instance.map do |student|
...
last_pass = student.memo_tests.where(:result => true).last.created_at.utc
difference_in_weeks = ((last_pass.to_i - current_date.to_i) / 1.week).round
...
end
This leads to a single SQL query for each student. And since I have over 300+ students, leads to very slow load times and over 300+ SQL queries.
Am I right in thinking that this is caused by the .where() method. I think this because I have checked everything else and these are the two lines that cause all of the queries.
More importantly, is there a better way to do this that reduces these queries to a single query?
The moment you ask where, the statement is translated to a query. Normally, the result should be sql-cached...
Anyway, in order to be sure, you can instead add programming logic to your statement. That way, you are not requesting a NEW sql statement.
last_pass = student.memo_tests.map {|m| m.created_at if m.result}.compact.sort.last
EDIT
I see the OP's question does not require sorting... So, leaving the sorting out:
last_pass = student.memo_tests.map {|m| m.created_at if m.result}.compact.last
compact is required to remove nil results from the array.
I have a table KmRelationship which associates Keywords and Movies
In keyword index I would like to list all keywords that appear most frequently in the KmRelationships table and only take(20)
.order doesn't seem to work no matter how I use it and where I put it and same for sort_by
It sounds relatively straight forward but i just can't seem to get it to work
Any ideas?
Assuming your KmRelationship table has keyword_id:
top_keywords = KmRelationship.select('keyword_id, count(keyword_id) as frequency').
order('frequency desc').
group('keyword_id').
take(20)
This may not look right in your console output, but that's because rails doesn't build out an object attribute for the calculated frequency column.
You can see the results like this:
top_keywords.each {|k| puts "#{k.keyword_id} : #{k.freqency}" }
To put this to good use, you can then map out your actual Keyword objects:
class Keyword < ActiveRecord::Base
# other stuff
def self.most_popular
KmRelationship.
select('keyword_id, count(keyword_id) as frequency').
order('frequency desc').
group('keyword_id').
take(20).
map(&:keyword)
end
end
And call with:
Keyword.most_popular
#posts = Post.select([:id, :title]).order("created_at desc").limit(6)
I have this listed in my controller index method which allows the the order to show the last post with a limit of 6. It might be something similar to what you are trying to do. This code actually reflects a most recent post on my home page.
When I use a column alias in a query with find_by_sql, it doesn't appear to be set in the result objects, even when I add an attr_accessor for the property.
class Country < ActiveRecord::Base
attr_accessor :average_score
def self.sorted_by_average_score
sql = "SELECT country_id, AVG(score) AS average_score, countries.name " +
"FROM players " +
"INNER JOIN countries ON players.country_id = countries.id " +
"GROUP BY country_id "
Country.find_by_sql(sql)
end
end
I would expect to be able to do this:
countries = Country.sorted_by_average_score.first.average_score
...but it always returns nil, even though a value is definitely returned from the query.
Can anyone explain to me why the attribute isn't set in the object?
You don't need to use attr_accessor, see neutrino's explanation. You just need to access your virtual column by using the attributes hash. This question is the same as Rails: find_by_sql and virtual column. Sample code for your example:
countries = Country.sorted_by_average_score.first.attributes['average_score']
Because attr_accessors have nothing to do with the way ActiveRecord treats your columns. I've shown it in this question. Basically all the stuff going on in finders that touches columns, works with the attributes hash, not with the instance variables that your accessors declare.
edit: actually my answer doesn't answer the question completely. it just explains why attr_accessor could not be of any help here. looking forward to see someone do the rest of the job :)