The query below in Rails console:
i = Comment.group('user_id').count
gives me output like this:
{1=>3, 2=>6, 3=>2, 4=>8}
where 1,2,3,4 are user ids and 3,6,2,8 are the count of the rows with these user ids. Please shed some light on me on how I can use these data. As i.count gives me total no.
I want to access these individual user row counts. Again, i[0].count or i[1].count gives me an error.
The expression gives you a Ruby Hash of key/value pairs, which you can access using the following:
i[1] # => 3
i[2] # => 6
... etc ...
You don't need to call count on them, the value of calling i[2] will be the count.
If I understand your question, you can access the hash with i[1] to get the comment count for the user with ID #1, instead of adding .count as you did in your example.
Related
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.
So I'm trying to get some specific data out of my database but I've searching online and can't find how to do this (probably because I'm searching for the wrong terms).
I start with getting all the participants with a specific id like this :
contributions = Participant.where(user_id: params[:id])
This will give me a json result like this :
0: {id_request: "1", user_id: "titivermeesch#gmail.com"}
1: {id_request: "2", user_id: "titivermeesch#gmail.com"}
So here I have all the requests (there is a Request class) that have that specific user_id.
Now I want to do this :
all = Request.where(id: id_request)
This obviously don't work but how would I get all those requests that have all those id's that come from the first database query?
So with the results above I should get Request 1 and 2, but how? Can anyone guide me?
How about
contributions = Participant.where(user_id: params[:id])
# Assuming the above is an active record query and id_request is a property of Participant
all = Request.where(id: contributions.map(&:id_request))
This is the equivalent of the SQL
select * from requests where id in (array_of_request_ids)
If You added associations in your model? it's very easy to retrieve the records
This should work:
Request.joins(:participants).where("participants.user_id = ?", params[:id])
Also you might want to read the following part (on joins)
I have small problem. Lets say I have users 1, 2 and 4. And now I'm trying to use this query:
User.select(:channel).where(:id => rand(1..User.count)).first.channel
Which in this case works like this:
User.select(:channel).where(:id => rand(1..3)).first.channel
And well, thats my problem. I can select user 1 and user 2. User 4 is unreachable. And if it try to take user 3 it returns me that there is no .channel method, because everything is nil then... What should I do so it could reach users 1, 2, 4 and ignore user 3 which is nil?
You could first get the list of available record IDs and then pick a random id:
ids = User.pluck(:id)
User.select(:channel).where(:id => ids.sample).first.channel
But of course that requires two queries so if that isn't efficient enough you could try telling the DB itself to select a random record. For example, if you're using MySQL you could so something like this:
User.order('rand()').limit(1)
I'm trying to select the User Id as a variable in the console however I keep ending up with:
[#<User id: 4>]
The find statement I have tried is:
userid = User.select('id').where('username = ?', 'uwZgf')
I've also tried with find_by_sql with same result.
What do I need to get the value out instead of the hash?
What you've got there is an array of one User object.
User.select(...).where(...).first.id
Would do the trick (you'd probably want to check the value returned by first before trying to call id on it.
You might find
User.find_by_username('foo').try(:id)
more readable.
In rails 3, I would like to do the following:
SomeModel.where(:some_connection_id => anArrayOfIds).select("some_other_connection_id")
This works, but i get the following from the DB:
[{"some_other_connection_id":254},{"some_other_connection_id":315}]
Now, those id-s are the ones I need, but I am uncapable of making a query that only gives me the ids. I do not want to have to itterate over the resulst, only to get those numbers out. Are there any way for me to do this with something like :
SomeModel.where(:some_connection_id => anArrayOfIds).select("some_other_connection_id").values()
Or something of that nautre?
I have been trying with the ".select_values()" found at Git-hub, but it only returns "some_other_connection_id".
I am not an expert in rails, so this info might be helpful also:
The "SomeModel" is a connecting table, for a many-to-many relation in one of my other models. So, accually what I am trying to do is to, from the array of IDs, get all the entries from the other side of the connection. Basicly I have the source ids, and i want to get the data from the models with all the target ids. If there is a magic way of getting these without me having to do all the sql myself (with some help from active record) it would be really nice!
Thanks :)
Try pluck method
SomeModel.where(:some => condition).pluck("some_field")
it works like
SomeModel.where(:some => condition).select("some_field").map(&:some_field)
SomeModel.where(:some_connection_id => anArrayOfIds).select("some_other_connection_id").map &:some_other_connection_id
This is essentially a shorthand for:
results = SomeModel.where(:some_connection_id => anArrayOfIds).select("some_other_connection_id")
results.map {|row| row.some_other_connection_id}
Look at Array#map for details on map method.
Beware that there is no lazy loading here, as it iterates over the results, but it shouldn't be a problem, unless you want to add more constructs to you query or retrieve some associated objects(which should not be the case as you haven't got the ids for loading the associated objects).